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C++ Programming Code Examples

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Copy files

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/* Copy files */ #include <iostream> #include <stdlib.h> #include <fstream> using namespace std; int main(int argc, char **argv) { char buffer[1]; ifstream input(argv[1], ios::in); if (input.fail()) { cout << "Error opening the file " << argv[1]; exit(1); } ofstream output(argv[2], ios::out); if (output.fail()) { cout << "Error opening the file " << argv[2]; exit(1); } do { input.read(buffer, sizeof(buffer)); if (input.good()) output.write(buffer, sizeof(buffer)); } while (! input.eof()); input.close(); output.close(); }
IOS Library eof() Function in C++
Check whether eofbit is set. Returns true if the eofbit error state flag is set for the stream. This flag is set by all standard input operations when the End-of-File is reached in the sequence associated with the stream. Note that the value returned by this function depends on the last operation performed on the stream (and not on the next). Operations that attempt to read at the End-of-File fail, and thus both the eofbit and the failbit end up set. This function can be used to check whether the failure is due to reaching the End-of-File or to some other reason.
Syntax for IOS eof() Function in C++
bool eof() const;
This function does not accept any parameter. Function returns true if the stream's eofbit error state flag is set (which signals that the End-of-File has been reached by the last input operation). false otherwise.
Data races
Accesses the stream object. Concurrent access to the same stream object may cause data races.
Exception safety
Strong guarantee: if an exception is thrown, there are no changes in the stream.
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/* The eof() method of ios class in C++ is used to check if the stream is has raised any EOF (End Of File) error. It means that this function will check if this stream has its eofbit set. */ // C++ code example to demonstrate the working of eof() function #include <iostream> #include <fstream> int main () { std::ifstream is("example.txt"); char c; while (is.get(c)) std::cout << c; if (is.eof()) std::cout << "[EoF reached]\n"; else std::cout << "[error reading]\n"; is.close(); return 0; }
IOS Library good() Function in C++
Check whether state of stream is good. Returns true if none of the stream's error state flags (eofbit, failbit and badbit) is set. ios::good() and ios::bad() functions in C++ are used to check the state of the stream whether it is good or bad to do our task. Both of these are defined in ios library. The good() method of ios class in C++ is used to check if the stream is good enough to work. It means that this function will check if this stream has raised any error or not. Notice that this function is not the exact opposite of member bad, which only checks whether the badbit flag is set.
Syntax for IOS good() Function in C++
#include <iostream> bool good() const;
This function does not accept any parameter. Function returns true if none of the stream's state flags are set. Function returns false if any of the stream's state flags are set (badbit, eofbit or failbit). Whether specific error flags are set, can be checked with member functions eof, fail, and bad: iostate value indicates functions to check state flags (member constant) good() eof() fail() bad() rdstate() goodbit No errors (zero value iostate) true false false false goodbit eofbit End-of-File reached on input operation false true false false eofbit failbit Logical error on i/o operation false false true false failbit badbit Read/writing error on i/o operation false false true true badbit eofbit, failbit and badbit are member constants with implementation-defined values that can be combined (as if with the bitwise OR operator). goodbit is zero, indicating that none of the other bits is set.
Data races
Accesses the stream object. Concurrent access to the same stream object may cause data races.
Exception safety
Strong guarantee: if an exception is thrown, there are no changes in the stream.
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/* IOS Library good() is used to check whether state of stream is good. */ /* ios::good() and ios::bad() functions in C++ are used to check the state of the stream whether it is good or bad to do our task. Both of these are defined in ios library. */ #include <iostream> #include <sstream> void print_state (const std::ios& stream) { std::cout << " good()=" << stream.good(); std::cout << " eof()=" << stream.eof(); std::cout << " fail()=" << stream.fail(); std::cout << " bad()=" << stream.bad(); } int main () { std::stringstream stream; stream.clear (stream.goodbit); std::cout << "goodbit:"; print_state(stream); std::cout << '\n'; stream.clear (stream.eofbit); std::cout << " eofbit:"; print_state(stream); std::cout << '\n'; stream.clear (stream.failbit); std::cout << "failbit:"; print_state(stream); std::cout << '\n'; stream.clear (stream.badbit); std::cout << " badbit:"; print_state(stream); std::cout << '\n'; return 0; }
exit() Function in C++
The exit function terminates the program normally. Automatic objects are not destroyed, but static objects are. Then, all functions registered with atexit are called in the opposite order of registration. The code is returned to the operating system. An exit code of 0 or EXIT_SUCCESS means successful completion. If code is EXIT_FAILURE, an indication of program failure is returned to the operating system. Other values of code are implementation-defined.
Syntax for exit() Function in C++
void exit (int status);
status
Status code. If this is 0 or EXIT_SUCCESS, it indicates success. If it is EXIT_FAILURE, it indicates failure. Calls all functions registered with the atexit() function, and destroys C++ objects with static storage duration, all in last-in-first-out (LIFO) order. C++ objects with static storage duration are destroyed in the reverse order of the completion of their constructor. (Automatic objects are not destroyed as a result of calling exit().) Functions registered with atexit() are called in the reverse order of their registration. A function registered with atexit(), before an object obj1 of static storage duration is initialized, will not be called until obj1's destruction has completed. A function registered with atexit(), after an object obj2 of static storage duration is initialized, will be called before obj2's destruction starts. Normal program termination performs the following (in the same order): • Objects associated with the current thread with thread storage duration are destroyed (C++11 only). • Objects with static storage duration are destroyed (C++) and functions registered with atexit are called. • All C streams (open with functions in <cstdio>) are closed (and flushed, if buffered), and all files created with tmpfile are removed. • Control is returned to the host environment. Note that objects with automatic storage are not destroyed by calling exit (C++). If status is zero or EXIT_SUCCESS, a successful termination status is returned to the host environment. If status is EXIT_FAILURE, an unsuccessful termination status is returned to the host environment. Otherwise, the status returned depends on the system and library implementation. Flushes all buffers, and closes all open files. All files opened with tmpfile() are deleted. Returns control to the host environment from the program. exit() returns no values.
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/* terminate the process normally, performing the regular cleanup for terminating programs by exit() function code example */ #include<iostream> using namespace std; int main() { int i; cout<<"Enter a non-zero value: "; //user input cin>>i; if(i) // checks whether the user input is non-zero or not { cout<<"Valid input.\n"; } else { cout<<"ERROR!"; //the program exists if the value is 0 exit(0); } cout<<"The input was : "<<i; }
read() Function in C++
Read block of data. Extracts n characters from the stream and stores them in the array pointed to by s. This function simply copies a block of data, without checking its contents nor appending a null character at the end. If the input sequence runs out of characters to extract (i.e., the end-of-file is reached) before n characters have been successfully read, the array pointed to by s contains all the characters read until that point, and both the eofbit and failbit flags are set for the stream. Internally, the function accesses the input sequence by first constructing a sentry object (with noskipws set to true). Then (if good), it extracts characters from its associated stream buffer object as if calling its member functions sbumpc or sgetc, and finally destroys the sentry object before returning. The number of characters successfully read and stored by this function can be accessed by calling member gcount.
Syntax for read() Function in C++
#include <fstream> istream& read (char* s, streamsize n);
s
Pointer to an array where the extracted characters are stored.
n
Number of characters to extract. streamsize is a signed integral type. Function returns the istream object (*this). Errors are signaled by modifying the internal state flags: • eofbit: The function stopped extracting characters because the input sequence has no more characters available (end-of-file reached). • failbit: Either the function could not extract n characters or the construction of sentry failed. • badbit: Error on stream (such as when this function catches an exception thrown by an internal operation). When set, the integrity of the stream may have been affected. Multiple flags may be set by a single operation. If the operation sets an internal state flag that was registered with member exceptions, the function throws an exception of member type failure.
Data races
Modifies the elements in the array pointed to by s and the stream object. Concurrent access to the same stream object may cause data races, except for the standard stream object cin when this is synchronized with stdio (in this case, no data races are initiated, although no guarantees are given on the order in which extracted characters are attributed to threads).
Exception safety
Basic guarantee: if an exception is thrown, the object is in a valid state. It throws an exception of member type failure if the resulting error state flag is not goodbit and member exceptions was set to throw for that state. Any exception thrown by an internal operation is caught and handled by the function, setting badbit. If badbit was set on the last call to exceptions, the function rethrows the caught exception.
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/* The function read() in <iostream> header file extracts n characters from the stream and stores them in the array pointed to the stream. Unlike functions put() and get() it is usually used to handle the data in binary form. */ /* extracts n characters from the stream and stores them in the array pointed by s with read() function code example. */ #include <iostream> #include <fstream> int main () { std::ifstream is ("test.txt", std::ifstream::binary); if (is) { is.seekg (0, is.end); int length = is.tellg(); is.seekg (0, is.beg); char * buffer = new char [length]; std::cout << "Reading " << length << " characters... "; is.read (buffer,length); if (is) std::cout << "all characters read successfully."; else std::cout << "error: only " << is.gcount() << " could be read"; is.close(); delete[] buffer; } return 0; }
C++ Files and Streams
In C++ programming we are using the iostream standard library, it provides cin and cout methods for reading from input and writing to output respectively. To read and write from a file we are using the standard C++ library called fstream. Let us see the data types define in fstream library is: • ofstream: This data type represents the output file stream and is used to create files and to write information to files. • ifstream: This data type represents the input file stream and is used to read information from files. • fstream: This data type represents the file stream generally, and has the capabilities of both ofstream and ifstream which means it can create files, write information to files, and read information from files. To perform file processing in C++, header files <iostream> and <fstream> must be included in your C++ source file.
Opening a File
A file must be opened before you can read from it or write to it. Either ofstream or fstream object may be used to open a file for writing. And ifstream object is used to open a file for reading purpose only. File streams in C++ are basically the libraries that are used in the due course of programming. The programmers generally use the iostream standard library in the C++ programming as it provides the cin and cout methods that are used for reading from the input and writing to the output respectively. In order to read and write from a file, the programmers are generally using the standard C++ library that is known as the fstream. Following is the standard syntax for open() function, which is a member of fstream, ifstream, and ofstream objects.
void open(const char *filename, ios::openmode mode);
Here, the first argument specifies the name and location of the file to be opened and the second argument of the open() member function defines the mode in which the file should be opened. ios::app Append mode. All output to that file to be appended to the end. ios::ate Open a file for output and move the read/write control to the end of the file. ios::in Open a file for reading. ios::out Open a file for writing. ios::trunc If the file already exists, its contents will be truncated before opening the file.
Closing a File
When a C++ program terminates it automatically flushes all the streams, release all the allocated memory and close all the opened files. But it is always a good practice that a programmer should close all the opened files before program termination. Following is the standard syntax for close() function, which is a member of fstream, ifstream, and ofstream objects.
void close();
Writing to a File
While doing C++ programming, you write information to a file from your program using the stream insertion operator (<<) just as you use that operator to output information to the screen. The only difference is that you use an ofstream or fstream object instead of the cout object.
Reading from a File
You read information from a file into your program using the stream extraction operator (>>) just as you use that operator to input information from the keyboard. The only difference is that you use an ifstream or fstream object instead of the cin object. 'ifstream' data type of 'fstream' library is used to read the files of C++. But before reading, there are several tasks which are performed sequentially like opening the file, reading and closing it. Different data types are used for the specific purpose. We can simply read the information from the file using the operator ( >> ) with the name of the file. We need to use the fstream or ifstream object in C++ in order to read the file. Reading of the file line by line can be done by simply using the while loop along with the function of ifstream 'getline()'.
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/* C++ files and streams */ /* writing to a file and reading from a file in C++ language */ #include <fstream> #include <iostream> using namespace std; int main () { char data[100]; // open a file in write mode. ofstream outfile; outfile.open("afile.dat"); cout << "Writing to the file" << endl; cout << "Enter your name: "; cin.getline(data, 100); // write inputted data into the file. outfile << data << endl; cout << "Enter your age: "; cin >> data; cin.ignore(); // again write inputted data into the file. outfile << data << endl; // close the opened file. outfile.close(); // open a file in read mode. ifstream infile; infile.open("afile.dat"); cout << "Reading from the file" << endl; infile >> data; // write the data at the screen. cout << data << endl; // again read the data from the file and display it. infile >> data; cout << data << endl; // close the opened file. infile.close(); return 0; }
Ostream Library put() Function in C++
Put character. Inserts character c into the stream. Internally, the function accesses the output sequence by first constructing a sentry object. Then (if good), it inserts c into its associated stream buffer object as if calling its member function sputc, and finally destroys the sentry object before returning.
Syntax for Ostream put() Function in C++
ostream& put (char c);
c
Character to write Function returns the ostream object (*this). Errors are signaled by modifying the internal state flags: • eofbit - • failbit May be set if the construction of sentry failed. • badbit Either the insertion on the stream failed, or some other error happened (such as when this function catches an exception thrown by an internal operation). When set, the integrity of the stream may have been affected. Multiple flags may be set by a single operation. If the operation sets an internal state flag that was registered with member exceptions, the function throws an exception of member type failure. Through the previous study, we know that C++ programs generally use the cout output stream object of the ostream class and the << output operator to achieve output, and the cout output stream has a corresponding buffer in memory. But sometimes users have special output requirements, such as outputting only one character. In this case, you can use the put() member method provided by this class to achieve.
Data races
Modifies the stream object. Concurrent access to the same stream object may cause data races, except for the standard stream objects (cout, cerr, clog) when these are synchronized with stdio (in this case, no data races are initiated, although no guarantees are given on the order in which characters from multiple threads are inserted).
Exception safety
Basic guarantee: if an exception is thrown, the object is in a valid state. It throws an exception of member type failure if the resulting error state flag is not goodbit and member exceptions was set to throw for that state. Any exception thrown by an internal operation is caught and handled by the function, setting badbit. If badbit was set on the last call to exceptions, the function rethrows the caught exception.
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/* It is used to inserts character c into the stream.this function accesses the output sequence by first constructing a sentry object. Then (if good), it inserts c into its associated stream buffer object as if calling its member function sputc, and finally destroys the sentry object before returning. */ //C++ code example to Writing data to a file using put() function and ios::out mode #include<iostream> #include<fstream> #include<cstring> using namespace std; int main() { //Creating an output stream to write data to a file ofstream ofstream_ob; //Opens/creates a file named File2.txt ofstream_ob.open("File2.txt", ios::out); char arr[100] = "Hello World. We wish you best in everything. Never give up!"; int length = strlen(arr); char ch; //Reading the char array i.e. a character at a time and writing it to the file for(int i=0; i<length; i++) { ch = arr[i]; ofstream_ob.put(ch); //Writing a character to file, by using put() function } //Closing the output stream ofstream_ob.close(); return 0; }
If Else Statement in C++
In computer programming, we use the if statement to run a block code only when a certain condition is met. An if statement can be followed by an optional else statement, which executes when the boolean expression is false. There are three forms of if...else statements in C++: • if statement, • if...else statement, • if...else if...else statement,
Syntax for If Statement in C++
if (condition) { // body of if statement }
The if statement evaluates the condition inside the parentheses ( ). If the condition evaluates to true, the code inside the body of if is executed. If the condition evaluates to false, the code inside the body of if is skipped.
Syntax for If...Else Statement
if (condition) { // block of code if condition is true } else { // block of code if condition is false }
The if..else statement evaluates the condition inside the parenthesis. If the condition evaluates true, the code inside the body of if is executed, the code inside the body of else is skipped from execution. If the condition evaluates false, the code inside the body of else is executed, the code inside the body of if is skipped from execution. The if...else statement is used to execute a block of code among two alternatives. However, if we need to make a choice between more than two alternatives, we use the if...else if...else statement.
Syntax for If...Else...Else If Statement in C++
if (condition1) { // code block 1 } else if (condition2){ // code block 2 } else { // code block 3 }
• If condition1 evaluates to true, the code block 1 is executed. • If condition1 evaluates to false, then condition2 is evaluated. • If condition2 is true, the code block 2 is executed. • If condition2 is false, the code block 3 is executed. There can be more than one else if statement but only one if and else statements. In C/C++ if-else-if ladder helps user decide from among multiple options. The C/C++ if statements are executed from the top down. As soon as one of the conditions controlling the if is true, the statement associated with that if is executed, and the rest of the C else-if ladder is bypassed. If none of the conditions is true, then the final else statement will be executed.
Syntax for If Else If Ladder in C++
if (condition) statement 1; else if (condition) statement 2; . . else statement;
Working of the if-else-if ladder: 1. Control falls into the if block. 2. The flow jumps to Condition 1. 3. Condition is tested. If Condition yields true, goto Step 4. If Condition yields false, goto Step 5. 4. The present block is executed. Goto Step 7. 5. The flow jumps to Condition 2. If Condition yields true, goto step 4. If Condition yields false, goto Step 6. 6. The flow jumps to Condition 3. If Condition yields true, goto step 4. If Condition yields false, execute else block. Goto Step 7. 7. Exits the if-else-if ladder. • The if else ladder statement in C++ programming language is used to check set of conditions in sequence. • This is useful when we want to selectively executes one code block(out of many) based on certain conditions. • It allows us to check for multiple condition expressions and execute different code blocks for more than two conditions. • A condition expression is tested only when all previous if conditions in if-else ladder is false. • If any of the conditional expression evaluates to true, then it will execute the corresponding code block and exits whole if-else ladder.
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/* If Else Statement in C++ Language */ #include <iostream> using namespace std; int main () { // local variable declaration: int a = 100; // check the boolean condition if( a < 20 ) { // if condition is true then print the following cout << "a is less than 20;" << endl; } else { // if condition is false then print the following cout << "a is not less than 20;" << endl; } cout << "value of a is : " << a << endl; return 0; }
#include Directive in C++
#include is a way of including a standard or user-defined file in the program and is mostly written at the beginning of any C/C++ program. This directive is read by the preprocessor and orders it to insert the content of a user-defined or system header file into the following program. These files are mainly imported from an outside source into the current program. The process of importing such files that might be system-defined or user-defined is known as File Inclusion. This type of preprocessor directive tells the compiler to include a file in the source code program.
Syntax for #include Directive in C++
#include "user-defined_file"
Including using " ": When using the double quotes(" "), the preprocessor access the current directory in which the source "header_file" is located. This type is mainly used to access any header files of the user's program or user-defined files.
#include <header_file>
Including using <>: While importing file using angular brackets(<>), the the preprocessor uses a predetermined directory path to access the file. It is mainly used to access system header files located in the standard system directories. Header File or Standard files: This is a file which contains C/C++ function declarations and macro definitions to be shared between several source files. Functions like the printf(), scanf(), cout, cin and various other input-output or other standard functions are contained within different header files. So to utilise those functions, the users need to import a few header files which define the required functions. User-defined files: These files resembles the header files, except for the fact that they are written and defined by the user itself. This saves the user from writing a particular function multiple times. Once a user-defined file is written, it can be imported anywhere in the program using the #include preprocessor. • In #include directive, comments are not recognized. So in case of #include <a//b>, a//b is treated as filename. • In #include directive, backslash is considered as normal text not escape sequence. So in case of #include <a\nb>, a\nb is treated as filename. • You can use only comment after filename otherwise it will give error.
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/* using #include directive in C language */ #include <stdio.h> int main() { /* * C standard library printf function * defined in the stdio.h header file */ printf("I love you Clementine"); printf("I love you so much"); printf("HappyCodings"); return 0; }
While Loop Statement in C++
In while loop, condition is evaluated first and if it returns true then the statements inside while loop execute, this happens repeatedly until the condition returns false. When condition returns false, the control comes out of loop and jumps to the next statement in the program after while loop. The important point to note when using while loop is that we need to use increment or decrement statement inside while loop so that the loop variable gets changed on each iteration, and at some point condition returns false. This way we can end the execution of while loop otherwise the loop would execute indefinitely. A while loop that never stops is said to be the infinite while loop, when we give the condition in such a way so that it never returns false, then the loops becomes infinite and repeats itself indefinitely.
Syntax for While Loop Statement in C++
while (condition) { // body of the loop }
• A while loop evaluates the condition • If the condition evaluates to true, the code inside the while loop is executed. • The condition is evaluated again. • This process continues until the condition is false. • When the condition evaluates to false, the loop terminates. Do not forget to increase the variable used in the condition, otherwise the loop will never end!
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/* While Loop Statement in C++ language */ // program to find the sum of positive numbers // if the user enters a negative number, the loop ends // the negative number entered is not added to the sum #include <iostream> using namespace std; int main() { int number; int sum = 0; // take input from the user cout << "Enter a number: "; cin >> number; while (number >= 0) { // add all positive numbers sum += number; // take input again if the number is positive cout << "Enter a number: "; cin >> number; } // display the sum cout << "\nThe sum is " << sum << endl; return 0; }
write() Function in C++
Write block of data. Inserts the first n characters of the array pointed by s into the stream. This function simply copies a block of data, without checking its contents: The array may contain null characters, which are also copied without stopping the copying process. Internally, the function accesses the output sequence by first constructing a sentry object. Then (if good), it inserts character into its associated stream buffer object as if calling its member function sputc until n characters have been written or until an insertion fails (in this case it sets the badbit flag). Finally, it destroys the sentry object before returning.
Syntax for write() Function in C++
#include <fstream> ostream& write (const char* s, streamsize n);
s
Pointer to an array of at least n characters.
n
Number of characters to insert. Integer value of type streamsize representing the size in characters of the block of data to write. streamsize is a signed integral type. Function returns the ostream object (*this). Errors are signaled by modifying the internal state flags: • eofbit: - • failbit: May be set if the construction of sentry failed. • badbit: Either an insertion on the stream failed, or some other error happened (such as when this function catches an exception thrown by an internal operation). When set, the integrity of the stream may have been affected. Multiple flags may be set by a single operation. If the operation sets an internal state flag that was registered with member exceptions, the function throws an exception of member type failure.
Data races
Access up to n characters pointed by s. Modifies the stream object. Concurrent access to the same stream object may cause data races, except for the standard stream objects (cout, cerr, clog) when these are synchronized with stdio (in this case, no data races are initiated, although no guarantees are given on the order in which characters from multiple threads are inserted).
Exception safety
Basic guarantee: if an exception is thrown, the object is in a valid state. It throws an exception of member type failure if the resulting error state flag is not goodbit and member exceptions was set to throw for that state. Any exception thrown by an internal operation is caught and handled by the function, setting badbit. If badbit was set on the last call to exceptions, the function rethrows the caught exception.
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/* write block of data by ostream::write function code example */ /* in order to perform a binary input/output operation using the read() and write() functions, C++ provides us a few file stream classes */ // Copy a file #include <fstream> // std::ifstream, std::ofstream int main () { std::ifstream infile ("test.txt",std::ifstream::binary); std::ofstream outfile ("new.txt",std::ofstream::binary); // get size of file infile.seekg (0,infile.end); long size = infile.tellg(); infile.seekg (0); // allocate memory for file content char* buffer = new char[size]; // read content of infile infile.read (buffer,size); // write to outfile outfile.write (buffer,size); // release dynamically-allocated memory delete[] buffer; outfile.close(); infile.close(); return 0; }
IOS Library fail() Function in C++
Check whether either failbit or badbit is set. Returns true if either (or both) the failbit or the badbit error state flags is set for the stream. At least one of these flags is set when an error occurs during an input operation. failbit is generally set by an operation when the error is related to the internal logic of the operation itself; further operations on the stream may be possible. While badbit is generally set when the error involves the loss of integrity of the stream, which is likely to persist even if a different operation is attempted on the stream. badbit can be checked independently by calling member function bad.
Syntax for IOS fail() Function in C++
bool fail() const;
This function does not accept any parameter. Function returns true if badbit and/or failbit are set. false otherwise. iostate value indicates functions to check state flags (member constant) good() eof() fail() bad() rdstate() goodbit No errors (zero value iostate) true false false false goodbit eofbit End-of-File reached on input operation false true false false eofbit failbit Logical error on i/o operation false false true false failbit badbit Read/writing error on i/o operation false false true true badbit eofbit, failbit and badbit are member constants with implementation-defined values that can be combined (as if with the bitwise OR operator). goodbit is zero, indicating that none of the other bits is set. Reaching the End-of-File sets the eofbit. But note that operations that reach the End-of-File may also set the failbit if this makes them fail (thus setting both eofbit and failbit). This function is a synonym of ios::operator!.
Data races
Accesses the stream object. Concurrent access to the same stream object may cause data races.
Exception safety
Strong guarantee: if an exception is thrown, there are no changes in the stream.
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/* The fail() method of ios class in C++ is used to check if the stream is has raised any fail error. It means that this function will check if this stream has its failbit set. */ /* check whether either failbit or badbit is set by ios::fail function code example */ #include <iostream> #include <fstream> #include <cstdlib> int main() { std::ifstream file("test.txt"); if(!file) // operator! is used here { std::cout << "File opening failed\n"; return EXIT_FAILURE; } // typical C++ I/O loop uses the return value of the I/O function // as the loop controlling condition, operator bool() is used here for(int n; file >> n; ) { std::cout << n << ' '; } std::cout << '\n'; if (file.bad()) std::cout << "I/O error while reading\n"; else if (file.eof()) std::cout << "End of file reached successfully\n"; else if (file.fail()) std::cout << "Non-integer data encountered\n"; }
main() Function in C++
A program shall contain a global function named main, which is the designated start of the program in hosted environment. main() function is the entry point of any C++ program. It is the point at which execution of program is started. When a C++ program is executed, the execution control goes directly to the main() function. Every C++ program have a main() function.
Syntax for main() Function in C++
void main() { ............ ............ }
void
void is a keyword in C++ language, void means nothing, whenever we use void as a function return type then that function nothing return. here main() function no return any value.
main
main is a name of function which is predefined function in C++ library. In place of void we can also use int return type of main() function, at that time main() return integer type value. 1) It cannot be used anywhere in the program a) in particular, it cannot be called recursively b) its address cannot be taken 2) It cannot be predefined and cannot be overloaded: effectively, the name main in the global namespace is reserved for functions (although it can be used to name classes, namespaces, enumerations, and any entity in a non-global namespace, except that a function called "main" cannot be declared with C language linkage in any namespace). 3) It cannot be defined as deleted or (since C++11) declared with C language linkage, constexpr (since C++11), consteval (since C++20), inline, or static. 4) The body of the main function does not need to contain the return statement: if control reaches the end of main without encountering a return statement, the effect is that of executing return 0;. 5) Execution of the return (or the implicit return upon reaching the end of main) is equivalent to first leaving the function normally (which destroys the objects with automatic storage duration) and then calling std::exit with the same argument as the argument of the return. (std::exit then destroys static objects and terminates the program). 6) (since C++14) The return type of the main function cannot be deduced (auto main() {... is not allowed). 7) (since C++20) The main function cannot be a coroutine.
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/* simple code example by main() function in C++ */ #include <iostream> using namespace std; int main() { int day = 4; switch (day) { case 1: cout << "Monday"; break; case 2: cout << "Tuesday"; break; case 3: cout << "Wednesday"; break; case 4: cout << "Thursday"; break; case 5: cout << "Friday"; break; case 6: cout << "Saturday"; break; case 7: cout << "Sunday"; break; } return 0; }
sizeof() Operator in C++
The sizeof() is an operator that evaluates the size of data type, constants, variable. It is a compile-time operator as it returns the size of any variable or a constant at the compilation time. The size, which is calculated by the sizeof() operator, is the amount of RAM occupied in the computer. The sizeof is a keyword, but it is a compile-time operator that determines the size, in bytes, of a variable or data type. The sizeof operator can be used to get the size of classes, structures, unions and any other user defined data type.
Syntax for sizeof() Operator in C++
sizeof(data_type);
data_type
data type whose size is to be calculated The data_type can be the data type of the data, variables, constants, unions, structures, or any other user-defined data type. If the parameter of a sizeof() operator contains the data type of a variable, then the sizeof() operator will return the size of the data type. sizeof() may give different output according to machine, we have run our program on 32 bit gcc compiler.
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/* The sizeof() is an operator in C and C++. It is an unary operator which assists a programmer in finding the size of the operand which is being used. */ #include <iostream> using namespace std; int main() { int arr[]={10,20,30,40,50}; std::cout << "Size of the array 'arr' is : "<<sizeof(arr) << std::endl; cout << "Size of char : " << sizeof(char) << endl; cout << "Size of int : " << sizeof(int) << endl; cout << "Size of short int : " << sizeof(short int) << endl; cout << "Size of long int : " << sizeof(long int) << endl; cout << "Size of float : " << sizeof(float) << endl; cout << "Size of double : " << sizeof(double) << endl; cout << "Size of wchar_t : " << sizeof(wchar_t) << endl; return 0; }
Namespaces in C++ Language
Consider a situation, when we have two persons with the same name, jhon, in the same class. Whenever we need to differentiate them definitely we would have to use some additional information along with their name, like either the area, if they live in different area or their mother's or father's name, etc. Same situation can arise in your C++ applications. For example, you might be writing some code that has a function called xyz() and there is another library available which is also having same function xyz(). Now the compiler has no way of knowing which version of xyz() function you are referring to within your code. A namespace is designed to overcome this difficulty and is used as additional information to differentiate similar functions, classes, variables etc. with the same name available in different libraries. Using namespace, you can define the context in which names are defined. In essence, a namespace defines a scope.
Defining a Namespace
A namespace definition begins with the keyword namespace followed by the namespace name as follows:
namespace namespace_name { // code declarations }
To call the namespace-enabled version of either function or variable, prepend (::) the namespace name as follows:
name::code; // code could be variable or function.
Using Directive
You can also avoid prepending of namespaces with the using namespace directive. This directive tells the compiler that the subsequent code is making use of names in the specified namespace.
Discontiguous Namespaces
A namespace can be defined in several parts and so a namespace is made up of the sum of its separately defined parts. The separate parts of a namespace can be spread over multiple files. So, if one part of the namespace requires a name defined in another file, that name must still be declared. Writing a following namespace definition either defines a new namespace or adds new elements to an existing one:
namespace namespace_name { // code declarations }
Nested Namespaces
Namespaces can be nested where you can define one namespace inside another name space as follows:
namespace namespace_name1 { // code declarations namespace namespace_name2 { // code declarations } }
• Namespace is a feature added in C++ and not present in C. • A namespace is a declarative region that provides a scope to the identifiers (names of the types, function, variables etc) inside it. • Multiple namespace blocks with the same name are allowed. All declarations within those blocks are declared in the named scope. • Namespace declarations appear only at global scope. • Namespace declarations can be nested within another namespace. • Namespace declarations don't have access specifiers. (Public or private) • No need to give semicolon after the closing brace of definition of namespace. • We can split the definition of namespace over several units.
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/* namespaces in C++ language */ // A C++ code to demonstrate that we can define // methods outside namespace. #include <iostream> using namespace std; // Creating a namespace namespace ns { void display(); class happy { public: void display(); }; } // Defining methods of namespace void ns::happy::display() { cout << "ns::happy::display()\n"; } void ns::display() { cout << "ns::display()\n"; } // Driver code int main() { ns::happy obj; ns::display(); obj.display(); return 0; }


To reverse a number in C++, then you have to ask to the user to enter a number. Now, start reversing that number to find its reverse and then display its reverse on the screen. Make a
C++ Program to implement "Tower Of Hanoi" alogithm using recursion. Shows Movements of disk from one tower to another when a key is pressed. Entering the fibonacci number and
First Input the length of a side of the triangle. Then Input the 'length of another side' of the triangle, then input the angle between these sides of the triangle. The area of the Scalene
2 const variables row & col are used to define size. If we do not make both const then error found because without "const reserve word" they are behaving as variable. Before placing