do-while loop

 

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Executes a statement repeatedly, until the value of expression becomes false. The test takes place after each iteration.

Syntax

attr (optional) do statement while ( expression ) ;

attr	-	(since C++11) any number of attributes
expression	-	any expression which is contextually convertible to bool. This expression is evaluated after each iteration, and if it yields false, the loop is exited.
statement	-	any statement, typically a compound statement, which is the body of the loop

Explanation

statement is always executed at least once, even if expression always yields false. If it should not execute in this case, a while or for loop may be used.

If the execution of the loop needs to be terminated at some point, a break statement can be used as terminating statement.

If the execution of the loop needs to be continued at the end of the loop body, a continue statement can be used as shortcut.

Notes

As part of the C++ forward progress guarantee, the behavior is undefined if a loop, unless it is a trivial infinite loop,(since C++26) has no observable behavior (does not make calls to I/O functions, access volatile objects, or perform atomic or synchronization operations) does not terminate. Compilers are permitted to remove such loops.

A trivial infinite loop is one of the following forms： while ( true-condition ) ; do ; while ( true-condition ) ; do ; while ( true-condition ) { } for ( declaration-or-expression (optional) ; true-condition (optional) ; expression (optional) ) ; for ( declaration-or-expression (optional) ; true-condition (optional) ; expression (optional) ) { } and the true-condition is a constant expression that evaluates to true, then its statement is replaced with std::this_thread::yield();; this replacement is implementation-defined in the freestanding implementation.

(since C++26)

Keywords

do, while

Example

Run this code

#include <algorithm>
#include <iostream>
#include <string>
 
int main()
{
    int j = 2;
    do // compound statement is the loop body
    {
        j += 2;
        std::cout << j << ' ';
    }
    while (j < 9);
    std::cout << '\n';
 
    // common situation where do-while loop is used
    std::string s = "aba";
    std::sort(s.begin(), s.end());
 
    do std::cout << s << '\n'; // expression statement is the loop body
    while (std::next_permutation(s.begin(), s.end()));
}

Output:

4 6 8 10
aab
aba
baa