while loop

 

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Conditionally executes a statement repeatedly.

Syntax

attr (optional) while ( condition ) statement

attr	-	(since C++11) any number of attributes
condition	-	a condition
statement	-	a statement (typically a compound statement)

Condition

A condition can either be an expression or a simple declaration. If it can be syntactically resolved as either an expression or a declaration, it is interpreted as the latter.

When control reaches condition, the condition will yield a value of type bool, which is used to determine whether statement will be executed.

Expression

If condition is an expression, the value it yields is the the value of the expression contextually converted to bool. If that conversion is ill-formed, the program is ill-formed.

Declaration

If condition is not an expression, it is a simple declaration with the following restrictions:

• It has only one declarator.
• The declarator cannot specify a function or an array.
• The declarator must have an initializer, which cannot be of syntax (3).
• The declaration specifier sequence can only contain type specifiers and constexpr(since C++11), and it cannot define a class or enumeration.

In this case, the value which condition yields is the value of the declared variable contextually converted to bool. If that conversion is ill-formed, the program is ill-formed.

Explanation

A while statement is equivalent to

/* label */ : { if ( condition ) { statement goto /* label */ ; } }

If condition is a declaration, the variable it declares is destroyed and created with each iteration of the loop.

If the loop needs to be terminated within statement, a break statement can be used as terminating statement.

If the current iteration needs to be terminated within statement, a continue statement can be used as shortcut.

Notes

Regardless of whether statement is a compound statement, it always introduces a block scope. Variables declared in it are only visible in the loop body, in other words,

while (--x >= 0)
    int i;
// i goes out of scope

is the same as

while (--x >= 0)
{
    int i;
} // i goes out of scope

As part of the C++ forward progress guarantee, the behavior is undefined if a loop that is not a trivial infinite loop(since C++26) without observable behavior does not terminate. Compilers are permitted to remove such loops.

Keywords

while

Example

Run this code

#include <iostream>
 
int main()
{
    // while loop with a single statement
    int i = 0;
    while (i < 10)
         i++;
    std::cout << i << '\n';
 
    // while loop with a compound statement
    int j = 2;
    while (j < 9)
    {
        std::cout << j << ' ';
        j += 2;
    }
    std::cout << '\n';
 
    // while loop with a declaration condition
    char cstr[] = "Hello";
    int k = 0;
    while (char c = cstr[k++])
        std::cout << c;
    std::cout << '\n';
}

Output:

10
2 4 6 8
Hello