Conditionally executes a statement repeatedly.

[edit]Syntax

[edit]Condition

Acondition can either be anexpression or asimple declaration.

• If it can be syntactically resolved as an expression, it is treated as an expression. Otherwise, it is treated as a declaration that is not a structured binding declaration(since C++26).

When control reaches condition, the condition will yield a value, which is used to determine whetherstatement will be executed.

[edit]Expression

Ifcondition is an expression, the value it yields is the the value of the expression contextually converted tobool. If that conversion is ill-formed, the program is ill-formed.

[edit]Declaration

Ifcondition is a simple declaration, the value it yields is the value of the decision variable (see below) contextually converted tobool. If that conversion is ill-formed, the program is ill-formed.

[edit]Non-structured binding declaration

The declaration has the following restrictions:

• Syntactically conforms to the following form:

• The declarator cannot specify afunction or anarray.
• Thetype specifier sequence(until C++11)declaration specifier sequence can only contain type specifiers andconstexpr, and it(since C++11) cannot define aclass orenumeration.

The decision variable of the declaration is the declared variable.

[edit]Explanation

Awhile statement is equivalent to

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

If the loop needs to be terminated withinstatement, abreak statement can be used as terminating statement.

If the current iteration needs to be terminated withinstatement, acontinue statement can be used as shortcut.

[edit]Notes

Regardless of whetherstatement is a compound statement, it always introduces ablock 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 isundefined if a loop that is not atrivial infinite loop(since C++26) withoutobservable behavior does not terminate. Compilers are permitted to remove such loops.

[edit]Keywords

while

[edit]Example

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

102 4 6 8Hello

[edit]See also

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