while loop

 

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Statements

Labels

label : statement

Expression statements

expression ;

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{statement... }

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if

switch

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while

do while

for

range for(C++11)

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contract_assert(C++26)

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declaration ;

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try block

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synchronized, atomic_commit, etc (TM TS)

 

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 a structured binding declaration, it is interpreted as a structured binding declaration.

(since C++26)

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

Non-structured binding declaration

The declaration has the following restrictions:

• Syntactically conforms to the following form:

type-specifier-seqdeclarator=assignment-expression	(until C++11)
attribute-specifier-seq(optional)decl-specifier-seqdeclaratorbrace-or-equal-initializer	(since C++11)

• The declarator cannot specify a function or an array.
• The type specifier sequence(until C++11)declaration specifier sequence can only contain type specifiers and constexpr, and it(since C++11) cannot define a class or enumeration.

The decision variable of the declaration is the declared variable.

Structured binding declaration The declaration has the following restrictions: The expression in its initializer cannot be of an array type. The declaration specifier sequence can only contain type specifiers and constexpr. The decision variable of the declaration is the invented variable eintroduced by the declaration.

(since C++26)

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 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';}

Output:

10 2 4 6 8 Hello

See also

C documentation for while