while loop

Conditionally executes a statement repeatedly.

[edit] Syntax

[edit] Condition

A condition can either be an expression or a simple 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 whether statement will be executed.

[edit] 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.

[edit] 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.

[edit] Non-structured binding declaration

The declaration has the following restrictions:

• Syntactically conforms to the following form:

• 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.

[edit] Explanation

A while statement is equivalent to

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.

[edit] 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.

[edit] Keywords

while

[edit] Example

#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';
}

10
2 4 6 8
Hello

[edit] See also

• In other languages

• Deutsch
• Español
• Français
• Italiano
• 日本語
• Português
• Русский
• 中文