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while loop - cppreference.com

Conditionally executes a statement repeatedly.

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.

• 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.

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.

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.

The declaration has the following restrictions:

• Syntactically conforms to the following form:

• type-specifier-seq declarator = assignment-expression

• attribute-specifier-seq(optional) decl-specifier-seq declarator brace-or-equal-initializer

• 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.

A while statement is equivalent to

{

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.

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.

while

#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:

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