for loop

 

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Executes init-statement-seq once, then executes statement and iteration-expression repeatedly, until the value of condition becomes false. The test takes place before each iteration.

Syntax

attr(optional) for ( init-statement-seq condition(optional) ; iteration-expression(optional) ) statement

attr	-	(since C++11) any number of attributes
init-statement-seq	-	(since C++11) sequence of one or more init-statement
init-statement	-	one of an expression statement (which may be a null statement ";") a simple declaration, typically a declaration of a loop counter variable with initializer, but it may declare arbitrary many variables or structured bindings(since C++17) an alias declaration (since C++23) Note that any init-statement must end with a semicolon ;, which is why it is often described informally as an expression or a declaration followed by a semicolon.
condition	-	either an expression which is contextually convertible to bool. This expression is evaluated before each iteration, and if it yields false, the loop is exited. a declaration of a single variable with a brace-or-equals initializer. the initializer is evaluated before each iteration, and if the value of the declared variable converts to false, the loop is exited.
iteration-expression	-	any expression, which is executed after every iteration of the loop and before re-evaluating condition. Typically, this is the expression that increments the loop counter
statement	-	any statement, typically a compound statement, which is the body of the loop

Explanation

The above syntax produces code equivalent to:

{ init-statement-seq while ( condition ) { statement iteration-expression ; } }

Except that

1) Names declared by the init-statement (if init-statement is a declaration) and names declared by condition (if condition is a declaration) are in the same scope (which is also the scope of statement).

2) continue in the statement will execute iteration-expression

3) Empty condition is equivalent to while(true)

If the execution of the loop needs to be terminated at some point, break statement can be used as terminating statement.

If the execution of the loop needs to be continued at the end of the loop body, continue statement can be used as shortcut.

As is the case with while loop, if statement is a single statement (not a compound statement), the scope of variables declared in it is limited to the loop body as if it was a compound statement.

for (;;)
    int n;
// n goes out of scope

Keywords

for

Notes

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 in C++, the scope of the init-statement and the scope of statement are one and the same, in C the scope of statement is nested within the scope of init-statement:

for (int i = 0; ; ) {
    long i = 1;   // valid C, invalid C++
    // ...
}

Example

Run this code

#include <iostream>
#include <vector>
 
int main()
{
    std::cout << "1) typical loop with a single statement as the body:\n";
    for (int i = 0; i < 10; ++i)
        std::cout << i << ' ';
 
    std::cout << "\n\n" "2) init-statement can declare multiple names, as "
                 "long as they can use the same decl-specifier-seq:\n";
    for (int i = 0, *p = &i; i < 9; i += 2) {
        std::cout << i << ':' << *p << ' ';
    }
 
    std::cout << "\n\n" "3) condition may be a declaration:\n";
    char cstr[] = "Hello";
    for (int n = 0; char c = cstr[n]; ++n)
        std::cout << c;
 
    std::cout << "\n\n" "4) init-statement can use the auto type specifier:\n";
    std::vector<int> v = {3, 1, 4, 1, 5, 9};
    for (auto iter = v.begin(); iter != v.end(); ++iter) {
        std::cout << *iter << ' ';
    }
 
    std::cout << "\n\n" "5) init-statement can be an expression:\n";
    int n = 0;
    for (std::cout << "Loop start\n";
         std::cout << "Loop test\n";
         std::cout << "Iteration " << ++n << '\n')
        if(n > 1)
            break;
 
    std::cout << "\n" "6) constructors and destructors of objects created "
                 "in the loop's body are called per each iteraton:\n";
    struct S {
        S(int x, int y) { std::cout << "S::S(" << x << ", " << y << "); "; }
        ~S() { std::cout << "S::~S()\n"; }
    };
    for (int i{0}, j{5}; i < j; ++i, --j)
        S s{i, j};
}

Output:

1) typical loop with a single statement as the body:
0 1 2 3 4 5 6 7 8 9 
 
2) init-statement can declare multiple names, as long as they can use the same decl-specifier-seq:
0:0 2:2 4:4 6:6 8:8 
 
3) condition may be a declaration:
Hello
 
4) init-statement can use the auto type specifier:
3 1 4 1 5 9 
 
5) init-statement can be an expression:
Loop start
Loop test
Iteration 1
Loop test
Iteration 2
Loop test
 
6) constructors and destructors of objects created in the loops's body are called per each iteraton:
S::S(0, 5); S::~S()
S::S(1, 4); S::~S()
S::S(2, 3); S::~S()

See also

range-for loop(C++11)	executes loop over range
C documentation for for