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/C++

for loop

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

Syntax

formal syntax:
attr (optional) `for (` init-statement condition (optional) `;` iteration-expression (optional) `)` statement
informal syntax:
attr (optional) `for (` declaration-or-expression (optional) `;` condition (optional) `;` expression (optional) `)` statement

attr - (since C++11) any number of attributes.

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 its value converts to 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

while ( condition ) {

statement

iteration-expression ;

}

Except that.

1) The scope of init-statement and the scope of condition are the same.

2) The scope of statement and the scope of iteration-expression are disjoint and nested within the scope of init-statement and condition.

3) continue in statement will execute iteration-expression.

4) 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 has no observable behavior (does not make calls to I/O functions, access volatile objects, or perform atomic or synchronization operations) does not terminate. Compilers are permitted to remove such loops. While in C names declared in the scope of init-statement and condition can be shadowed in the scope of statement, it is forbidden in C++:

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

Example

#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\n"
                 "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\n"
                 "in the loop's body are called per each iteration:\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 loop's body are called per each iteration:
S::S(0, 5); S::~S()
S::S(1, 4); S::~S()
S::S(2, 3); S::~S()