for loop

Conditionally executes a statement repeatedly, where the statement does not need to manage the loop condition.

[edit] Syntax

[edit] Condition

A condition can either be an expression or a simple declaration. If it can be syntactically resolved as either an expression or a declaration, it is interpreted as the latter.

When control reaches condition, the condition will yield a value of type bool, 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 not an expression, it is a simple declaration with the following restrictions:

• It has only one declarator.
• The declarator cannot specify a function or an array.
• The declarator must have an initializer, which cannot be of syntax (3).
• The declaration specifier sequence can only contain type specifiers and constexpr(since C++11), and it cannot define a class or enumeration.

In this case, the value which condition yields is the value of the declared variable contextually converted to bool. If that conversion is ill-formed, the program is ill-formed.

[edit] Explanation

A for statement equivalent to:

Except that

• The scope of init-statement and the scope of condition are the same.
• The scope of statement and the scope of expression are disjoint and nested within the scope of init-statement and condition.
• Executing a continue statement in statement will evaluate expression.
• Empty condition is equivalent to true.

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

As is the case with while loop, if statement is 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

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 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++
    // ...
}

[edit] Keywords

for

[edit] 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};
 
    std::cout << "\n" "7) init-statement can use structured bindings:\n";
    long arr[]{1, 3, 7};
    for (auto [i, j, k] = arr; i + j < k; ++i)
        std::cout << i + j << ' ';
    std::cout << '\n';
}

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()
 
7) init-statement can use structured bindings:
4 5 6

[edit] See also