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for loop - cppreference.com

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

• an expression statement (which may be a null statement ;)
• a simple declaration (typically a declaration of a loop counter variable with initializer), it may declare arbitrary many variables or structured bindings(since C++17)

Note that any init-statement must end with a semicolon. This is why it is often described informally as an expression or a declaration followed by a semicolon.

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 for statement equivalent to:

init-statement

while ( condition )

{

statement

expression ;

}

}

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.

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++
    // ...
}

for

#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';
}

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

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