for loop
Conditionally executes a statement repeatedly, where the statement does not need to manage the loop condition.
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[edit] Syntax
attr (optional) for ( init-statement condition (optional) ; expression (optional) ) statement
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| attr | - | (since C++11) any number of attributes | ||
| init-statement | - | one of
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. | ||
| condition | - | a condition | ||
| expression | - | an expression (typically an expression that increments the loop counter) | ||
| statement | - | a statement (typically a compound statement) |
[edit] Condition
A condition can either be an expression or a simple declaration.
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(since C++26) |
- 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.
[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 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.
[edit] Non-structured binding declaration
The declaration has the following restrictions:
- Syntactically conforms to the following form:
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(until C++11) |
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(since C++11) |
- 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 varaiable of the declaration is the declared variable.
Structured binding declarationThe declaration has the following restrictions:
The decision variable of the declaration is the invented variable e introduced by the declaration. |
(since C++26) |
[edit] Explanation
A for statement equivalent to:
{
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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
[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'; }
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
[edit] See also
range-for loop(C++11)
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executes loop over range |
| C documentation for for
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