- 2.1 Expression
- 2.2 Declaration
- 2.2.1 Non-structured binding declaration
- 2.2.2 Structured binding declaration
[edit] Syntax
attr (optional) for ( init-statement condition (optional) ; expression (optional) ) statement - 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.
[edit] Condition
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 either an expression or a declaration that is not a structured binding declaration (since C++26) , it is interpreted as the latter.
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:
- 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.
The decision varaiable of the declaration is the declared variable.
Structured binding declaration
The declaration has the following restrictions:
- The expression in its initializer cannot be of an array type.
- The declaration specifier sequence can only contain type specifiers and constexpr .
The decision variable of the declaration is the invented variable e introduced by the declaration.
[edit] Explanation
A for statement equivalent to:
- 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
Run this code#include #include 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::vectorint> 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
