Difference between revisions of "cpp/language/range-for"

Revision as of 07:48, 19 June 2024

Executes a for loop over a range.

Used as a more readable equivalent to the traditional for loop operating over a range of values, such as all elements in a container.

Syntax

for (auto&& [first, second] : mymap)
{
    // use first and second
}

Explanation

The above syntax produces code equivalent to the following except for the lifetime expansion of temporaries of range-expression (see below)(since C++23). The variables __range, __begin and __end are for exposition only.

range-expression is evaluated to determine the sequence or range to iterate. Each element of the sequence, in turn, is dereferenced and is used to initialize the variable with the type and name given in range-declaration.

begin-expr and end-expr are defined as follows:

• If range-expression is an expression of array type, then begin-expr is __range and end-expr is (__range + __bound), where __bound is the number of elements in the array (if the array has unknown size or is of an incomplete type, the program is ill-formed);
• If range-expression is an expression of a class type C that has both a member named begin and a member named end (regardless of the type or accessibility of such member), then begin-expr is __range.begin() and end-expr is __range.end();
• Otherwise, begin-expr is begin(__range) and end-expr is end(__range), which are found via argument-dependent lookup (non-ADL lookup is not performed).

Just as with a traditional loop, a break statement can be used to exit the loop early and a continue statement can be used to restart the loop with the next element.

If a name introduced in init-statement is redeclared in the outermost block of loop-statement, the program is ill-formed:

for (int i : {1, 2, 3})
    int i = 1; // error: redeclaration

Temporary range expression

If range-expression returns a temporary, its lifetime is extended until the end of the loop, as indicated by binding to the forwarding reference __range. Lifetimes of all temporaries within range-expression are not(until C++23) extended if they would otherwise be destroyed at the end of range-expression(since C++23).

// if foo() returns by value
for (auto& x : foo().items()) { /* .. */ } // until C++23 undefined behavior

for (T thing = foo(); auto& x : thing.items()) { /* ... */ } // OK

using T = std::list<int>;
const T& f1(const T& t) { return t; }
const T& f2(T t)        { return t; } // always returns a dangling reference
T g();
 
void foo()
{
    for (auto e : f1(g())) {} // OK: lifetime of return value of g() extended
    for (auto e : f2(g())) {} // UB: lifetime of f2's value parameter ends early
}

Notes

If the initializer (range-expression) is a braced-init-list, __range is deduced to be std::initializer_list<>&&.

It is safe, and in fact, preferable in generic code, to use deduction to forwarding reference, for (auto&& var : sequence).

The member interpretation is used if the range type has a member named begin and a member named end. This is done regardless of whether the member is a type, data member, function, or enumerator, and regardless of its accessibility. Thus a class like class meow { enum { begin = 1, end = 2 }; /* rest of class */ }; cannot be used with the range-based for loop even if the namespace-scope begin/end functions are present.

While the variable declared in the range-declaration is usually used in the loop-statement, doing so is not required.

When used with a (non-const) object that has copy-on-write semantics, the range-based for loop may trigger a deep copy by (implicitly) calling the non-const begin() member function.

struct cow_string { /* ... */ }; // a copy-on-write string
cow_string str = /* ... */;
 
// for (auto x : str) { /* ... */ } // may cause deep copy
 
for (auto x : std::as_const(str)) { /* ... */ }

Keywords

for

Example

#include <iostream>
#include <vector>
 
int main()
{
    std::vector<int> v = {0, 1, 2, 3, 4, 5};
 
    for (const int& i : v) // access by const reference
        std::cout << i << ' ';
    std::cout << '\n';
 
    for (auto i : v) // access by value, the type of i is int
        std::cout << i << ' ';
    std::cout << '\n';
 
    for (auto&& i : v) // access by forwarding reference, the type of i is int&
        std::cout << i << ' ';
    std::cout << '\n';
 
    const auto& cv = v;
 
    for (auto&& i : cv) // access by f-d reference, the type of i is const int&
        std::cout << i << ' ';
    std::cout << '\n';
 
    for (int n : {0, 1, 2, 3, 4, 5}) // the initializer may be a braced-init-list
        std::cout << n << ' ';
    std::cout << '\n';
 
    int a[] = {0, 1, 2, 3, 4, 5};
    for (int n : a) // the initializer may be an array
        std::cout << n << ' ';
    std::cout << '\n';
 
    for ([[maybe_unused]] int n : a)  
        std::cout << 1 << ' '; // the loop variable need not be used
    std::cout << '\n';
 
    for (auto n = v.size(); auto i : v) // the init-statement (C++20)
        std::cout << --n + i << ' ';
    std::cout << '\n';
 
    for (typedef decltype(v)::value_type elem_t; elem_t i : v)
    // typedef declaration as init-statement (C++20)
        std::cout << i << ' ';
    std::cout << '\n';
 
    for (using elem_t = decltype(v)::value_type; elem_t i : v)
    // alias declaration as init-statement (C++23)
        std::cout << i << ' ';
    std::cout << '\n';
}

0 1 2 3 4 5 
0 1 2 3 4 5 
0 1 2 3 4 5 
0 1 2 3 4 5 
0 1 2 3 4 5 
0 1 2 3 4 5 
1 1 1 1 1 1 
5 5 5 5 5 5 
0 1 2 3 4 5 
0 1 2 3 4 5

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also