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

Revision as of 16:02, 21 March 2019

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 (__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 assigned to 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).

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, but beware that the lifetime of any temporary within range_expression is not extended.

for (auto& x : foo().items()) { /* .. */ } // undefined behavior if foo() returns by value
for (T thing = foo(); auto& x : thing.items()) { /* ... */ } // OK

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.

attr represents an optional number of attributes.

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.

As of C++17, the types of the begin_expr and the end_expr do not have to be the same, and in fact the type of the end_expr does not have to be an iterator: it just needs to be able to be compared for inequality with one. This makes it possible to delimit a range by a predicate (e.g. "the iterator points at a null character").

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. If that is undesirable (for instance because the loop is not actually modifying the object), std::as_const can be used:

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

Defect reports

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

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 (int n : a)  
        std::cout << 1 << ' '; // the loop variable need not be used
    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
1 1 1 1 1 1

See also