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Range-based for loop (since C++11)

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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.

Contents

Syntax

attr (optional) for ( init-statement (optional) range-declaration : range-expression )

loop-statement

attr - any number of attributes
init-statement - (since C++20) either
(since C++23)
Note that any init-statement must end with a semicolon ;, which is why it is often described informally as an expression or a declaration followed by a semicolon.
range-declaration - a declaration of a named variable, whose type is the type of the element of the sequence represented by range-expression, or a reference to that type. Often uses the auto specifier for automatic type deduction
range-expression - any expression that represents a suitable sequence (either an array or an object for which begin and end member functions or free functions are defined, see below) or a braced-init-list.
loop-statement - any statement, typically a compound statement, which is the body of the loop


range-declaration may be a structured binding declaration: 

for (auto&& [first, second] : mymap)
{
    // use first and second
}
 (since C++17)

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.

{

auto && __range = range-expression ;
for (auto __begin = begin-expr, __end = end-expr; __begin != __end; ++__begin)
{
range-declaration = *__begin;
loop-statement
}

}

 (until C++17)

{

auto && __range = range-expression ;
auto __begin = begin-expr ;
auto __end = end-expr ;
for ( ; __begin != __end; ++__begin)
{
range-declaration = *__begin;
loop-statement
}

}

 (since C++17)
(until C++20)

{

init-statement
auto && __range = range-expression ;
auto __begin = begin-expr ;
auto __end = end-expr ;
for ( ; __begin != __end; ++__begin)
{
range-declaration = *__begin;
loop-statement
}

}

 (since C++20)

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

This problem may be worked around using init-statement: 

for (T thing = foo(); auto& x : thing.items()) { /* ... */ } // OK
 (since C++20)

Note that even in C++23 non-reference parameters of intermediate function calls do not get a lifetime extension (because in some ABIs they are destroyed in the callee, not in the caller), but that is only a problem for functions that are buggy anyway: 

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
}
 (since C++23)

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").

(since C++17)

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), it can be avoided by using std::as_const: 

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)) { /* ... */ }
 (since C++17)
Feature-test macro Value Std Feature
__cpp_range_based_for 200907L (C++11) Range-based for loop
201603L (C++17) Range-based for loop with different begin/end types
202211L (C++23) Lifetime extension for all temporary objects in range-expression

Keywords

for

Example

Run this code
#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';
}

Output: 

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.

DR Applied to Behavior as published Correct behavior
CWG 1442 C++11 it was unspecified whether the lookup of non-member
begin and end includes usual unqualified lookup 		no usual unqualified lookup
CWG 2220 C++11 the names introduced in init-statement could be redeclared the program is ill-formed in this case
P0962R1 C++11 member interpretation was used if
either member begin and end is present 		only used if both are present

See also

 applies a function to a range of elements
(function template) [edit]
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