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

Latest revision as of 23:24, 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.

[edit] Syntax

[edit] Explanation

The above syntax produces code equivalent to the following except for the lifetime expansion of temporaries of range-initializer (see below)(since C++23) (the variables and expressions wrapped in /* */ are for exposition only):

range-initializer is evaluated to initialize 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 item-declaration.

item-declaration can be one of the following:

• a simple declaration with the following restrictions:
• It has only one declarator.
• The declarator must have no initializer.
• The declaration specifier sequence can only contain type specifiers and constexpr, and it cannot define a class or enumeration.

Exposition-only expressions /* begin-expr */ and /* end-expr */ are defined as follows:

• If the type of /* range */ is a reference to an array type R:

• If R is of bound N, /* begin-expr */ is /* range */ and /* end-expr */ is /* range */ + N.
• If R is an array of unknown bound or an array of incomplete type, the program is ill-formed.

• If the type of /* range */ is a reference to a class type C, and searches in the scope of C for the names “begin” and “end” each find at least one declaration, then /* begin-expr */ is /* range */.begin() and /* end-expr */ is /* range */.end().
• Otherwise, /* begin-expr */ is begin(/* range */) and /* end-expr */ is end(/* range */), where “begin” and “end” are found via argument-dependent lookup (non-ADL lookup is not performed).

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.

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

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

[edit] Temporary range initializer

If range-initializer 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-initializer are not extended unless they would otherwise be destroyed at the end of range-initializer(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
}

[edit] Notes

If the range-initializer is a braced-enclosed initializer list, /* range */ is deduced to be a reference to a 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 item-declaration is usually used in the 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)) { /* ... */ }

[edit] Keywords

for

[edit] 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-enclosed initializer 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

[edit] Defect reports

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

[edit] See also