Executes afor loop over a range.

Used as a more readable equivalent to the traditionalfor 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 ofrange-initializer (seebelow)(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 initem-declaration.

item-declaration can be one of the following:

• asimple declaration with the following restrictions:
• It has only onedeclarator.
• The declarator must have noinitializer.
• Thedeclaration specifier sequence can only contain type specifiers andconstexpr, and it cannot define aclass orenumeration.

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

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

• IfR is of boundN,/* begin-expr */ is/* range */ and/* end-expr */ is/* range */+ N.
• IfR 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 typeC, and searches in the scope ofC 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 */ isbegin(/* range */) and/* end-expr */ isend(/* range */), where “begin” and “end” are found viaargument-dependent lookup (non-ADL lookup is not performed).

If the loop needs to be terminated withinstatement, abreak statement can be used as terminating statement.

If the current iteration needs to be terminated withinstatement, acontinue statement can be used as shortcut.

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

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

[edit]Temporary range initializer

Ifrange-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 withinrange-initializer are not extended unless they would otherwise be destroyed at the end ofrange-initializer(since C++23).

// if foo() returns by valuefor(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 referenceT g(); void foo(){for(auto e: f1(g())){}// OK: lifetime of return value of g() extendedfor(auto e: f2(g())){}// UB: lifetime of f2's value parameter ends early}

[edit]Notes

If therange-initializer is abraced-enclosed initializer list,/* range */ is deduced to be a reference to astd::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 likeclass meow{enum{ begin=1, end=2};/* rest of class */}; cannot be used with the range-basedfor loop even if the namespace-scope “begin”/“end” functions are present.

While the variable declared in theitem-declaration is usually used in thestatement, doing so is not required.

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

struct cow_string{/* ... */};// a copy-on-write stringcow_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(constint& i: v)// access by const referencestd::cout<< i<<' ';std::cout<<'\n'; for(auto i: v)// access by value, the type of i is intstd::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'; constauto& 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 liststd::cout<< n<<' ';std::cout<<'\n'; int a[]={0,1,2,3,4,5};for(int n: a)// the initializer may be an arraystd::cout<< n<<' ';std::cout<<'\n'; for([[maybe_unused]]int n: a)std::cout<<1<<' ';// the loop variable need not be usedstd::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

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