Executes a for loop over a range.
Used as a more readable equivalent to the traditional
operating over a range of values, such as all elements in a container.
Syntax
attr(optional)for (init-statement(optional)item-declaration:range-initializer)statementExplanation
The above syntax produces code equivalent to the following except for the lifetime expansion of temporaries of range-initializer (see
)(since C++23) (the variables and expressions wrapped in /* */ are for exposition only):
{
auto&&/* range */=range-initializer;
for (auto/* begin */=/* begin-expr */,/* end */=/* end-expr */;
/* begin */!=/* end */; ++/* begin */)
{item-declaration= */* begin */;
statement
}
}
(until C++17){
auto&&/* range */=range-initializer;
auto/* begin */=/* begin-expr */;
auto/* end */=/* end-expr */;
for ( ;/* begin */!=/* end */; ++/* begin */)
{item-declaration= */* begin */;
statement
}
}
(since C++17)
(until C++20){
init-statement
auto&&/* range */=range-initializer;
auto/* begin */=/* begin-expr */;
auto/* end */=/* end-expr */;
for ( ;/* begin */!=/* end */; ++/* begin */)
{item-declaration= */* begin */;
statement
}
}
(since C++20)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.
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
(non-ADL lookup is not performed).
As a loop, a range-based for loop can contain
to exit the loop early and
to end the current iteration and continue with the next element.
If a name introduced in init-statement is redeclared in the outermost block of statement, the program is ill-formed:
for(inti:{1,2,3})inti=1;// error: redeclarationTemporary 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 valuefor(auto&x:foo().items()){/* ... */}// until C++23 undefined behaviorThis problem may be worked around by binding the temporary to a variable in the init-statement:
for(Tthing=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:
usingT=std::list<int>;constT&f1(constT&t){returnt;}constT&f2(Tt){returnt;}// always returns a dangling referenceTg();voidfoo(){for(autoe:f1(g())){}// OK: lifetime of return value of g() extendedfor(autoe:f2(g())){}// UB: lifetime of f2's value parameter ends early}(since C++23)Notes
If the range-initializer is a
braced-enclosed initializer list
, /* range */ is deduced to be a reference to a
.
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 classmeow{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.
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
:
structcow_string{/* ... */};// a copy-on-write stringcow_stringstr=/* ... */;// for (auto x : str) { /* ... */ } // may cause deep copyfor(autox:std::as_const(str)){/* ... */}(since C++17)Feature-test macro ValueStdFeature
(C++11)
(C++17)Range-based for loop with
types
(C++23)Lifetime extension for all temporary objects in range-initializerKeywords
Example
Run this code
#include<iostream>#include<vector>intmain(){std::vector<int>v={0,1,2,3,4,5};for(constint&i:v)// access by const referencestd::cout<<i<<' ';std::cout<<'\n';for(autoi: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(intn:{0,1,2,3,4,5})// the initializer may be a// braced-enclosed initializer liststd::cout<<n<<' ';std::cout<<'\n';inta[]={0,1,2,3,4,5};for(intn:a)// the initializer may be an arraystd::cout<<n<<' ';std::cout<<'\n';for([[maybe_unused]]intn:a)std::cout<<1<<' ';// the loop variable need not be usedstd::cout<<'\n';for(auton=v.size();autoi:v)// the init-statement (C++20)std::cout<<--n+i<<' ';std::cout<<'\n';for(typedefdecltype(v)::value_typeelem_t;elem_ti:v)// typedef declaration as init-statement (C++20)std::cout<<i<<' ';std::cout<<'\n';for(usingelem_t=decltype(v)::value_type;elem_ti: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
C++11 it was unspecified whether the lookup of non-member
“begin” and “end” includes usual unqualified lookup no usual unqualified lookup
C++11 the names introduced in init-statement could be redeclared the program is ill-formed in this case
C++11 if range-initializer is a brace-enclosed initializer list,
the non-member “begin” and “end” will be looked up will lookup member “begin”
and “end” in this case
C++11 member interpretation was used if either
member “begin” and “end” is present only used if both are present See also