Range-based for loop (since C++11) – cppreference.com

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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.

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Syntax

attr

(optional)

for (

init-statement

(optional)

range-declaration

:

range-expression

)

loop-statement

attr

–

any number of attributes

init-statement

–

(since C++20)

either

  • an expression statement (which may be a null statement “;“)
  • a simple declaration, typically a declaration of a variable with initializer, but it may declare arbitrarily many variables or be a structured binding declaration
  • an alias declaration

(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)

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Explanation

The above syntax produces code equivalent to the following (__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

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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, 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

This problem may be worked around using init-statement: 

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

(since C++20)

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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

Comment

__cpp_range_based_for

200907L

(C++11)

Range-based for loop

201603L

(C++17)

Range-based for loop with different begin/end types

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Keywords

for

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Example

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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

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See also

for_each

applies a function to a range of elements

(function template)

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