Structured binding declaration (since C++17)
Binds the specified names to subobjects or elements of the initializer.
Like a reference, a structured binding is an alias to an existing object. Unlike a reference, a structured binding does not have to be of a reference type.
attr (optional) cv-auto ref-qualifier (optional) [ sb-identifier-list ] = expression ;
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(1) | ||||||||
attr (optional) cv-auto ref-qualifier (optional) [ sb-identifier-list ]{ expression };
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(2) | ||||||||
attr (optional) cv-auto ref-qualifier (optional) [ sb-identifier-list ]( expression );
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(3) | ||||||||
attr | - | sequence of any number of attributes |
cv-auto | - | possibly cv-qualified type specifier auto, may also include storage class specifier static or thread_local; including volatile in cv-qualifiers is deprecated(since C++20) |
ref-qualifier | - | either & or &&
|
sb-identifier-list | - | list of comma-separated identifiers introduced by this declaration, each identifier may be followed by an attribute specifier sequence, at most one identifier may be preceded by an ellipsis(since C++26) |
expression | - | an expression that does not have the comma operator at the top level (grammatically, an assignment-expression), and has either array or non-union class type. If expression refers to any of the names from sb-identifier-list, the declaration is ill-formed. |
A structured binding declaration introduces all identifiers in the sb-identifier-list as names in the surrounding scope and binds them to subobjects or elements of the object denoted by expression. The bindings so introduced are called structured bindings.
Any identifier in the sb-identifier-list that is preceded with an ellipsis must declare a templated entity. Such identifier introduces a structured binding pack. |
(since C++26) |
A structured binding is an identifier in the sb-identifier-list that is not preceded with an ellipsis, or an element of a structured binding pack introduced in the same identifier list(since C++26).
Contents |
[edit] Binding process
A structured binding declaration first introduces a uniquely-named variable (here denoted by e) to hold the value of the initializer, as follows:
- If expression has array type
A
and no ref-qualifier is present, then e has type cvA
, where cv is the cv-qualifiers in the cv-auto sequence, and each element of e is copy-initialized (for (1)) or direct-initialized (for (2,3)) from the corresponding element of expression. - Otherwise e is defined as if by using its name instead of
[
sb-identifier-list]
in the declaration.
We use E
to denote the type of e. (In other words, E
is the equivalent of std::remove_reference_t<decltype((e))>.)
A structured binding size of E
is the number of structured bindings that need to be introduced by the structured binding declaration.
The number of identifiers in sb-identifier-list must match the structured binding size of E
provided that there is no structured binding pack(since C++26).
If there is a structured binding pack in a structured binding declaration, then the number of non-pack elements (referring to the identifiers not preceded by an ellipsis) must not exceed the structured binding size of The number of elements of the structured binding pack is equal to the structured binding size of E less the number of non-pack elements in the declaration. struct C { int x, y, z; }; template <class T> void now_i_know_my() { auto [a, b, c] = C(); // OK: a, b, c refer to x, y, z, respectively auto [d, ...e] = C(); // OK: d refers to x; ...e refers to y and z auto [...f, g] = C(); // OK: ...f refers x and y; g refers to z auto [h, i, j, ...k] = C(); // OK: the pack k is empty auto [l, m, n, o, ...p] = C(); // error: structured binding size is too small } |
(since C++26) |
A structured binding declaration performs the binding in one of three possible ways, depending on E
:
- Case 1: If
E
is an array type, then the names are bound to the array elements. - Case 2: If
E
is a non-union class type and std::tuple_size<E> is a complete type with a member namedvalue
(regardless of the type or accessibility of such member), then the "tuple-like" binding protocol is used. - Case 3: If
E
is a non-union class type but std::tuple_size<E> is not a complete type, then the names are bound to the accessible data members ofE
.
Each of the three cases is described in more detail below.
Each structured binding has a referenced type, defined in the description below. This type is the type returned by decltype
when applied to an unparenthesized structured binding.
[edit] Case 1: binding an array
Each structured binding in the sb-identifier-list becomes the name of an lvalue that refers to the corresponding element of the array. The structured binding size of E
is equal to the number of array elements.
The referenced type for each structured binding is the array element type. Note that if the array type E
is cv-qualified, so is its element type.
int a[2] = {1, 2}; auto [x, y] = a; // creates e[2], copies a into e, // then x refers to e[0], y refers to e[1] auto& [xr, yr] = a; // xr refers to a[0], yr refers to a[1]
[edit] Case 2: binding a type implementing the tuple operations
The expression std::tuple_size<E>::value must be a well-formed integer constant expression, and the structured binding size of E
is equal to std::tuple_size<E>::value.
For each structured binding, a variable whose type is "reference to std::tuple_element<i, E>::type" is introduced: lvalue reference if its corresponding initializer is an lvalue, rvalue reference otherwise. The initializer for the i-th variable is
- e.get<i>(), if lookup for the identifier
get
in the scope ofE
by class member access lookup finds at least one declaration that is a function template whose first template parameter is a non-type parameter - Otherwise, get<i>(e), where get is looked up by argument-dependent lookup only, ignoring non-ADL lookup.
In these initializer expressions, e is an lvalue if the type of the entity e is an lvalue reference (this only happens if the ref-qualifier is &
or if it is &&
and the initializer expression is an lvalue) and an xvalue otherwise (this effectively performs a kind of perfect forwarding), i is a std::size_t prvalue, and <i> is always interpreted as a template parameter list.
The variable has the same storage duration as e.
The structured binding then becomes the name of an lvalue that refers to the object bound to said variable.
The referenced type for the i-th structured binding is std::tuple_element<i, E>::type.
float x{}; char y{}; int z{}; std::tuple<float&, char&&, int> tpl(x, std::move(y), z); const auto& [a, b, c] = tpl; // using Tpl = const std::tuple<float&, char&&, int>; // a names a structured binding that refers to x (initialized from get<0>(tpl)) // decltype(a) is std::tuple_element<0, Tpl>::type, i.e. float& // b names a structured binding that refers to y (initialized from get<1>(tpl)) // decltype(b) is std::tuple_element<1, Tpl>::type, i.e. char&& // c names a structured binding that refers to the third component of tpl, get<2>(tpl) // decltype(c) is std::tuple_element<2, Tpl>::type, i.e. const int
[edit] Case 3: binding to data members
Every non-static data member of E
must be a direct member of E
or the same base class of E
, and must be well-formed in the context of the structured binding when named as e.name. E
may not have an anonymous union member. The structured binding size of E
is equal to the number of non-static data members.
Each structured binding in sb-identifier-list becomes the name of an lvalue that refers to the next member of e in declaration order (bit-fields are supported); the type of the lvalue is that of e.m_i, where m_i
refers to the ith member.
The referenced type of the i-th structured binding is the type of e.m_i if it is not a reference type, or the declared type of m_i
otherwise.
#include <iostream> struct S { mutable int x1 : 2; volatile double y1; }; S f() { return S{1, 2.3}; } int main() { const auto [x, y] = f(); // x is an int lvalue identifying the 2-bit bit-field // y is a const volatile double lvalue std::cout << x << ' ' << y << '\n'; // 1 2.3 x = -2; // OK // y = -2.; // Error: y is const-qualified std::cout << x << ' ' << y << '\n'; // -2 2.3 }
[edit] Initialization order
Let val_i be the object or reference named by the ith structured binding in sb-identifier-list :
- The initialization of e is sequenced before the initialization of any val_i.
- The initialization of each val_i is sequenced before the initialization of any val_j where i is less than j.
[edit] Notes
Structured bindings cannot be constrained: template<class T> concept C = true; C auto [x, y] = std::pair{1, 2}; // error: constrained |
(since C++20) |
The lookup for member get
ignores accessibility as usual and also ignores the exact type of the non-type template parameter. A private template<char*> void get(); member will cause the member interpretation to be used, even though it is ill-formed.
The portion of the declaration preceding [
applies to the hidden variable e, not to the introduced structured bindings:
The tuple-like interpretation is always used if std::tuple_size<E> is a complete type with a member named value
, even if that would cause the program to be ill-formed:
struct A { int x; }; namespace std { template<> struct tuple_size<::A> { void value(); }; } auto [x] = A{}; // error; the "data member" interpretation is not considered.
The usual rules for reference-binding to temporaries (including lifetime-extension) apply if a ref-qualifier is present and the expression is a prvalue. In those cases the hidden variable e is a reference that binds to the temporary variable materialized from the prvalue expression, extending its lifetime. As usual, the binding will fail if e is a non-const lvalue reference:
int a = 1; const auto& [x] = std::make_tuple(a); // OK, not dangling auto& [y] = std::make_tuple(a); // error, cannot bind auto& to rvalue std::tuple auto&& [z] = std::make_tuple(a); // also OK
decltype(x), where x denotes a structured binding, names the referenced type of that structured binding. In the tuple-like case, this is the type returned by std::tuple_element, which may not be a reference even though a hidden reference is always introduced in this case. This effectively emulates the behavior of binding to a struct whose non-static data members have the types returned by std::tuple_element, with the referenceness of the binding itself being a mere implementation detail.
std::tuple<int, int&> f(); auto [x, y] = f(); // decltype(x) is int // decltype(y) is int& const auto [z, w] = f(); // decltype(z) is const int // decltype(w) is int&
Structured bindings cannot be captured by lambda expressions: #include <cassert> int main() { struct S { int p{6}, q{7}; }; const auto& [b, d] = S{}; auto l = [b, d] { return b * d; }; // valid since C++20 assert(l() == 42); } |
(until C++20) |
A structured binding size is allowed to be 0 as long as the sb-identifier-list contains exactly one identifier. That identifier can only introduce a structured binding pack that is always an empty pack. auto return_empty() -> std::tuple<>; template <class> void test_empty() { auto [] = return_empty(); // error auto [...args] = return_empty(); // OK, args is an empty pack auto [one, ...rest] = return_empty(); // error, structured binding size is too small } |
(since C++26) |
Feature-test macro | Value | Std | Feature |
---|---|---|---|
__cpp_structured_bindings |
201606L |
(C++17) | Structured bindings |
202403L |
(C++26) | Structured bindings with attributes | |
202411L |
(C++26) | Structured bindings can introduce a pack |
[edit] Keywords
[edit] Example
#include <iomanip> #include <iostream> #include <set> #include <string> int main() { std::set<std::string> myset{"hello"}; for (int i{2}; i; --i) { if (auto [iter, success] = myset.insert("Hello"); success) std::cout << "Insert is successful. The value is " << std::quoted(*iter) << ".\n"; else std::cout << "The value " << std::quoted(*iter) << " already exists in the set.\n"; } struct BitFields { // C++20: default member initializer for bit-fields int b : 4 {1}, d : 4 {2}, p : 4 {3}, q : 4 {4}; }; { const auto [b, d, p, q] = BitFields{}; std::cout << b << ' ' << d << ' ' << p << ' ' << q << '\n'; } { const auto [b, d, p, q] = []{ return BitFields{4, 3, 2, 1}; }(); std::cout << b << ' ' << d << ' ' << p << ' ' << q << '\n'; } { BitFields s; auto& [b, d, p, q] = s; std::cout << b << ' ' << d << ' ' << p << ' ' << q << '\n'; b = 4, d = 3, p = 2, q = 1; std::cout << s.b << ' ' << s.d << ' ' << s.p << ' ' << s.q << '\n'; } }
Output:
Insert is successful. The value is "Hello". The value "Hello" already exists in the set. 1 2 3 4 4 3 2 1 1 2 3 4 4 3 2 1
[edit] 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 2285 | C++17 | expression could refer to the names from identifier-list | the declaration is ill-formed in this case |
CWG 2312 | C++17 | the meaning of mutable was lost in the binding-to-members case |
its meaning is still kept |
CWG 2386 | C++17 | the "tuple-like" binding protocol is used whenever std::tuple_size<E> is a complete type |
used only when std::tuple_size<E> has a member value
|
CWG 2635 | C++20 | structured bindings could be constrained | prohibited |
CWG 2867 | C++17 | the initialization order was unclear | made clear |
P0961R1 | C++17 | in the tuple-like case, member get isused if lookup finds a get of any kind
|
only if lookup finds a function template with a non-type parameter |
P0969R0 | C++17 | in the binding-to-members case, the members are required to be public |
only required to be accessible in the context of the declaration |
[edit] References
- C++23 standard (ISO/IEC 14882:2024):
- 9.6 Structured binding declarations [dcl.struct.bind] (p: 228-229)
- C++20 standard (ISO/IEC 14882:2020):
- 9.6 Structured binding declarations [dcl.struct.bind] (p: 219-220)
- C++17 standard (ISO/IEC 14882:2017):
- 11.5 Structured binding declarations [dcl.struct.bind] (p: 219-220)
[edit] See also
(C++11) |
creates a tuple of lvalue references or unpacks a tuple into individual objects (function template) |