View source for cpp/language/decltype

===Explanation===
@1@ If the argument is an unparenthesized {{rlp|identifiers|id-expression}} or an unparenthesized {{rlp|operator member access|class member access}} expression, then decltype yields the type of the {{spar|entity}} named by this expression. If there is no such entity, or if the argument names a set of overloaded functions, the program is ill-formed.
{{rev begin}}
{{rev|since=c++17|
If the argument is an unparenthesized {{rlp|identifiers|id-expression}} naming a {{rlp|structured binding|structured binding}}, then decltype yields the ''referenced type'' (described in the specification of the structured binding declaration).
}}
{{rev|since=c++20|
If the argument is an unparenthesized {{rlp|identifiers|id-expression}} naming a {{rlp|template parameters#Non-type template parameter|non-type template parameter}}, then decltype yields the type of the template parameter (after performing any necessary type deduction if the template parameter is declared with a placeholder type). The type is non-const even if the entity is a template parameter object (which is a const object).
}}
{{rev end}}
@2@ If the argument is any other expression of type {{tt|T}}, and
:@a@ if the {{rlp|value category}} of {{spar|expression}} is {{rlp|value category#xvalue|''xvalue''}}, then decltype yields {{c|T&&}};
:@b@ if the value category of {{spar|expression}} is {{rlp|value category#lvalue|''lvalue''}}, then decltype yields {{c|T&}};
:@c@ if the value category of {{spar|expression}} is {{rlp|value category#prvalue|''prvalue''}}, then decltype yields {{c|T}}.
{{rrev multi|until1=c++17|rev1=
If {{spar|expression}} is a function call which returns a prvalue of class type or is a {{rlp|operator other|comma expression}} whose right operand is such a function call, a temporary object is not introduced for that prvalue.
|rev2=
If {{spar|expression}} is a prvalue{{rev inl|since=c++20| other than a (possibly parenthesized) {{rlp|consteval|immediate invocation}}}}, a temporary object is not {{rlp|implicit_cast#Temporary_materialization|materialized}} from that prvalue: such prvalue has no result object.}}
Because no temporary object is created, the type need not be {{rlp|incomplete type|complete}} or have an available {{rlp|destructor}}, and can be {{rlp|abstract class|abstract}}. This rule doesn't apply to sub-expressions: in {{c|decltype(f(g()))}}, {{c|g()}} must have a complete type, but {{c|f()}} need not.

Note that if the name of an object is parenthesized, it is treated as an ordinary lvalue expression, thus {{c|decltype(x)}} and {{c|decltype((x))}} are often different types.

{{tt|decltype}} is useful when declaring types that are difficult or impossible to declare using standard notation, like lambda-related types or types that depend on template parameters.