[dcl.type.auto.deduct] (original) (raw)

9 Declarations [dcl]

9.2 Specifiers [dcl.spec]

9.2.9 Type specifiers [dcl.type]

9.2.9.7 Placeholder type specifiers [dcl.spec.auto]

9.2.9.7.2 Placeholder type deduction [dcl.type.auto.deduct]

Placeholder type deductionis the process by which a type containing a placeholder type is replaced by a deduced type.

A type T containing a placeholder type, and a corresponding initializer-clause E, are determined as follows:

• For a non-discarded return statement that occurs in a function declared with a return type that contains a placeholder type,T is the declared return type.
  • If the return statement has no operand, then E is void().
  • Otherwise, E is the operand of the return statement.
• For a variable declared with a type that contains a placeholder type,T is the declared type of the variable.
• For an explicit type conversion ([expr.type.conv]),T is the specified type, which shall be auto.
• For a constant template parameter declared with a type that contains a placeholder type,T is the declared type of the constant template parameter and E is the corresponding template argument.

T shall not be an array type.

If the placeholder-type-specifier is of the formtype-constraint auto, the deduced type replacing Tis determined using the rules for template argument deduction.

Obtain P fromT by replacing the occurrences oftype-constraint auto either with a new invented type template parameter U or, if the initialization is copy-list-initialization, withstd​::​initializer_list<U>.

If the deduction fails, the declaration is ill-formed.

Otherwise, is obtained by substituting the deduced U into P.

[Example 1: auto x1 = { 1, 2 }; auto x2 = { 1, 2.0 }; auto x3{ 1, 2 }; auto x4 = { 3 }; auto x5{ 3 }; — _end example_]

[Example 2: const auto &i = expr;

The type of i is the deduced type of the parameter u in the call f(expr) of the following invented function template:template <class U> void f(const U& u);

— _end example_]

The type deduced for T is determined as described in [dcl.type.decltype], as thoughE had been the operand of the decltype.

[Example 3: int i;int&& f();auto x2a(i); decltype(auto) x2d(i); auto x3a = i; decltype(auto) x3d = i; auto x4a = (i); decltype(auto) x4d = (i); auto x5a = f(); decltype(auto) x5d = f(); auto x6a = { 1, 2 }; decltype(auto) x6d = { 1, 2 }; auto *x7a = &i; decltype(auto)*x7d = &i; auto f1(int x) -> decltype((x)) { return (x); } auto f2(int x) -> decltype(auto) { return (x); } — _end example_]