std::convertible_to

 

C++

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Feature test macros (C++20)

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Technical specifications

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External libraries

 

Concepts library

Core language concepts same_as derived_from convertible_to common_reference_with common_with integral signed_integral unsigned_integral floating_point swappableswappable_with destructible constructible_from default_initializable move_constructible copy_constructible assignable_from

Comparison concepts equality_comparableequality_comparable_with totally_orderedtotally_ordered_with Object concepts movable copyable semiregular regular Callable concepts invocableregular_invocable predicate relation equivalence_relation strict_weak_order

 

Defined in header <concepts>
template <class From, class To> concept convertible_to =   std::is_convertible_v<From, To> &&   requires {     static_cast<To>(std::declval<From>());   };		(since C++20)

The concept convertible_to<From, To> specifies that an expression of the same type and value category as those of std::declval<From>() can be implicitly and explicitly converted to the type To, and the two forms of conversion are equivalent.

Semantic requirements

convertible_to<From, To> is modeled only if, given a function fun of type std::add_rvalue_reference_t<From>() such that the expression fun() is equality-preserving (see below),

• Either
  • To is neither an object type nor a reference-to-object type, or
  • static_cast<To>(fun()) is equal to []() -> To { return fun(); }(), and

• One of the following is true:
  • std::add_rvalue_reference_t<From> is not a reference-to-object type, or
  • std::add_rvalue_reference_t<From> is an rvalue reference to a non-const-qualified type, and the resulting state of the object referenced by fun() is valid but unspecified after either expression above; or
  • the object referred to by fun() is not modified by either expression above.

Equality preservation

An expression is equality preserving if it results in equal outputs given equal inputs.

• The inputs to an expression consist of its operands.
• The outputs of an expression consist of its result and all operands modified by the expression (if any).

In specification of standard concepts, operands are defined as the largest subexpressions that include only:

• an id-expression, and
• invocations of std::move, std::forward, and std::declval.

The cv-qualification and value category of each operand is determined by assuming that each template type parameter denotes a cv-unqualified complete non-array object type.

Every expression required to be equality preserving is further required to be stable: two evaluations of such an expression with the same input objects must have equal outputs absent any explicit intervening modification of those input objects.

Unless noted otherwise, every expression used in a requires-expression is required to be equality preserving and stable, and the evaluation of the expression may modify only its non-constant operands. Operands that are constant must not be modified.

See also

is_convertibleis_nothrow_convertible (C++11)(C++20)

checks if a type can be converted to the other type (class template)