Component bslmf_isswappable [Package bslmf]

Provide metafunction to identify swappable types. More...

Outline

• Purpose
• Classes
• Description
  • Usage
    • Example 1: Verify Class Types

Purpose:

Provide metafunction to identify swappable types.

Classes:

bsl::is_swappable	type-traits metafunction
bsl::is_swappable_v	the metafunction's result value

See also:

Component bslmf_integralconstant

Description:

This component defines a metafunction, bsl::is_swappable, and a variable template bsl::is_swappable_v that represents the result value of the bsl::is_swappable metafunction, which may be used to query whether swap(x,y); is well-formed. Note that this is only implemented for C++11 and above.

bsl::is_swappable meets the requirements of the is_swappable template defined in the C++17 standard.

Note that the template variable is_swappable_v is defined in the C++17 standard as an inline variable. If the current compiler supports the inline variable C++17 compiler feature, bsl::is_swappable_v is defined as an inline constexpr bool variable. Otherwise, if the compiler supports the variable templates C++14 compiler feature, bsl::is_swappable_v is defined as a non-inline constexpr bool variable. See BSLS_COMPILERFEATURES_SUPPORT_INLINE_VARIABLES and BSLS_COMPILERFEATURES_SUPPORT_VARIABLE_TEMPLATES macros in the bsls_compilerfeatures component for details.

Usage:

In this section we show intended use of this component.

Example 1: Verify Class Types:

Suppose that we want to assert whether a particular type is swappable.

First, we create two structs -- one swappable and one not.

  struct MyType1 {
      // trivial so swappable
  };

  struct MyType2 {
      // private assignement, so not swappable
    private:
      // NOT IMPLEMENTED
      MyType2& operator=(const MyType2&);
          // Assignment operator made private to prevent swappability.
  };

Now, we instantiate the bsl::is_swappable template for each of the structs and assert the value static data member of each instantiation:

  assert(true  == bsl::is_swappable<MyType1>::value);
  assert(false == bsl::is_swappable<MyType2>::value);

Note that if the current compiler supports the variable templates C++14 feature then we can re-write the snippet of code above using the bsl::is_swappable_v variable as follows:

#ifdef BSLS_COMPILERFEATURES_SUPPORT_VARIABLE_TEMPLATES
  assert(true  == bsl::is_swappable_v<MyType1>);
  assert(false == bsl::is_swappable_v<MyType2>);
#endif