Component bslmf_isclass [Package bslmf]

Provide a compile-time check for determining class types. More...

Namespaces
namespace	bslmf
namespace	bsl
Defines
#define	bslmf_IsClass   bslmf::IsClass

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Detailed Description

Outline

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

Purpose:

Provide a compile-time check for determining class types.

Classes:

bsl::is_class	standard meta-function for determining class types
bsl::is_class_v	the result value of the bsl::is_class meta-function
bslmf::IsClass	meta-function for determining class types

Description:

This component defines two meta-functions, bsl::is_class and BloombergLP::bslmf::IsClass and a template variable bsl::is_class_v, that represents the result value of the bsl::is_class meta-function. All these meta-functions may be used to query whether a type is a class, struct, or union, optionally qualified with const or volatile.

bsl::is_class meets the requirements of the is_class template defined in the C++11 standard [meta.unary.cat], while bslmf::IsClass was devised before is_class was standardized.

The two meta-functions are functionally equivalent. The major difference between them is that the result for bsl::is_class is indicated by the class member value, while the result for bslmf::IsClass is indicated by the class member VALUE.

Note that bsl::is_class should be preferred over bslmf::IsClass, and in general, should be used by new components.

Also note that the template variable is_class_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_class_v is defined as an inline constexpr bool variable. Otherwise, if the compiler supports the variable templates C++14 compiler feature, bsl::is_class_v is defined as a non-inline constexpr bool variable. See BSLS_COMPILERFEATURES_SUPPORT_INLINE_VARIABLES and BSLS_COMPILERFEATURES_SUPPORT_VARIABLE_TEMPLATES macros in 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 set of types are class types.

First, we create a class type MyClass:

  class MyClass
  {
  };

Now, we instantiate the bsl::is_class template for both a non-class type and the defined type MyClass, and assert the value static data member of each instantiation:

  assert(false == bsl::is_class<int>::value);
  assert(true  == bsl::is_class<MyClass>::value);

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

#ifdef BSLS_COMPILERFEATURES_SUPPORT_VARIABLE_TEMPLATES
  assert(false == bsl::is_class_v<int>);
  assert(true  == bsl::is_class_v<MyClass>);
#endif

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Define Documentation

#define bslmf_IsClass   bslmf::IsClass