bslmf_isvoid

Detailed Description

Outline

• Purpose
• Classes
• Description
  • Usage
    • Example 1: Determine Whether a Type is the void Type

Purpose

Provide a compile-time check for void types.

Classes

• bsl::is_void: standard meta-function for determining void types
• bsl::is_void_v: the result value of the bsl::is_void meta-function
• bslmf::IsVoid: meta-function for determining void types

See also

bslmf_integralconstant

Description

This component defines two meta-functions, bsl::is_void and BloombergLP::bslmf::IsVoid and a template variable bsl::is_void_v, that represents the result value of bsl::is_void meta-function. All these meta-functions may be used to query whether a type is the (possibly cv-qualified) void type.

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

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

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

Also note that the template variable is_void_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_void_v is defined as an inline constexpr bool variable. Otherwise, if the compiler supports the variable templates C++14 compiler feature, bsl::is_void_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: Determine Whether a Type is the void Type

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

First, we create two typedefs – the void type and another type:

typedef int  MyType;
typedef void MyVoidType;

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

assert(false == bsl::is_void<MyType>::value);
assert(true  == bsl::is_void<MyVoidType>::value);

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

#ifdef BSLS_COMPILERFEATURES_SUPPORT_VARIABLE_TEMPLATES
  assert(false == bsl::is_void_v<MyType>);
  assert(true  == bsl::is_void_v<MyVoidType>);
#endif