Provide a compile-time check for void
types.
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Detailed Description
- Outline
-
-
- Purpose:
- Provide a compile-time check for
void
types.
-
- Classes:
-
- See also:
- Component 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
typedef
s -- the void
type and another type: typedef int MyType;
typedef void MyVoidType;
Now, we instantiate the bsl::is_void
template for each of the typedef
s and assert the value
static data member of each instantiation: 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