bslmf_isconst

Detailed Description

Outline

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

Purpose

Provide a compile-time check for const-qualified types.

Classes

• bsl::is_const: meta-function for determining const-qualified types
• bsl::is_const_v: the result value of the bsl::is_const meta-function

See also

bslmf_integralconstant

Description

This component defines a meta-function, bsl::is_const and a template variable bsl::is_const_v, that represents the result value of the bsl::is_const meta-function, that may be used to query whether a type is const-qualified as defined in the C++ standard [basic.type.qualifier].

bsl::is_const meets the requirements of the is_const template defined in the C++11 standard [meta.unary.prop].

Note that the template variable is_const_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_const_v is defined as an inline constexpr bool variable. Otherwise, if the compiler supports the variable templates C++14 compiler feature, bsl::is_const_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 const Types

Suppose that we want to assert whether a particular type is const-qualified.

First, we create two typedefs – a const-qualified type and an unqualified type:

typedef int        MyType;
typedef const int  MyConstType;

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

assert(false == bsl::is_const<MyType>::value);
assert(true  == bsl::is_const<MyConstType>::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_const_v variable as follows:

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