bslmf_istriviallycopyable

Detailed Description

Outline

• Purpose
• Classes
• Description
  • bslmf::IsTriviallyCopyableCheck
  • Usage
    • Example 1: Verify Whether Types are Trivially Copyable

Purpose

Provide a meta-function for determining trivially copyable types.

Classes

• bsl::is_trivially_copyable: type-traits meta-function
• bsl::is_trivially_copyable_v: the result value of the meta-function

See also

bslmf_integralconstant, bslmf_nestedtraitdeclaration

Description

This component defines a meta-function, bsl::is_trivially_copyable and a template variable bsl::is_trivially_copyable_v, that represents the result value of the bsl::is_trivially_copyable meta-function, that may be used to query whether a type is trivially copyable as defined in section 3.9.3 of the C++11 standard [basic.types].

bsl::is_trivially_copyable has the same syntax as the is_trivially_copyable template from the C++11 standard [meta.unary.prop]. However, unlike the template defined in the C++11 standard, which can determine the correct value for all types without requiring specialization, bsl::is_trivially_copyable can, by default, determine the value for the following type categories only:

Type Category        Is Trivially Copyable
-------------        ---------------------
reference types      false
fundamental types    true
enumerated types     true
pointers             true
pointers to members  true

For all other types, bsl::is_trivially_copyable returns false, unless the type is explicitly specified to be trivially copyable. This can be done in 2 ways:

1. Define a template specialization for bsl::is_trivially_copyable having the type as the template parameter that inherits directly from bsl::true_type.
2. Use the BSLMF_NESTED_TRAIT_DECLARATION macro to define bsl::is_trivially_copyable as a trait in the class definition of the type.

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

bslmf::IsTriviallyCopyableCheck

In addition to bsl::is_trivially_copyable, we provide bslmf::IsTriviallyCopyableCheck, which is never intended to be specialized, but rather to be used to get the same return value as bsl::is_trivially_copyable, but perform a static assert that the bsl and std versions of is_trivially_copyable are equivalent.

Usage

In this section we show intended use of this component.

Example 1: Verify Whether Types are Trivially Copyable

Suppose that we want to assert whether a type is trivially copyable.

First, we define a set of types to evaluate:

typedef int  MyFundamentalType;
typedef int& MyFundamentalTypeReference;
 
class MyTriviallyCopyableType {
};
 
struct MyNonTriviallyCopyableType {
    //...
};

Then, since user-defined types cannot be automatically evaluated by is_trivially_copyable, we define a template specialization to specify that MyTriviallyCopyableType is trivially copyable:

namespace bsl {
 
template <>
struct is_trivially_copyable<MyTriviallyCopyableType> : bsl::true_type {
    // This template specialization for 'is_trivially_copyable' indicates
    // that 'MyTriviallyCopyableType' is a trivially copyable type.
};
 
}  // close namespace bsl

Now, we verify whether each type is trivially copyable using bsl::is_trivially_copyable:

assert(true  == bsl::is_trivially_copyable<MyFundamentalType>::value);
assert(false == bsl::is_trivially_copyable<
                                       MyFundamentalTypeReference>::value);
assert(true  == bsl::is_trivially_copyable<
                                          MyTriviallyCopyableType>::value);
assert(false == bsl::is_trivially_copyable<
                                       MyNonTriviallyCopyableType>::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(true  == bsl::is_trivially_copyable_v<MyFundamentalType>);
assert(false == bsl::is_trivially_copyable_v<MyFundamentalTypeReference>);
assert(true  == bsl::is_trivially_copyable_v<MyTriviallyCopyableType>);
assert(false == bsl::is_trivially_copyable_v<MyNonTriviallyCopyableType>);
#endif