bslmf_unwrapreference

Detailed Description

Provide a meta-function to unwrap reference wrappers.

Outline

• Purpose
• Classes
• Canonical Header
• Description
  • Usage
    • Example 1: Unwrap Reference Wrapped Types

Purpose

Provide a meta-function to unwrap reference wrappers.

Classes

• bsl::unwrap_reference: standard meta-function to unwrap reference wrappers
• bsl::unwrap_reference_t: alias to the return type of the meta-function

Canonical Header

bsl_functional.h, bsl_type_traits.h

See also

bslmf_referencewrapper, bslmf_unwrap_ref_decay

Description

This component defines a meta-function bsl::unwrap_reference that may be used to unwrap a bsl::reference_wrapper (which is an alias of std::reference_wrapper when that exists) of some type U, resulting in U&. In case the specified type template argument is not a specialization of bsl::reference_wrapper the result is the type itself.

bsl::unwrap_reference meets the requirements of the unwrap_reference template defined in the C++20 standard [meta.trans.other].

Usage

In this section we show intended use of this component.

Example 1: Unwrap Reference Wrapped Types

Suppose that we work in a programming environment where types may be presented wrapped in a bsl::reference_wrapper or its std equivalent. We would like to use a reference to the wrapped type, but use unwrapped types as they are. This is the exact use case for bsl::unwrap_reference.

First, we create types that represent both reference-wrapped, and normal type parameters:

typedef bsl::reference_wrapper<int *>  WrappedType;
typedef int                           *NotWrappedType;

Next, we create types that are references if they were wrapped:

typedef bsl::unwrap_reference<WrappedType>::type    UnwrappedWrapped;
typedef bsl::unwrap_reference<NotWrappedType>::type UnwrappedNotWrapped;

Finally we can verify that the wrapped type became a reference, while the other type is unchanged:

assert((true == bsl::is_same<UnwrappedWrapped,    int *&>::value));
assert((true == bsl::is_same<UnwrappedNotWrapped, int *>::value));

Note, that (when available) the bsl::unwrap_reference_t avoids the ::type suffix and typename prefix when we want to use the result of the bsl::unwrap_reference meta-function in templates.