Component bslalg_hastrait [Package bslalg]

Provide a meta-function to detect if a type has a given trait. More...

Namespaces
namespace	bslalg
Defines
#define	bslalg_HasTrait   bslalg::HasTrait

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Detailed Description

Outline

• Purpose
• Classes
• Description
  • Usage

Purpose:

Provide a meta-function to detect if a type has a given trait.

Deprecated:

Do not use.

Classes:

bslalg::HasTrait

legacy mechanism for detecting type traits

See also:

Component bslalg_typetraits

Description:

This component provides a meta-function, bslalg::HasTrait, that is a legacy mechanism for detecting traits. bslalg::HasTrait takes two template parameters, TYPE and TRAIT, and provides a Type member. bslalg::HasTrait<TYPE, TRAIT>Type evaluates to bslmf::MetaInt<1> if the (template parameter) TYPE has the (template parameter) TRAIT, and to bslmf::MetaInt<0> otherwise. (bslalg::HasTrait also provides a VALUE member where bslalg::HasTrait<TYPE, TRAIT>VALUE evaluates to 1 if the TYPE type has the TRAIT trait, and to 0 otherwise.)

bslalg::HasTrait is not a general-purpose trait detection facility. It is intended to be used in conjunction with the legacy (deprecated) bslalg traits only. In particular, the type supplied to bslalg::HasTrait as the TRAIT template parameter must be a bslalg trait, for example, bslalg::TypeTraitUsesBslmaAllocator. Supplying anything else for TRAIT will (more than likely) fail to compile. For example, the following errant use of bslalg::HasTrait will definitely fail to compile:

  bslalg::HasTrait<Foo, bsl::is_trivially_default_constructible>'

See bslalg_typetraits for a complete list of the bslalg traits that are supported by bslalg::HasTrait.

In lieu of the deprecated bslalg traits, the newer traits intended to replace them should be used instead. These successor traits are typically defined in bslmf (two exceptions are bslma::UsesBslmaAllocator and bslalg::HasStlIterators). bslalg::HasTrait and the bslalg traits that it supports are written in such a way that bslalg::HasTrait will correctly detect whether a type has a given trait regardless of whether the trait is ascribed to the type using the deprecated BSLALG_DECLARE_NESTED_TRAITS* or BSLMF_NESTED_TRAIT_DECLARATION macros (see bslalg_typetraits and bslmf_nestedtraitdeclaration, respectively), or whether the (preferred) C++11 idiom for defining traits is used (see bslmf_detectnestedtrait). In this way, bslalg::HasTrait provides a "bridge" between legacy bslalg type traits and their replacements that facilitates the migration to the newer traits.

Usage:

No Usage example is provided since this component is deprecated.

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Define Documentation

#define bslalg_HasTrait   bslalg::HasTrait