Component bslmf_decay [Package bslmf]

Convert a type to the type used for pass-by-value. More...

Classes
class	bsl::decay< t_TYPE >
struct	bsl::decay_imp< t_TYPE, t_IS_ARRAY, t_IS_FUNC >
struct	bsl::decay_imp< t_TYPE, true, false >
struct	bsl::decay_imp< t_TYPE, false, true >
Typedefs
typedef decay_imp< U, k_ISARRAY, k_ISFUNC >::type	bsl::decay::type

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Detailed Description

Outline

• Purpose
• Classes
• Description
  • Usage
    • Usage Example 1

Purpose:

Convert a type to the type used for pass-by-value.

Classes:

bsl::decay	type trait computing return type for type parameter
bsl::decay_t	alias to the return type of the bsl::decay meta-function

See also:

Component bslmf_removeextent

Description:

This component provides a metafunction, bsl::decay, that applies array-to-pointer and function-to-pointer conversion and cv-qualification removal to a type, thus modeling the decay of an argument type when passed by-value into a function. bsl::decay provides identical functionality to the C++11 standard metafunction std::decay. From the C++14, standard description of std::decay:

Let U be remove_reference_t<T>. If is_array<U>value is true, the member typedef type shall equal remove_extent_t<U>*. If is_function<U>value is true, the member typedef type shall equal add_pointer_t<U>. Otherwise the member typedef type equals remove_cv_t<U>. [ Note: This behavior is similar to the lvalue-to-rvalue (4.1), array-to-pointer (4.2), and function-to-pointer (4.3) conversions applied when an lvalue expression is used as an rvalue, but also strips cv-qualifiers from class types in order to more closely model by-value argument passing. - end note ]

Usage:

Usage Example 1:

A class template needs to cache a value of type T. There is nothing in the definition of the class that would prevent it from working for T of function type or array-of-unknown bound, but one cannot simply declare a member of either of those types. Instead, we use bsl::decay<T>type, which can be stored, copied, and compared as needed:

  #ifndef INCLUDED_BSLMF_DECAY
  #include <bslmf_decay.h>
  #endif

  template <class t_TYPE>
  class Thing {
    public:

#ifdef BSLS_COMPILERFEATURES_SUPPORT_ALIAS_TEMPLATES

Note that if the current compiler supports alias templates C++11 feature, we can use bsl::decay_t alias to the "result" type of the bsl::decay meta-function, that avoids the ::type suffix and typename prefix in the declaration of the function return type:

      using CacheType = bsl::decay_t<t_TYPE>;
#else
      typedef typename bsl::decay<t_TYPE>::type CacheType;
#endif

  private:
      CacheType d_cache;
      // ...

  public:
      CacheType cache() const { return d_cache; }
  };

Now verify that for function and array types, cache() will return a simple pointer:

  int main()
  {
      typedef const int A1[];
      typedef double A2[3][4];
      typedef void F1(int);
      typedef int (&F2)();

      assert((bsl::is_same<const int*,    Thing<A1>::CacheType>::value));
      assert((bsl::is_same<double(*)[4],  Thing<A2>::CacheType>::value));
      assert((bsl::is_same<void (*)(int), Thing<F1>::CacheType>::value));
      assert((bsl::is_same<int (*)(),     Thing<F2>::CacheType>::value));

      return 0;
  }

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Typedef Documentation

template<class t_TYPE>

typedef decay_imp<U, k_ISARRAY, k_ISFUNC>::type bsl::decay< t_TYPE >::type [inherited]