Component bslmf_nil [Package bslmf]

Provide a nil type. More...

Namespaces
namespace	bslmf
Defines
#define	bslmf_IsNil   bslmf::IsNil
Typedefs
typedef bslmf::Nil	bslmf_Nil

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Detailed Description

Outline

• Purpose
• Classes
• Description
  • Usage

Purpose:

Provide a nil type.

Classes:

bslmf::Nil	class representing a nil (non-existent) type.
bslmf::IsNil	meta-function to test for nil.

Description:

bslmf::Nil is mainly used for template meta-programming. It is useful for providing defaults for template parameters and terminating template recursions. bslmf::Nil can also be used to represent an unset state (i.e., nil) in components that require one.

Usage:

The following usage illustrates a typical use of bslmf::Nil as a default template parameter for another meta-function.

  template <int   CONDITION,
            class TRUE_TYPE = bslmf::Nil,
            class FALSE_TYPE = bslmf::Nil>
  struct my_If {
      typedef TRUE_TYPE Type;
  };

  template <class TRUE_TYPE, class FALSE_TYPE>
  struct my_If<0, TRUE_TYPE, FALSE_TYPE> {
      typedef FALSE_TYPE Type;
  };

Next, we can use the my_If meta-function to implement a print function that checks whether a type has a print method (using a fictitious meta-function my_IsPrintable), and print the value. Note the use of bslmf::Nil as the type for overloading:

  template <class TYPE>
  void print(const TYPE& value, int)
  {
      bsl::cout << value << bsl::endl;
  }

  template <class TYPE>
  void print(const TYPE&, bslmf::Nil)
  {
      bsl::cout << "Cannot be printed." << bsl::endl;
  }

  template <class TYPE>
  void print(const TYPE& value)
  {
      my_If<my_IsPrintable<TYPE>::value, int>::Type Type;  // default false
                                                           // type is
                                                           // 'bslmf::Nil'.
      print(value, Type());
  }

Using the above functions, the following code:

  int         x = 3;
  bslmf::Nil nil;

Will print the following to stdout:

  3
  Cannot be printed.

Finally, the bslmf::IsNil meta-function returns true if the type passed to it is bslmf::Nil, and false otherwise:

  assert(1 == bslmf::IsNil<bslmf::Nil>::value)
  assert(0 == bslmf::IsNil<int>::value)
  assert(0 == bslmf::IsNil<char>::value)

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Define Documentation

#define bslmf_IsNil   bslmf::IsNil

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Typedef Documentation

typedef bslmf::Nil bslmf_Nil