bslmf_nil

Macros
#define	bslmf_IsNil   bslmf::IsNil
	This alias is defined for backward compatibility.

Typedefs
typedef bslmf::Nil	bslmf_Nil
	This alias is defined for backward compatibility.

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)

Macro Definition Documentation

bslmf_IsNil

#define bslmf_IsNil   bslmf::IsNil

Typedef Documentation

bslmf_Nil

typedef bslmf::Nil bslmf_Nil