Meta-programming

Meta-programming is the use of the C++ template system to implement compile-time behaviors. Template meta-programming consists in providing functions working on types and/or to produce values using types. It must be noted that the value provided by a type is named, in ELM, "_" instead of "value" used in STL. More...

Classes
struct	n< x >
struct	em
struct	_if< c, T, E >
struct	is_supported< T, C >
struct	enable_if< B, T >

Functions
template<class T >
T &	declval ()

Detailed Description

Meta-programming is the use of the C++ template system to implement compile-time behaviors. Template meta-programming consists in providing functions working on types and/or to produce values using types. It must be noted that the value provided by a type is named, in ELM, "_" instead of "value" used in STL.

Included in the Meta-programming namespace, the following facilities are available:

• meta::true – type which "</em>" value is 1,
• meta::_false -- type which "_" value is 0,
• meta::n <N> -- type which "_" value is N,
• meta::if <C, T1, T2> – if C = true, produce type T1 else produce type T2,
• meta::declval <T> – generate an abstract value of type T,
• meta::is_same <T1, T2> – type which "</em>" value is 1 if T1 = T2, 0 else,
• meta::is_supported<T, C> – test if the type C is instantiable with T,
• meta::enable_if <C, T> – define a type if a condition is true.

Function Documentation

declval()

T & declval ( )

inline

#include <include/elm/meta.h>

Function generating an abstract value of type T. The returned value is abstract means it does not correspond to any value in memory and therefore must not be used in classic code. A more consistent use of declval() is in meta-programming when a value is needed, for instance, to select a function by parameter overloading.

For example, the code below selects the return type of a function f() taking no parameter:

template <class T>
decltype(meta::declval<T>().f()) ff(T *v) { return v.f(); }

Parameters

T	Type of the returned abstract value.