identity< T > Struct Template Reference

#include <deal.II/base/template_constraints.h>

Detailed Description

template<typename T>
struct identity< T >

A template class that simply exports its template argument as a local alias. This class, while at first appearing useless, makes sense in the following context: if you have a function template as follows:

template <typename T>
void f(T, T);

then it can't be called in an expression like f(1, 3.141) because the type T of the template can not be deduced in a unique way from the types of the arguments. However, if the template is written as

template <typename T>
void f(T, typename identity<T>::type);

then the call becomes valid: the type T is not deducible from the second argument to the function, so only the first argument participates in template type resolution.

The context for this feature is as follows: consider

template <typename RT, typename A>
void forward_call(RT (*p) (A), A a)
{
  p(a);
}

void h (double);

void g()
{
  forward_call(&h, 1);
}

This code fails to compile because the compiler can't decide whether the template type A should be double (from the signature of the function given as first argument to forward_call, or int because the expression 1 has that type. Of course, what we would like the compiler to do is simply cast the 1 to double. We can achieve this by writing the code as follows:

template <typename RT, typename A>
void forward_call(RT (*p) (A), typename identity<A>::type a)
{
  p(a);
}

void h (double);

void g()
{
  forward_call(&h, 1);
}

Author

Wolfgang Bangerth, 2008

Definition at line 241 of file template_constraints.h.

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The documentation for this struct was generated from the following file:

• deal.II/base/template_constraints.h