C++ Identify template arguments

Question

I would like to know is there a way to identify template argument? For example, let say I want to initialize a variable based on template argument. Prototype example is as follow

template<class T> 
      void initialise(T a)
      {
        if(T==int)a=0;
        else if(T=double)a=0.0;
        else if(T=complex<double>)a=T(0.,0);
        else print("unknown type"); 
      }

My question is how to identify that template argument "T"? Or, do I need to take help of preprocessor directives? It might be a repetitive question, but I didn't find its answer. Any kind of suggestions will be appreciated.

Answer 1

This is how you would "re-set" an argument to zero using a function template:

template<class T> 
void initialise(T& a)
{
  a = T(); // or T{};
}

If you want to restrict it to arithmetic types, you can use SFINAE and std::is_arithmetic (floating point and integral types), plus a specialization for std::complex .

template<class T> 
typename enable_if<std::is_arithmetic<T>::value, void >::type
void initialise(T& a)
{
  a = T();
}

template<class T>
void initialise(std::complex<T>& a)
{
  a = T();
}

Answer 2

You can use template specializations. Something like that:

template<class T> 
void initialise(T a)
{
    print("unknown type"); 
}

template<> 
void initialise<int>(int a)
{
    a=0;
}

template<> 
void initialise<double>(double a)
{
    a=0.0;
}

template<> 
void initialise<complex<double>>(complex<double> a)
{
    a=complex<double>(0.,0);
}

Of course, those functions do not make much sense, but I guess it is due to the fact that you tried to propose a minimal example.