Template type deduction for member variables and f

Question

Consider the following implementation of a template class:

template<class T>
class MyClass
{
public:
    void setVar1(const T& v1)
    {
        var1 = v1;
    }
    void setVar2(const T1& v2)
    {
        var2 = v2;
    }

    T var1;        
    T1 var2;
};

If the template parameter T is a fundamental type (like float, double, or long double) I would like to have T1 = T.

If the template parameter T is std::complex<float>, I would like to have T=std::complex<float> and T1 = float. Similarly for std::complex<double> and std::complex<long double>.

Deducing the type of the variable is discussed at Template type derivation

However, the additional member functions prevents usage of their solution in this context.

Answer 1

Based on Bo Personns comment and the answer provided at https://stackoverflow.com/questions/47334675/template-type-derivation

I got the following working example.

The content of my .h file is as follows.

#include <iostream>
#include <complex>
#include <typeinfo>


template <typename T>
class MyClass
 {


 template <typename T0>
struct myTypeTraits
 { using type = T0; };

template <typename T0>
struct myTypeTraits<std::complex<T0>>
 { using type = T0; };


public:

   using T0 = typename myTypeTraits<T>::type;

   void setVar1(const T0& v);

   void setVar2(const T& v);


T getVar2() const;




   void print() const;

   T0 var1;
   T  var2;
 };

The .cpp file contains the following lines of code.

template <class T>
void MyClass<T>::setVar1(const T0& v)
{
    var1 = v;
}


template <class T>
void MyClass<T>::setVar2(const T& v)
{
    var2 = v;
}



template <class T>
T MyClass<T>::getVar2() const
{
    return var2;
}


template <typename T>
void MyClass<T>::print() const
{
    std::cout<<"var1: "<<var1<<std::endl;
    std::cout<<"var2: "<<var2<<std::endl;

}



int main()
{

    MyClass<float> tmp;

    MyClass<std::complex<float> > tmp1;

    tmp.print();
    tmp1.print();
    return 0;
}

The above code works as intended.

However, I have another related question posted at Template type deduction in function return type

Answer 2

A custom traits class would suffice. You can leverage partial specializations to select the type you want/need. Example code

template<typename T, bool F> class Base;

template<typename T>
class Base <T, true> {
public:
  T var2;
};

template<typename T>
class Base <std::complex<T>, false> {
public:
  typename std::complex<T>::value_type var2;
};

template <typename T>
class BaseHelper : public Base<T, std::is_fundamental<T>::value> {};

template<class T>
class MyClass : public BaseHelper<T> {
public:

  using T1 = decltype(BaseHelper<T>::var2);

  void setVar1(const T& v1) {
    var1 = v1;
  }
  void setVar2(const T1& v2) {
    this->var2 = v2;
  }

  T var1;
};

Live Example

There are many other ways though.