Import type-alias from class in source

Question

How do I import the type alias of a class for general use in my source file?

I.e., consider I have a file myClass.h with

template<typenmae T>
class myClass {
  using vec = std::vector<double>;
  
  vec some_vec_var;
public:
  // some constructor and other functions
  myClass(T var);
 
  vec some_function(vec some_var);
  void some_other_fcn(vec some_var);
};

I've always written my sourcefile (myclass.cpp) in the following manner

#include "myClass.h"

template<typename T> 
myClass<T>::myClass(T var){/* */} // fine 

template<typename T>
void myclass<T>::some_function(vec some_var){/* */} // fine

// Error: vec was not declared in this scope ... 
template<typename T> 
vec myClass<T>::some_function(vec some_var){/* */} // not fine

How do I overcome this? Type aliases tend to become tediously long, so simply replacing vec as return-type with its declared alias is not a feasable solution for me.

Do I need to wrap it around a namespace scope? something like

namespace MyNameSpace{
  class myClass { ... };
}

as described here: C++: Namespaces -- How to use in header and source files correctly? ?

Or can I just import the namespace of the class somehow?

typename myClass<T>::vec myClass<T>::some_function(typename myClass<T>::vec some_var){/* */}. Frankly, I'm more concerned with the whole myClass being (a) a template, and (b) implemented anywhere other than the header file. Barring explicit instantiation intentions, that's where it belongs. — WhozCraig
– WhozCraig, Commented Apr 19, 2022 at 9:28
Could zou elaborate your concern a little more? I thought it is common practice to write two separate files *.h with class and function declarations and *.cpp with the actual code. — ro_go
– ro_go, Commented Apr 19, 2022 at 9:35
@ro_go yes but not with templated classes - these need to be defined in the header file as well, since the compiler instantiates it for the specific types used inside the template; see Why can templates only be implemented in header file — codeling
– codeling, Commented Apr 19, 2022 at 11:22

Jarod42 · Accepted Answer · 2022-04-19 09:34:09Z

2

Trailing return type (C++11) might help:

template<typename T> 
auto myClass<T>::some_function(vec some_var) -> vec
{/* */}

Else you have to fully qualify:

template<typename T> 
typename myClass<T>::vec myClass<T>::some_function(vec some_var)
{/* */}

answered Apr 19, 2022 at 9:34

Jarod42

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