Odtnhj

I have a templated class which is split into a .hpp file and a .ipp which has quite a long type in it. While refactoring I wanted to write a type alias for that type so the code could be a lot more readable. But no matter what I do I cannot get the type alias to be recognized in the .ipp file.

Here is a simple example of what I am trying to do:

container.hpp:

template <class T>
class container 
public:
 using TypeAlias = T;
 container(const TypeAlias& a);
 const TypeAlias& getA() const;
private:
 TypeAlias a;
;

#include "container.ipp"

container.ipp:

template <class T>
container<T>::container(const TypeAlias& a) : a(a)




template <class T>
const TypeAlias& container<T>::getA() const

 return this->a;

When compiling this the following error occurs:

./container.ipp:8:7: error: unknown type name 'TypeAlias'
const TypeAlias& container<T>::getA() const

Obviously in this case it doesn't make much sense to introduce the TypeAlias but in my actual program it could make a huge difference. Is there anything I can do to make the TypeAlias available in the .ipp file?

When you define a class member like container<T>::getA, things after the name of the member (getA here) are looked up in the scope of the class, but things before it are not. Essentially, if the compiler tries to understand things in order, when the declaration begins with const TypeAlias&, it doesn't yet know that it should look inside container<T> to figure out what TypeAlias could mean.

So using normal function syntax, you would need to specify that TypeAlias is a member type:

template <class T>
const typename container<T>::TypeAlias& container<T>::getA() const

 return this->a;

But this is one advantage of the "trailing return type" syntax (valid in C++11 and later): Just by changing the order of things, it lets you use members of the class within the return type more simply:

template <class T>
auto container<T>::getA() const -> const TypeAlias&

 return this->a;