Creating template classes in a Class Library project C++ - c++

Question

I have a C++ class library project that is commonly used by other C++ projects. To be able to use classes inside my class library project, I wrote a header file like the example given below
#pragma once
#ifdef MYLIB
# define MYLIB_EXPORT __declspec(dllexport)
#else
# define MYLIB_EXPORT __declspec(dllimport)
#endif
No problem until I want to create a template class inside my class library project. The problem is I can't export my template class.
MyClass.h
template<class T>
class MYLIB_EXPORT MyClass
{
void myMethod();
// ...
}
template<class T>
void MyClass::myMethod()
{
// ...
}
In this case I am getting compilaton errors saying "definition of dllimport function not allowed". I know what causes the problem and I understand it. Other projects using my class library project converts the MYLIB_EXPORT keyword to __declspec(dllimport). Therefore, they are expecting the methods of MyClass to be defined in a DLL. But, then the compiler sees the definition inside the header.
How can I overcome this situation and be able to export my template classes that are defined inside my class library project?

Answer 1

Uninstantiated templates can't be compiled directly - they are code generators, so they are actually translated to binary instructions only when they are instantiated; for this reason, you can't export a template "in binary form" as if it was a "regular" function/class (on the other hand, at least in theory you could export an instantiation of a template).
Long story short: just leave the templates in a header to be included by library clients.
Notice that this is the exact reason why you keep templates in headers and you don't normally separate their implementation in .cpp files.

Answer 2

Just remove the MYLIB_EXPORT statement on template class. Then you can define class functions outside the class (but still in *.h or *.hpp header files).
MyClass.h
template <typename T>
class MyClass // MYLIB_EXPORT removed
{
void myMethod();
// ...
};
template <typename T>
void MyClass<T>::myMethod()
{
// ...
}
I got this issue. After a long time, I realized removing MYLIB_EXPORT fixed it. Hope this answer will save time to others :-)