Converting library class into template library class

[rozick1]

Hi

I have been developing a library which has the following structure (simplified for the purposes of the explanation):

// myLibrary.h

#include "HelperClass.h"
  
class myLibrary {

  private:

    struct Details {
      string nm;
      int age;
    } details;

    HelperClass some_data;

  public:
  
    int getNum();
    HelperClass getData();

};

//myLibrary.cpp

int myLibrary::getNum() {
  return num;
}

HelperClass myLibrary::getData() {
  return some_data;
}

//main.cpp

myLibrary mLib;

myLibrary and HelperClass are compiled to form a static library ("myLibrary.a" file) which the client can link to when using myLibrary.

I'd like to make myLibrary a templated class so that the client using the library can supply a type T for the details member i.e.

// myLibrary.h
  
class myLibrary {

  private:

    T details;

    ...
};

// main.cpp

struct custom_details{
  string nm;
  bool isSomething;
};

myLibrary <custom_details> mLib;

However, I understand that, for templated classes, the interface (header file) cannot be separated from the implementation file (cpp file). So my options are either to:

1.) Make explicit the instantiations of T - this doesn't work as the client could supply any type for T.

2.) Put the method definitions of myLibrary<T> in myLibrary.h. Either:
(a) Directly in myLibrary.h; or
(b) Creating an intermediate file (a ".tpp" file) with the definitions and including this intermediate file at the end of myLibrary.h.

Option 2(b) is more convenient for me as I will not have to move all of the method definitions from myLibrary.cpp to myLibrary.h and I can rename myLibrary.cpp to myLibrary.tpp (and add template definitions to each method). It also helps keep the appearance of a interface/implementation distinction which aids readability.

For 2(b), I think myLibrary becomes a header only library and so the client would simply include myLibrary.h rather than linking to myLibrary.a.

Given the above, I had a couple of questions:

(A) Is the understanding set out above correct? Have I missed any easier solutions? I'd considered using something like std::any for details but I understand that using std::any is discouraged.


(B) Will HelperClass, which will not be a templated class, need to be header only as well or would the client still link to something like HelperClass.a?


Thanks very much

[kbw]

It's not unusual to have a header only library in C++. For example: https://github.com/gabime/spdlog or https://github.com/Tencent/rapidjson

Such libraries just distributed as header files.

With C++17, static members in template classes can be defined in the class. So if you go down this route, you may have no need for .cpp files.

I haven't answered your points directly because they assume you'll have .cpp files, which isn't the case.