Templated C++ classes with friend functions and implementation-header separation

In the last two posts we saw how to call a constructor from another constructor using friend functions and how to separate the implementation from the header when C++ templates are used. In this post, I'm explaining how to do both, i.e., to use friend functions and templates with the implementation in a separate file from the header. One would expect that this is a pretty straightforward extensions, but, unfortunately, there are a few details that need to be carefully considered.

C++ Templates: Separate definition and implementation

Templates in C++ often force the developer to either merge the definition of the function prototypes and the implementation in a single header file (which is awkward), or to #include the implementation file in their application (i.e., do something like #include "MyClass.cpp", which is even more awkward!). The use of extern and export has been introduced in C++11, but here I'm going to present a neat way to deal with this peculiarity through an easy example.

C++: calling a constructor from within a constructor

Constructors within constructors? This is business as usual for the Java programmer, but what about C++? Unfortunately, it turns out that that's not so straightforward a task... In this post we are going to work this around with a simple example: a class which can be used to read data from text files. The key in this approach answers to the name of friend function! A friend function is a function which (i) is not a member function of a class, but (ii) has access to the private fields of objects that are passed to it as arguments.