Solved

c++ class template (not inline)

Posted on 2014-01-22
4
405 Views
Last Modified: 2014-01-22
I have coded a C++ class template that is a descendent of the STL map template. The code compiles and runs as expected. I have coded it inline in a header file. How do I code the member functions in a separate cpp file, using the header file for the prototype declaration only ?
The code in the header file is a follows :

#include <map.h>

template<class T1,class T2>
class MyMap : public map<T1,T2>
{
    private:
    map<T1,T2> &mx;
    T2 *x;
    public:
    MyMap()  : mx(*this){};
    ~MyMap(void){};
    map<T1,T2> operator = (map<T1,T2>  m)
    {
        mx = m;
        return m;
    };
    T2& operator[](T1 k)
    {
        if(mx.count(k)==0)
        {
            x = &new T1();
            mx[k]= *x;
        }
        return  mx[k];
    }
};
0
Comment
Question by:alcindor
  • 2
4 Comments
 
LVL 32

Expert Comment

by:phoffric
Comment Utility
Normally, you do not code the template member functions in a separate cpp file since the templates are not actual code that is run until the compiler statically instantiates them. If another module uses your header and tries to instantiate them, it will not know how since it doesn't have direct access to the template .cpp file.

Is there a reason why you want to separate them?
0
 
LVL 32

Accepted Solution

by:
phoffric earned 400 total points
Comment Utility
Here is a  more detailed elaboration of why you don't separate the template body code from the template declarations.
   Why can't I separate the definition of my templates class from its declaration and put it inside a .cpp file?

But, if you really must, there is a way that requires extra maintenance when you need to add a new static instantiation of your template class.
Separate Template Class Definition from Declaration
The message from this last link is that in your .cpp file, you will need to add as many specific static instantiations as you  program requires - something like:
  template class MyMap <int, int>;
  template class MyMap <string, int>;
  etc.

When you compile this .cpp file, you get the body of all the different signatures your program needs. If you later need a <string, string>, then you have to go back and add
    template class MyMap <string, string>;
to the template's .cpp file.
0
 
LVL 86

Assisted Solution

by:jkr
jkr earned 100 total points
Comment Utility
>>How do I code the member functions in a separate cpp file, using the header file for the
>>prototype declaration only ?

Just like with any other class that would not be templated. The caveat just is: While the C++ Standard technically allows doing that, most compilers don't support it. So, if that already works for you, the question basically is: Why putting in the effort if chances are that you will have to add a '#include "MyMap.cpp"' to get it to work anyway?
0
 
LVL 2

Author Closing Comment

by:alcindor
Comment Utility
Thanks for your comments. I have a better understanding now and will not be coding the definition from the declaration.

Roger
0

Featured Post

Highfive Gives IT Their Time Back

Highfive is so simple that setting up every meeting room takes just minutes and every employee will be able to start or join a call from any room with ease. Never be called into a meeting just to get it started again. This is how video conferencing should work!

Join & Write a Comment

Introduction This article is the first in a series of articles about the C/C++ Visual Studio Express debugger.  It provides a quick start guide in using the debugger. Part 2 focuses on additional topics in breakpoints.  Lastly, Part 3 focuses on th…
C++ Properties One feature missing from standard C++ that you will find in many other Object Oriented Programming languages is something called a Property (http://www.experts-exchange.com/Programming/Languages/CPP/A_3912-Object-Properties-in-C.ht…
The goal of the tutorial is to teach the user how to use functions in C++. The video will cover how to define functions, how to call functions and how to create functions prototypes. Microsoft Visual C++ 2010 Express will be used as a text editor an…
The viewer will learn how to clear a vector as well as how to detect empty vectors in C++.

763 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

13 Experts available now in Live!

Get 1:1 Help Now