Solved

Template derivation

Posted on 2001-08-10
5
501 Views
Last Modified: 2008-03-03
Hi,

I created a template class :

template<class T>
class CBaseT
{
 T *m_pT;
public :
 CBase();
 ~CBase();
 T *Get();
 void Set(T *pT);
};

And I derived a class from it :

class Derived : public CBaseT<char>
{
 public :
 Derived() {Set("A");} // Correction, instead of: Set('A');
 ~Derived();
};

The problem is that I got a link error which says that
the class CBaseT<char> is not found.

My question is : Why is that happened , and how can I
fix it that CBaseT<char> will be found ?

 Thanks
 Michael
0
Comment
Question by:MichaelMaromm
5 Comments
 
LVL 4

Expert Comment

by:IainHere
ID: 6371737
Set('A') passes a const char, but your definition of Set() requires a char*.

You could change the definition to

void Set(T pT);

and it should work.
0
 

Expert Comment

by:Walluce
ID: 6371821
Or you could simply pass in "A" instead of 'A'.
"A" will be a char * because it creates a string of two characters (A and \0) whereas 'A' is just one char.
0
 
LVL 30

Accepted Solution

by:
Axter earned 50 total points
ID: 6371847
>>Why is that happened , and how can I fix it that
>>CBaseT<char> will be found ?

This is most likely happening because you put the implementation for your code in the *.cpp file.

In other words, you have the {void Set(T *pT)} function defined in the *.cpp file.

For template classes you need to define the functions in the header.

Continue.......
0
 
LVL 30

Expert Comment

by:Axter
ID: 6371874
Continuation:

Although the current C++ standards say that you can put the functions in the *.cpp file, most compilers do not support this capability.

If you put the code in the *.cpp file, then only code that includes the *.cpp file can declare new types of your template class.

An alternative to this is to put a special forward declaration for each type that you will use for the template class.

The syntax for this is the following:

template CBaseT<char>;

The above forward declaration would be put into the *.cpp file that has your template function defined.
You would have to do this for each type if you want to use this method.

I do not recommend using the above method unless you really have a good reason for keeping the template function definition inside your *.cpp file.

It is far easier to just keep the template function difinition in the *.h header file, and this is the common method used by most programmers for template classes.
0
 

Author Comment

by:MichaelMaromm
ID: 6373017
Thank you Axter,
You realy helped me.

Michael
0

Featured Post

Free Tool: Subnet Calculator

The subnet calculator helps you design networks by taking an IP address and network mask and returning information such as network, broadcast address, and host range.

One of a set of tools we're offering as a way of saying thank you for being a part of the community.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

In days of old, returning something by value from a function in C++ was necessarily avoided because it would, invariably, involve one or even two copies of the object being created and potentially costly calls to a copy-constructor and destructor. A…
Templates For Beginners Or How To Encourage The Compiler To Work For You Introduction This tutorial is targeted at the reader who is, perhaps, familiar with the basics of C++ but would prefer a little slower introduction to the more ad…
The goal of the video will be to teach the user the concept of local variables and scope. An example of a locally defined variable will be given as well as an explanation of what scope is in C++. The local variable and concept of scope will be relat…
The viewer will learn how to use the return statement in functions in C++. The video will also teach the user how to pass data to a function and have the function return data back for further processing.

830 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