Question

I am trying to implement a leftist tree using heaps as a base class. The following is the contents of heap.h:

template <class T>

class heap {
public:
    virtual void init(T*) = 0;
    virtual void insert(T*) = 0;
    virtual T delete_min() = 0;
};

The following is the contents of leftist.cpp:

#include "heap.h"

template <class T>
class leftist_tree : public heap<T> {
private:
    T* root;
public:
    void init(T* a) {}
    void insert(T* a) {}
    T delete_min() {T a; return a;}
};

I am passing another class leftist_node as a parameter to this class using the following definition:

leftist_tree<leftist_node> mytree;

I am getting an LNK 2001 unresolved external symbol error for functions init, insert and delete_min. What am I doing wrong?

Edit:

Okay the example I have given at this point is far too complex. I have tried to reproduce the same error on a smaller scale so that someone can identify the problem more easily. I have created the following sample files.

try.cpp

#include "stdafx.h"
#include "myclass.h"

int _tmain(int argc, _TCHAR* argv[])
{
    myclass<int> a;
    a.hello(3);
    return 0;
}

myclass.h

template <class T>

class myclass {
public:
    void hello(T);
};

myclass.cpp

#include "myclass.h"
#include <iostream>
using namespace std;

template <class T>
void myclass<T>::hello(T a){
    cout<<a<<endl;
    system("pause");
}

I get a similar error message:

1>try.obj : error LNK2001: unresolved external symbol “public: void __thiscall myclass::hello(int)” (?hello@?$myclass@H@@QAEXH@Z)
1>c:\users\meher anand\documents\visual studio 2010\Projects\try\Debug\try.exe : fatal error LNK1120: 1 unresolved externals

Can you tell me where I am going wrong right now? Thanks

Answer 1

Template functions are treated a little differently from regular functions. The compiler doesn’t actually compile the function until you try to use it. If the only place you try to use it is a .cpp where the body of the function is undefined, it assumes it must be compiled somewhere else and makes a reference for the linker to fill in later.

In your case you defined the body of the functions in leftist.cpp, but you didn’t use it there, you used it somewhere else. If you had defined it in the .h file then the definition would have been available everywhere you tried to use it and all would be well. If you had used the functions somewhere in leftist.cpp then the functions would have been created and the linker would have fixed everything up.

The general rule is to define the body of all template functions in the header files.