C++ class template undefined reference to function [duplicate]

Question

I keep getting undefined reference when i call the two functions from my template class "add" and "greater" in my main function.

So, i have: number.h

#ifndef NUMBER_H
#define NUMBER_H

template <class T>
class number {
public:
    T x;
    T y;

    number (int a, int b){
        x=a; y=b;}
    int add (T&);
    T greater ();
};

#endif

number.cpp

#include "number.h"

template <class T>
int number<T>::add (T& rezAdd){
    rezAdd = x+y;
    return 1;
}

template <class T>
T number<T>::greater (){
        return x>y? x : y;
}

And my main file is: resolver.cpp

#include <stdio.h>
#include <stdlib.h>
#include "number.h"

int main (int argc, char **argv) {
    int aux;
    number<int> c(3,5);

    c.add(aux);
    printf ("number added [%d]\n", c.add(aux));
    printf ("greater number: [%d]\n", c.greater());

    return 0;
}

The errors that i keep getting are:

g++ -Wall -o tema1 resolver.cpp number.cpp
/tmp/ccX483J4.o: In function `main':
resolver.cpp:(.text+0x34): undefined reference to `number<int>::add(int&)'
resolver.cpp:(.text+0x47): undefined reference to `number<int>::add(int&)'
resolver.cpp:(.text+0x64): undefined reference to `number<int>::greater()'
collect2: ld returned 1 exit status
make: *** [all] Error 1

Thanks for the help in advance!

Answer 1

I prefer to have all of my functions in the .cpp file, regardless of whether they are template functions or regular functions. And there is a way to do that with some basic #ifndef magic. Here's what you can do:

main.cpp

#include "myclass.hpp"

int main()
{
  // ...
}

myclass.hpp

#ifndef MYCLASS
#define MYCLASS

template<class T>
class MyClass
{
  T val;
public:
  MyClass(T val_);
}

#define MYCLASS_FUNCTIONS
#include "myclass.cpp"

#endif

myclass.cpp

#ifndef MYCLASS_FUNCTIONS
#include "myclass.hpp"

// regular functions:
// ...

#else

// template functions:
template<class T>
MyClass<T>::MyClass(T val_)
    :val(val_)
{}

// ...
#endif

Here's how the precompiler sees it. We have two .cpp files.

1. When we compile main.cpp we:
   1. include myclass.hpp
   2. check that MYCLASS is undefined, and it is
   3. define it
   4. give compiler the definitions of the generated class (from template class)
   5. include myclass.cpp
   6. define MYCLASS_FUNCTIONS
   7. check if MYCLASS_FUNCTIONS is defined, it is
   8. give compiler the definitions of the generated functions (from template functions)
2. When we compile myclass.cpp
   1. check if MYCLASS_FUNCTIONS is defined, it isn't
   2. include myclass.hpp
   3. check that MYCLASS is undefined, and it is
   4. define it
   5. give compiler the definitions of the class
   6. include myclass.cpp
   7. include myclass.hpp again
   8. this time MYCLASS is defined so do nothing inside, return to myclass.cpp
   9. check if MYCLASS_FUNCTIONS is defined, it is
   10. give compiler the definition of the generated functions (from template functions)
   11. exit include twice
   12. pass to the compiler all the regular functions

Answer 2

Your class is named wrong. Your class is named cai where all your functions belong to a class named number: http://ideone.com/ZayX0c

One more thing.. you cannot have templates in the .cpp file. Template functions/defintions go in the header along with the class declaration. This is the reason for your undefined function error. Non-template functions go in the .cpp.

#include <cstdio>
#include <cstdlib>

template <class T>
class number {
public:
    T x;
    T y;

    number (int a, int b){
        x=a; y=b;}
    int add (T&);
    T greater ();
};

template <class T>
int number<T>::add (T& rezAdd){
    rezAdd = x+y;
    return 1;
}

template <class T>
T number<T>::greater (){
        return x>y? x : y;
}


int main (int argc, char **argv) {
    int aux;
    number<int> c(3,5);

    c.add(aux);
    printf ("number added [%d]\n", c.add(aux));
    printf ("greater number: [%d]\n", c.greater());

    return 0;
}

Answer 3

Move the definitions of the add and greater function templates into your number.h.

Remember that add and greater aren't functions, they're function templates. To create actual functions, the compiler has to instantiate the template for specific types, such as int, and it can only do that if it has access to the template's definition at the point where it discovers that an instance is needed.

When you compile number.cpp, the compiler has access to the templates' definitions, but it doesn't see any code that requires a specific instance (such as number<int>), so it doesn't generate instances.

When you compile resolver.cpp, the compiler sees that it needs to instantiate those templates for the int type, but it can't since it doesn't have their definitions. So it generates "external references", basically notes telling the linker to look for those functions in some other object file.

The result is that the function templates don't get instantiated in either object file — in one because the compiler didn't know that it should, and in the other because it couldn't — so when the linker goes looking for them (to resolve those external references), it can't find them. That's why you get the error.

Moving the template function definitions into the header makes them visible to the compiler while it's compiling main.cpp, so it's able to instantiate those functions for the int type. Function templates typically need to be defined in header files, rather than .cpp files, for exactly this reason.