templates - undefined reference error - c++

Question

I have the following simple template code:
#ifndef CLUSTER_H
#define CLUSTER_H
#include <iostream>
#include <vector>
template <typename T, size_t K>
class Cluster
{
public:
void Print() const;
private:
std::vector<T> objects;
};
template <typename T, size_t K>
void Cluster<T,K>::Print() const
{
for (int i=0; i<objects.size(); i++)
{
T curr=objects[i];
std::cout << curr << " ";
}
std::cout << std::endl;
}
#endif
and for some reason I get the following error: "undefined reference to 'Cluster<int, 5u>::Print() const'. What could be the cause for this?
Thanks!

Answer 1

So, I'm going to go out on a limb here and say that you've defined a template function in a CPP file, which means it will end up in a different translation unit. Here's a simple example:
A header, example.h
#ifndef EXAMPLE_H
#define EXAMPLE_H
template<int TValue>
class example
{
public:
int get_tvalue();
};
#endif
A source file, example.cpp
#include "example.h"
template<int TValue>
int example<TValue>::get_tvalue()
{
return TValue;
}
And another source file, main.cpp
#include "example.h"
int main()
{
example<5> instance;
instance.get_tvalue();
return 0;
}
If I compile these together using GCC, I get undefined reference to 'example<5>::get_tvalue()'. This is because of the way template classes are instantiated. A template class definition is just that... a template, not an actual class. The actual class definition is created when a parameterised (or specifically, fully specialised) definition of that class occurs, in this case, example<5>. That fully specialised class definition only exists in main.cpp... there's no such class inside example.cpp! Example.cpp contains only the template, and no specialisations. This means the function, get_tvalue is not defined for example<5> in main.cpp, hence the error.
You can fix this in one of two ways. The first way is to always have your entire template class defined in its header file. This is the way its done with STL containers, for example. The alternative is to force creation of a parameterised class in example.cpp... you can do this by adding
template class example<5>;
to the end of example.cpp. Because there's now an actual class definition for example<5> in example.cpp, you will also get an actual function definition for example<5>::get_tvalue and when your translation units main.o and example.o are linked together at the end of the compilation step everything will be fine.
Obviously, this would be a poor approach in most cases, but under circumstances where your template parameters take only a small range of values it can work. Putting your whole class in the header file is probably easiest, safest and most flexible though.