"Undefined symbols" linker error with simple template class

Question

Been away from C++ for a few years and am getting a linker error from the following code:

Gene.h

#ifndef GENE_H_INCLUDED
#define GENE_H_INCLUDED

template <typename T>
class Gene {
    public:
    T getValue();
    void setValue(T value);
    void setRange(T min, T max);

    private:
    T value;
    T minValue;
    T maxValue;
};

#endif // GENE_H_INCLUDED

Gene.cpp

#include "Gene.h"

template <typename T>
T Gene<T>::getValue() {
    return this->value;
}

template <typename T>
void Gene<T>::setValue(T value) {
    if(value >= this->minValue && value <= this->minValue) {
        this->value = value;
    }
}

template <typename T>
void Gene<T>::setRange(T min, T max) {
    this->minValue = min;
    this->maxValue = max;
}

Using Code::Blocks and GCC if it matters to anyone. Also, clearly porting some GA stuff to C++ for fun and practice.

Answer 1

The template definition (the cpp file in your code) has to be included prior to instantiating a given template class, so you either have to include function definitions in the header, or #include the cpp file prior to using the class (or do explicit instantiations if you have a limited number of them).

Answer 2

Including the cpp file containing the implementations of the template class functions works. However, IMHO, this is weird and awkward. There must surely be a slicker way of doing this?

If you have only a few different instances to create, and know them beforehand, then you can use "explicit instantiation"

This works something like this:

At the top of gene.cpp add the following lines

template class Gene<int>;
template class Gene<float>;