Question

I have the following method declared in the public area of a class like that:

In the header file:

class EntityManager
{
    public:
        ...
        template <typename ComponentType>
        bool addComponentToEntity(const Entity in_Entity, const shared_ptr<ComponentType> in_ComponentInstance);
        ...
}

In the source file:

template <typename ComponentType>
bool EntityManager::addComponentToEntity(const Entity in_Entity, const shared_ptr<ComponentType> in_ComponentInstance)
{
    ...
}

Then I try to use it like this:

Entity l_Entity = 1;
shared_ptr<TestComponent> l_TestComponent(new TestComponent());
EntityManager* l_EntityManager = new EntityManager();

l_EntityManager->addComponentToEntity<TestComponent>(l_Entity, l_TestComponent);

This results in the compiler complaining:

undefined reference to `bool EntityManager::addComponentToEntity<TestComponent>(unsigned long, boost::shared_ptr<TestComponent>)'|

I am aware that this is probably because I am not very experienced in C++ programming, but I can’t see a reason why the function is undefined. I ommitted some other code that calls other functions of the EntityManager class and that works perfectly well.

I also tried rewriting the function with regular pointers, references and even passes by value with the same result.

Answer 1

Template methods/functions must be defined in the header itself (unless the method’s class itself is defined in a source file). This is also the case for all methods of a template class (even if the methods themselves aren’t template methods).

Most people don’t even bother defining them below the class definition, they just define them inline inside the class:

class EntityManager 
{ 
    public: 
        ... 
        template <typename ComponentType> 
        bool addComponentToEntity(const Entity in_Entity, const shared_ptr<ComponentType> in_ComponentInstance)
        {
            ...
        }
        ... 

}