模板中的虚拟 function 继承 class

Question

I am new to templates and I have searched the web for this error but I don't know how to fix it it. Already checked Why can templates only be implemented in the header file?

State.h

template <class entityType>
class State
{
public:
    State() = default;
    virtual void Enter(entityType * owner);

};

EnterMine.h

#include "State.h"

class Miner;
class EnterMine : public State<Miner>
{
public:
    EnterMine() = default;
    virtual void Enter(Miner *) {

    };
};

and Miner.cpp is blank

and the problem appears in main.cpp

#include "EnterMine.h"

int main()
{
    EnterMine a;
}

The error I get is a linking error:

LNK2001 unresolved external symbol "public: virtual void __thiscall State::Enter(class Miner *)" (?Enter@?$State@VMiner@@@@UAEXPAVMiner@@@Z)

Answer 1

(Note: this answer was written for the original question, it has been completely rewritten after that.)

Every function that is declared and used, should be defined somewhere.

It seems that you declare EnterMine::EnterMine() but never define it. If this constructor does nothing, either omit it (it will be implicitly defined by a compiler ), or mark it as = default; .

class EnterMine : public State<Miner>
{
public:
    EnterMine() = default;
    ...
};

This also applies to the State::State() constructor.

Answer 2

Even though it's a singleton, you're still calling the constructor. Thus, you will still need to define the constructor.

In fact, you need to define every function you declare in your header.