c++ 使用模板实现接口的编译错误

Question

I am trying to implement interface with template for some class. I have code like this:

file.h

#pragma once;

class MySpecificClass {
    std::string data;
    unsigned int x;
    unsigned int y;
    unsigned int z;

public:
    MySpecificClass(): data(""), x(0), y(0), z(0) {
    }
    MySpecificClass(std::string s, unsigned int xx, unsigned int yy, unsigned zz) : data(s), x(xx), y(yy), z(zz) {
    }
};

template <class T>
class IFileClass {
public:
    IFileClass(std::string f) : fileName(f) {
    }
    virtual void save(T c);
protected:
    std::string fileName;
};

template <class T>
class FileWithClass : public IFileClass<T> {
public:
    FileWithClass(std::string fn) : IFileClass<T>(fn) {
    }
    void save(T c) override {
        std::cout << "FileWithClass save" << std::endl;
    }
};

When I trying use it in main

main.cpp

#include "file.h"
int main() {
    // create object to save
    MySpecificClass msc = {"My Test", 100, 200, 300};
    FileWithClass<MySpecificClass> fsv = {"test.txt"};
    fsv.save(msc);
}

I get a compile error like this:

undefined reference to `IFileClass<MySpecificClass>::save(MySpecificClass)'

What is wrong?

Answer 1

The template is not the problem here. Virtual function without a definition is.

The C++ Standard specifies that all virtual methods of a class that are not pure-virtual must be defined. In your base class you declare a virtual function save. If it were not virtual then it would be OK to leave it without a definition (if it is not used - the linker wouldn't complain).

But since it is virtual there are to options - give it an empty definition with {} or make it pure virtual with =0. Both variants are legal depending on what you want.