“错误：未解析的外部符号”每当我使用纯虚函数 [英] "Error: Unresolved external symbol" whenever I use a pure virtual function

问题描述

我觉得我做了一些令人难以置信的蠢事，但我根本无法弄清楚我的代码有什么问题。我甚至做了一个超级简化版本的代码，错误仍然发生：

I feel like I'm doing something incredibly stupid, but I simply can't figure out what's wrong with my code. I even made a super simplified version of the code and the error still occurs:

#include <iostream>

using namespace std;

class c1{
public:
    c1(){}
    ~c1(){}

    virtual int add(int a, int b);

private:
protected:


};

class c2 : c1{
public:
    c2(){}
    ~c2(){}

    int add(int a, int b){
        return a+b;
    }

};

int main(){

    c2 c;
    c.add(5,6);

}

任何人都可以发现我肯定是最愚蠢的错误的所有时间？

Can anyone spot what I'm sure is the silliest error of all time?

以下是确切的错误讯息：

Here's the exact error message:

1>main.obj : error LNK2001: unresolved external symbol "public: virtual int __thiscall c1::add(int,int)" (?add@c1@@UAEHHH@Z)

推荐答案

virtual int add(int a, int b);

这不是纯虚函数的声明。它只是一个虚函数的声明。它缺少一个定义，这就是为什么你得到的错误。

This is not a declaration of a pure virtual function. It's just a declaration of a virtual function. It lacks a definition, which is why you get the error.

virtual int add(int a, int b) = 0;

这是一个纯虚函数的声明。它不需要定义，这就是为什么不会得到错误。

This is a declaration of a pure virtual function. It does not require a definition, which is why won't get the error.

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