[轻笔记]C++实现类自注册时出现“multiple definition”（重定义）错误

一、背景

• 1. factory.h

#pragma once
#include <map>
#include <iostream>
#include <string>
#include <functional>
#include <memory>

class Message
{
    
    
public:
    virtual ~Message() {
    
    }

    virtual void foo()
    {
    
    

    }
};

struct factory
{
    
    
    template<typename T>
    struct register_t
    {
    
    
        register_t(const std::string& key)
        {
    
    
            factory::get().map_.emplace(key, [] {
    
     return new T(); });
        }

        template<typename... Args>
        register_t(const std::string& key, Args... args)
        {
    
    
            factory::get().map_.emplace(key, [&] {
    
     return new T(args...); });
        }
    };

    static Message* produce(const std::string& key)
    {
    
    
        if (map_.find(key) == map_.end())
            throw std::invalid_argument("the message key is not exist!");

        return map_[key]();
    }

    static std::unique_ptr<Message> produce_unique(const std::string& key)
    {
    
    
        return std::unique_ptr<Message>(produce(key));
    }

    static std::shared_ptr<Message> produce_shared(const std::string& key)
    {
    
    
        return std::shared_ptr<Message>(produce(key));
    }

private:
    factory() {
    
    };
    factory(const factory&) = delete;
    factory(factory&&) = delete;

    static factory& get()
    {
    
    
        static factory instance;
        return instance;
    }

    static std::map<std::string, std::function<Message*()>> map_;
};

std::map<std::string, std::function<Message*()>> factory::map_;

#define REGISTER_MESSAGE_VNAME(T) reg_msg_##T##_
#define REGISTER_MESSAGE(T, key, ...) static factory::register_t<T> REGISTER_MESSAGE_VNAME(T)(key, ##__VA_ARGS__);

• 2.message.h

#pragma once
#include "factory.h"



class Message1 : public Message
{
    
    
public:

    Message1()
    {
    
    
        std::cout << "message1" << std::endl;
    }

    Message1(int a)
    {
    
    
        std::cout << "message1" << std::endl;
    }

    ~Message1()
    {
    
    
    }

    void foo() override
    {
    
    
        std::cout << "message1" << std::endl;
    }
};

//REGISTER_MESSAGE(Message1, "message1", 2);
REGISTER_MESSAGE(Message1, "message1");

• 3. main.cc

#include "message.h"

int main()
{
    
    

    Message* p = factory::produce("message1");
    p->foo();   //Message1
    
    auto p2 = factory::produce_unique("message1");
    p2->foo();  

  return 0;
}

运行没有问题。

二、问题

当我把message.h分成两个文件：message.h与message.cc时：

• 2.1 message.h

class Message
{
    
    
public:
    virtual ~Message() {
    
    }

    virtual void foo()
    {
    
    

    }
};

#include "factory.h"

class Message1 : public Message
{
    
    
public:

    Message1()
    {
    
    
        std::cout << "message1" << std::endl;
    }

    Message1(int a)
    {
    
    
        std::cout << "message1" << std::endl;
    }

    ~Message1()
    {
    
    
    }

    void foo() override;
};

//REGISTER_MESSAGE(Message1, "message1", 2);
REGISTER_MESSAGE(Message1, "message1");

• 2.2 message.cc

#include "message.h"
void Message1::foo() {
    
    
	std::cout << "message1" << std::endl;
}

编译时报错：

三、原因

由于factory.h中有一个变量定义：

std::map<std::string, std::function<Message*()>> factory::map_;

当程序编译时，message.h与message.cc都会调用一次factory.h，这样会导致多次编译上面这句代码，造成重复定义错误。

所以，当头文件中存在一个定义语句时要特别注意。

四、解决方法

新建factory.cc，把变量定义的代码从h文件中删除，放到cc文件下：

#include "factory.h"
std::map<std::string, std::function<Message*()>> factory::map_;

致谢

谢谢建哥！