你在头中定义一些东西，然后它将在每个包含该头的翻译单元中定义 - 在你的情况下， AccountHandler 和 main 。您应该在标题中声明（如果需要从多个单元访问），并在一个源文件中定义。

假设它是一个静态类成员（我不得不猜测，因为你忘了给我们代码），你想要的东西：

  // header 
 class AccountHandler 
 {
 public：
 static size_t number_of_account; // declaration 

 //其他成员... 
}; 

 //源文件
 size_t AccountHandler :: number_of_account; //定义
 

可能在您的代码中，该定义在标题中。

这假设它应该是静态的;它独立于任何特定帐户（例如，其代表存在的帐户的数量），而不是与每个帐户相关联（例如它代表帐号）。如果它不应该是静态的，那么确保它没有声明 static 。

包括守卫不会帮助有了这个;它们防止标题在每个翻译单元中被包括多于一次，但仍然允许它们被包括在多个单元中。

vio@!@#$:~/cpp/OOP/6$ g++ -o main main.o NormalAccount.o HighCreditAccount.o Account.o AccountHandler.o
AccountHandler.o:(.bss+0x0): multiple definition of `AccountHandler::account_number'
main.o:(.bss+0x0): first defined here
collect2: ld returned 1 exit status

I got the error message above. But I couldn't find the code where it is multiply defined, so I changed all account_number to number_of_account in 'account.h' and 'AccountHandler.cpp' and

vio@!@#$:~/cpp/OOP/6$ vi AccountHandler.cpp 
vio@!@#$:~/cpp/OOP/6$ g++ -c AccountHandler.cpp 
vio@!@#$:~/cpp/OOP/6$ g++ -o main main.o NormalAccount.o HighCreditAccount.o Account.o AccountHandler.o
vio@!@#$:~/cpp/OOP/6$

it compiled well.

After that, I changed main.cpp a little

vio@!@#$:~/cpp/OOP/6$ g++ -c main.cpp
vio@!@#$:~/cpp/OOP/6$ g++ -o main main.o NormalAccount.o HighCreditAccount.o Account.o AccountHandler.o
AccountHandler.o:(.bss+0x0): multiple definition of `AccountHandler::number_of_account'
main.o:(.bss+0x0): first defined here
collect2: ld returned 1 exit status

and error message emerged again.

I used #ifndef #define #define in all header file and when I changed the variable in AccountHandler.cpp and accounthandler.h, it compiled well again, so I wonder why it happens

Here is the code:

#ifndef __ACCOUNTHANDLER_H__
#define __ACCOUNTHANDLER_H__

#include "account.h"

class AccountHandler
{
private:
    Account* account[100];
    static int number_of_account;
public:
    AccountHandler(){}

    void show_menu();
    void make_account();
    void deposit_money();
    void withdraw_money();
    void show_all_account_info();

    ~AccountHandler();
};

int AccountHandler::number_of_account=0;

#endif

If you define something in a header, then it will be defined in every translation unit that includes that header - in your case, both AccountHandler and main. You should declare it in a header (if it needs to be accessed from multiple units), and define it in just one source file.

Assuming that it's a static class member (I'm having to guess, since you forgot to show us the code), you want something like:

// header
class AccountHandler 
{
public:
    static size_t number_of_account; // declaration

    // other members...
};

// source file
size_t AccountHandler::number_of_account; // definition

Presumably, in your code, that definition is in the header.

That's assuming it's supposed to be static at all; that it's independent of any particular account (e.g. it represents the number of accounts that exist), rather than being associated with each account (e.g. it represents an account number). If it's not supposed to be static, then make sure it's not declared static.

Include guards won't help with this; they prevent the header from being included more than once in each translation unit, but still allow them to be included from multiple units.