VS 2010 c++ LNK2005使用#pragma一次和#ifndef时的错误

Question

1>Deck.obj : error LNK2005: "class Card card" (?card@@3VCard@@A) already defined in Card.obj
1>PokerTester.obj : error LNK2005: "class Card card" (?card@@3VCard@@A) already defined in Card.obj
1>PokerTester.obj : error LNK2005: "class Deck deck" (?deck@@3VDeck@@A) already defined in Deck.obj
1>C:DevPokerDebugPoker.exe : fatal error LNK1169: one or more multiply defined symbols found

我已经知道为什么这些错误发生的谷歌，但我不知道为什么他们仍然发生，当我尝试#pragma和#ifndef保护的东西。

这是我的名片。

#pragma once
#ifndef CARD_H
#define CARD_H
#include <iostream>
#include <string>
using namespace std;
class Card
{
public:
    Card(int cardSuit = 0, int cardValue = 2); //constructor will create a two of hearts by default
    ~Card(void);
    int getSuit(); //returns the suit of the Card
    int getValue(); //returns the value of the Card
    int getColor(); //returns the color of the Card
    friend ostream& operator<< (ostream &out, Card &cCard);
private:
    int suit; //card suit
    int value; //card value
    int color; //card color
} card;
#endif 

and my Deck.h

#pragma once
#ifndef DECK_H
#define DECK_H
#include "Card.h"
#include <vector>
using namespace std;
class Deck
{
public:
     Deck(void);
    ~Deck(void);
    void newDeck(); //regenerates the full 52 card deck (e.g. cards are missing)
    void shuffle(); //shuffles the deck
    int cardsInDeck(); //returns the number of cards remaining in the deck
    Card takeTopCard(); //returns the top card and removes it from the deck
private:
    vector<Card> myDeck; //vector of 52 Card objects that make up the deck
} deck;
#endif

这可能很明显，但我就是想不明白…

，这里是Card.cpp:

#include "Card.h"
Card::Card(int cardSuit, int cardValue)
{
card.suit = cardSuit;
card.value = cardValue;
if(cardSuit == 0 || cardSuit == 1) card.color = 0;
if(cardSuit == 2 || cardSuit == 3) card.color = 1;
}
//returns the card's color
int Card::getColor() 
{
return card.color;
}
//returns the card's suit
int Card::getSuit()
{
return card.suit;
}
//returns the card's value
int Card::getValue()
{
return card.value;
}

，下面是我用来测试它们的代码:

#include "Deck.h"
int main() 
{
Deck testDeck = *new Deck();
Card testCardCreation = *new Card();
Card testCard = testDeck.takeTopCard();
testDeck.shuffle();
Card testShuf = testDeck.takeTopCard();
cout << testCard << endl << testShuf << endl;
return 0;
}

Answer 1

对象card和deck在头中定义。当您将头文件包含到翻译单元中时，它将创建该对象的另一个定义。您可能应该从类定义中删除card和deck。如果确实需要定义这些对象，可以使用

extern Card card;

Answer 2

它们是链接器错误，它们与c++代码本身无关。

问题出在Card和Deck定义的末尾。为什么有:

} card;

:

} deck;

?

Answer 3

1)如果多次定义"class Card"，您将得到编译错误，而不是链接错误。

2)在大多数情况下，您的头文件看起来很好。事实上，您并不需要两者都需要:#pragma once或#ifndef中的一个或另一个应该足够了。

3)问题:从你的类定义中去掉最后的"card"one_answers"deck"!