C++ 运算符<<的多重定义
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c++ multiple definitions of operator<<
提问by ewok
I am attempting to override the <<operator for a class. The purpose is basically to implement a toString()like behavior for my class, so that sending it to coutwill produce useful output. Using a dummy example, I have the code below. When I attempt to compile, I get the foollowing error:
我试图覆盖<<一个类的运算符。目的基本上是toString()为我的类实现类似的行为，以便将其发送到cout将产生有用的输出。使用一个虚拟示例，我有下面的代码。当我尝试编译时，我得到了一个愚蠢的错误：
$ g++ main.cpp Rectangle.cpp
/tmp/ccWs2n6V.o: In function `operator<<(std::basic_ostream<char, std::char_traits<char> >&, CRectangle const&)':
Rectangle.cpp:(.text+0x0): multiple definition of `operator<<(std::basic_ostream<char, std::char_traits<char> >&, CRectangle const&)'
/tmp/ccLU2LLE.o:main.cpp:(.text+0x0): first defined here
I can't figure out why this is happening. my code is below:
我不明白为什么会这样。我的代码如下：
Rectangle.h:
矩形.h：
#include <iostream>
using namespace std;
class CRectangle {
private:
int x, y;
friend ostream& operator<<(ostream& out, const CRectangle& r);
public:
void set_values (int,int);
int area ();
};
ostream& operator<<(ostream& out, const CRectangle& r){
return out << "Rectangle: " << r.x << ", " << r.y;
}
Rectangle.cpp:
矩形.cpp：
#include "Rectangle.h"
using namespace std;
int CRectangle::area (){
return x*y;
}
void CRectangle::set_values (int a, int b) {
x = a;
y = b;
}
main.cpp:
主.cpp：
#include <iostream>
#include "Rectangle.h"
using namespace std;
int main () {
CRectangle rect;
rect.set_values (3,4);
cout << "area: " << rect.area();
return 0;
}
回答by Luchian Grigore
You're breaking the one definition rule. A quick-fix is:
你打破了一个定义规则。一个快速修复是：
inline ostream& operator<<(ostream& out, const CRectangle& r){
return out << "Rectangle: " << r.x << ", " << r.y;
}
Others are:
其他有：
- declaring the operator in the header file and moving the implementation to
Rectangle.cppfile. - define the operator inside the class definition.
- 在头文件中声明操作符并将实现移动到
Rectangle.cpp文件中。 - 在类定义中定义运算符。
.
.
class CRectangle {
private:
int x, y;
public:
void set_values (int,int);
int area ();
friend ostream& operator<<(ostream& out, const CRectangle& r){
return out << "Rectangle: " << r.x << ", " << r.y;
}
};
Bonus:
奖金：
- use include guards
- remove the
using namespace std;from the header.
- 使用包括警卫
using namespace std;从标题中删除。
回答by Matteo Italia
You are putting the definitionof a function in a .hfile, which means that it will appear in every translation unit, violating the One Definition Rule (=> you defined operator<<in every object module, so the linker doesn't know which is "the right one").
您将函数的定义放在.h文件中，这意味着它会出现在每个翻译单元中，这违反了一个定义规则（=> 您operator<<在每个目标模块中定义，因此链接器不知道哪个是“正确的”一”）。
You can either:
您可以：
- write just the declarationof your operator (i.e. its prototype) in the .h file and move its definition to
rectangle.cpp - make
operator<<inline-inlinefunctions are allowed to be defined more than once, as long as all the definitions are identical.
- 在 .h 文件中只写你的操作符（即它的原型）的声明，并将其定义移动到
rectangle.cpp - make
operator<<inline-inline允许多次定义函数，只要所有定义都相同。
(Also, you should use header guards in your includes.)
（此外，您应该在包含中使用标题保护。）
