C++ 重载小于运算符
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Overloading less than operator
提问by user997112
I am overloading a less than operator for a class like so:
我正在为一个类重载一个小于运算符，如下所示：
#include<string>
using namespace std;
class X{
public:
X(long a, string b, int c);
friend bool operator< (X& a, X& b);
private:
long a;
string b;
int c;
};
and then the implementation file:
然后是实现文件：
#include "X.h"
bool operator < (X const& lhs, X const& rhs)
{
return lhs.a< rhs.a;
}
However it is not letting me access the adata member in the implementation file because ais declared as a private data member, even though its through an Xobject?
但是它不允许我访问a实现文件中的数据成员，因为它a被声明为私有数据成员，即使它是通过一个X对象？
回答by Pierre Fourgeaud
The friend function does not have the same signature as the function defined function:
友元函数与函数定义函数的签名不同：
friend bool operator< (X& a, X& b);
and
和
bool operator < (X const& lhs, X const& rhs)
// ^^^^^ ^^^^^
You should just change the line in your header file to:
您应该将头文件中的行更改为：
friend bool operator< ( X const& a, X const& b);
// ^^^^^ ^^^^^
As you don't modify the objects inside the comparison operators, they should take const-references.
由于您不修改比较运算符内的对象，因此它们应该采用常量引用。
回答by juanchopanza
You have declared a different friend function to the one you are trying to use. You need
您已经声明了一个与您尝试使用的不同的朋友函数。你需要
friend bool operator< (const X& a, const X& b);
// ^^^^^ ^^^^^
In any case, it would make no sense for a comparison operator to take non-const references.
在任何情况下，比较运算符采用非常量引用都是没有意义的。
