How to overload operator==() for a pointer to the

问题:

I have a class called AString. It is pretty basic:

class AString
{
public:
    AString(const char *pSetString = NULL);
    ~AString();
    bool operator==(const AString &pSetString);
    ...

protected:
    char *pData;
    int   iDataSize;
}

Now I want to write code like this:

AString *myString = new AString("foo");
if (myString == "bar") {
    /* and so on... */
}

However, the existing comparison operator only supports

if (*myString == "bar")

If I omit that asterisk, the compiler is unhappy.

Is there a way to allow the comparison operator to compare *AString with const char*?

回答1:

No, there is not.

To overload operator==, you must provide a user-defined type as one of the operands and a pointer (either AString* or const char*) does not qualify.
And when comparing two pointers, the compiler has a very adequate built-in operator==, so it will not consider converting one of the arguments to a class type.

回答2:

Not unless you wrap it in some sort of smart-pointer class, but that would make the semantics weird. What's wrong with if (*myString == "bar")?

回答3:

 if (myString == "bar")

even if you get it to work, is very confusing for others. You are comparing a pointer to an object with a string literal. A much clearer way to get this working is dereference the pointer, and provide an overload like

bool operator==(const char* pSetString);

回答4:

I think what you want is wrong since it obscures the type system of C++. myString is a pointer to a AString and not a AString. Dont't try to hide the fact that it's a pointer. It's an entry point for ugly bugs and if you're coding in a team everyone else would be nothing but confused!

回答5:

[ Original answer was wrong and thus corrected below ]

As pointed out by Oli Charlesworth, in a comment below, this is impossible.

You would need to define an operator like

   bool operator==(const AString *as, const char *cs); // Note: C++ will not do that

but you cannot overload an operator unless one of its parameters is non-primitive type - and pointers (both pointers to AString and pointers to char) are primitive types.