为后缀重载一元运算符

Question

Say I have a class as such

class foo
{
public:
    char* operator+ ()           //unary operator - prefix before instance
    {
        return "SomeChar";
    }
};

Now I can use it as such

foo d;
std::cout << +d; //unary operator used as prefix allowed 

Now if I wanted to use the unary operator as postfix then according to this page

As you can see in this case we use int as a dummy argument for post-fix, when we redefine the functions for the unary increment (++) and decrement (--) overloaded operators. You must remember that int isn't an integer, but just a dummy argument. You can see it as a signal to the compiler to create the post-fix notation of the operator.

I am using + instead of ++ since both are unary operators

I could do the following

class foo
{
public:
    char* operator+ ()           //unary operator - prefix before instance
    {
        return "SomeChar";
    }

    //Added this for postfix unary operation
    char* operator+ (int)       //unary operator - postfix before instance
    {
       return "SomeChar";
    }
};

However this (postfix) does not work

foo d;
std::cout << (d+) ; //unary operator used as post fix ERROR (Expected an expression)

Any suggestions/comments on this issue ?

Answer 1

+ cannot be a postfix operator. Only ++ and -- can.

Answer 2

C++ does not allow you to add new operators. You can only overload existing ones. There is no postfix + operator in the language, so you cannot add it. You can overload prefix ++ and postfix ++ , because they both exist.

Answer 3

There's no such thing as a postfix + operator, this user-defined conversion:

char* operator+ (int)       //unary operator - postfix before instance
    {
       return "SomeChar";
    }

has an ' int ' parameter not by chance, try to write:

foo d;
std::cout << (d+3) ; // No longer an "expression required" error

A recommended reading is §5.3.1/Unary operators