Overloading Operators and Templates in C++

[Alboin]

Hello,

I have a class that overloads the [] operator. The overloading function is:

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template <class C>
C &operator [](string key) {
	return static_cast<C>(data);
}

However, I have been unable to call said overloader with:

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A<int>["AX"] = 12;

(A being an instance of the class.)

Instead, I get:

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reg.cc:27: error: expected primary-expression before 'int'
reg.cc:27: error: expected `;' before 'int'

Anyone know how to call it? It does compile without the call, so I think it's legal C++...

[os64dev]

I have to ask what is the type of string?.

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template <class C>
C &operator [](string key) {
   return static_cast<C>(data);
} 

A<int>["AX"] = 12; 

In his example would be translated to 'A<int>& operator[](const char *)'.
I'll look into it a bit more and give an update.

update:

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#include <stdio.h>
typedef const char * string;

template <typename T>
class A {
  public:
    T & operator [](string key) {
       printf("using index: '%s'\n", key);
       return data;
    }
  private:
    T data;
};

int main(void) {
    A<int> x;
    
    x["AX"] = 12; 
    int w = x["ax"];
}

this works. but the templated version of the operator [] doesn't seem to work.

[Alboin]

I have to ask what is the type of string?.

It's a standard C++ string. (ie. using namespace std.)

[os64dev]

It's a standard C++ string. (ie. using namespace std.)

then this should work. not sure though.

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A<int> x;
x[std:string("AX")] = 12;

[Alboin]

I still get the expected primary-expression error. Here, this is the entire class:

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template <class T>
class Register {
	public:
		Register(char name) {
		}
	
		template <class C>
		C &operator [](string key) {
		}
};

Template C is a function template, but I don't know how to call [] with it...

[jnc100]

What happens if you try calling the operator function like:

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Register<T> a;
int b;
b = a.operator[]<int>("AX");

If that doesn't work it should give you a slightly more helpful error message.

Regards,
John.

[Alboin]

Hey, thanks, that works.

However, is there anyway to call it normally?

[jnc100]

According to http://www.thescripts.com/forum/thread461406.html there doesn't seem to be. There are some ways around it, however:

1) Define a member function

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template <class C> C& Item(string key)

and use that instead of operator overloading.

2) Define the class as

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template <class T, class C> {
...
C& operator[](string key)

although this limits you to using only one return type for operator[] for the entire lifetime of your class.

Regards,
John.

[Alboin]

According to http://www.thescripts.com/forum/thread461406.html there doesn't seem to be. There are some ways around it, however:

1) Define a member function

Code: Select all

template <class C> C& Item(string key)

and use that instead of operator overloading.

2) Define the class as

Code: Select all

template <class T, class C> {
...
C& operator[](string key)

although this limits you to using only one return type for operator[] for the entire lifetime of your class.

Regards,
John.

That's what I feared. 'Means I'm going top have to change my plans....

I wonder why it isn't allowed. Maybe they'll fix it in the future....

Thanks again.

[Candy]

That's what I feared. 'Means I'm going top have to change my plans....

I wonder why it isn't allowed. Maybe they'll fix it in the future....

Doubt it. Operator overloading is intended for cases in which you can easily see why you use the given operator because it's really intuitive. If you have to specify the type of return value somehow it's not intuitive anymore, so I can figure you can't call it using the inline syntax then.