Operator [] Overloading

[Mikey_H]

I have made a few wrapper classes for arrays to include dynamic resizing, string indexing etc. The classes worked fine when used on their own in an app. Now i have attempted to make a Hashtable class which uses an Array object of AssocArray objects. When I attempt to access data by using code like ' array[x][str] = y; ' I get the following error... error C2679: binary '[' : no operator found which takes a right-hand operand of type 'std::string' (or there is no acceptable conversion)

#include <string>
using std::string;

int x = 0;
int y = 1;

string = "Foo";

array = new Array<AssocArray<int>>();

// Error C2679
array[x][str] = y;

Function for overloaded operators is very similar for both Array, and AssocArray. I have removed all code inside function as it produced the same error when commented out.

template <class TType>
TType& AssocArray<TType>::operator [](string s)
{
return new TType;
}

It is my understanding that the line ' array[x][str] = y; ' would be computed like this...

AssocArray ax = array[x];
ax[str] = y;

Am I correct about this? I'm not sure what is going on here, and assume this is all the relevant code, If you would like to see any more just ask.

[CPallini]

Mikey_H wrote:

#include using std::string; int x = 0; int y = 1; string = "Foo"; array = new Array<AssocArray<int>>(); // Error C2679 array[x][str] = y;

does it stands for:

int x = 0;
int y = 1;

string str = "Foo";
Array<AssocArray<int>> * array;

array = new Array<AssocArray<int>>();
(*array)[x][str] = y;

?

Mikey_H wrote:

template TType& AssocArray<TType>::operator [](string s) { return new TType; }

does it stands for

TType& AssocArray<TType>::operator [](string s)
{
return *(new TType);
}

? Have you C# (or Java) roots? :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]

[Stuart Dootson]

Mikey_H wrote:

template TType& AssocArray::operator [](string s) { return new TType; }

That doesn't look quite right - typo somewhere?

Mikey_H wrote:

array = new Array<AssocArray<int>>(); // Error C2679 array[x][str] = y;

Similarly here - you can't use operators directly with pointers - but now I understand the error message. The type of array is Array<AssocArray<int>>*. So, the type of array[x] is Array<AssocArray<int>>& - and that has an operator[] that takes an integer index, not a string index. So, either declare array as Array<AssocArray<int>> array(); or use (*array)[x][str]

Java, Basic, who cares - it's all a bunch of tree-hugging hippy cr*p

[Mikey_H]

Stuart Dootson wrote:

you can't use operators directly with pointers

Ok, I did not think about that. So (*varPointer) will derefernce this? (Is this only for user defined types? A pointer to a standard C array does not have to be derferenced does it?) That first issue is solved with the corrections, another small one exists tho, probably a similar issue.

(*array)[x][str] = y;

will give the error.... error C2440: 'return' : cannot convert from 'int' to 'AssocArray<TType> &' Do I need to dereference the AssocArray somehow too? Thank you for your responses :)

modified on Tuesday, May 26, 2009 5:27 AM

[Mikey_H]

Few typos there, C# :laugh: I have made a reply to the other response.

[Cedric Moonen]

Is there a specific reason why you don't want to use the containers from the STL (std::map, std::vector, ...) ? All these classes already provide support for what you are doing, so why do you want to rewrite that ?

Cédric Moonen Software developer
Charting control [v2.0] OpenGL game tutorial in C++

[Stuart Dootson]

Mikey_H wrote:

(Is this only for user defined types? A pointer to a standard C array does not have to be derferenced does it?)

Quite right - pointers are special cases :-) Think about it - you're applying an operator defined on a class. The operator syntax a[b] is equivalent to a.operator[](b). So, with a pointer to the object, you can see that the method call syntax would be pa->operator[](b), or (*pa).operator[](b).

Mikey_H wrote:

That first issue is solved with the corrections, another small one exists tho, probably a similar issue. (*array)[x][str] = y; will give the error.... error C2440: 'return' : cannot convert from 'int' to 'AssocArray &' Do I need to dereference the AssocArray somehow too?

Here's some sample code I wrote that has the operator[] layout you want:

#include <string>
#include <iostream>

template<class Element>
class Array
{
public:
Element& operator[](int index) { return e; }
private:
Element e;
};

template<class Element>
class AssocArray
{
public:
Element& operator[](std::string const& s) { return e; }
private:
Element e;
};

int main()
{
Array< std::string > as;
AssocArray< std::string > aas;
Array< AssocArray< std::string > > aaas;

as[2];
aas["Hello"];

aaas[2]["Hello"] = "Test";
std::cout << aaas[2]["Hello"] << std::endl;
}

Java, Basic, who cares - it's all a bunch of tree-hugging hippy cr*p

[Mikey_H]

Ok, I got it working, I had it right by the last response, the next error that came up was somewhere else unrelated and I was too tired to notice. Thank you very much for your help.

[Mikey_H]

Just an attempt to learn a little more.