C++ compilation error, ambigous operator overloads

Question

I have a template array class overloading both operator [] to access items and operator T * to get direct buffer access.

template< typename T > class buffer
{
    const T & operator [] ( size_t ) const;
          T & operator [] ( size_t );

    operator const T * () const;
    operator       T * ();
};

For some reason, using operator [] on an instance of this class generates a compilation error saying that there is 4 possible overloads.

buffer< int > buf;
buf[ some_position ] = 0; // Generates an error

Error :

Error   3   error C2666: 'XXX::buffer<T>::operator []' : 4 overloads have similar conversions   c:\XXX.cpp  3886

Is the operator [] trying to convert my instance buf to a T * ? Why are 4 overloads detected instead of 2 ? Thank you. :)

EDIT : Actually it is : buf[ some_position ] = 0; // Generates an error

Please post the _full_ error message, including all the overloads it lists and any other related notes generated by compiler. — Petr, Jun 19 '15 at 14:14
This is the full error message. I'm using Visual Studio 2005. — Virus721, Jun 19 '15 at 14:15
It explains [here](https://ideone.com/bel9Rm) why you are getting your errors. Look in the compilation info section at the bottom. — NathanOliver, Jun 19 '15 at 14:17
@Virus721, hm, ok, that old VS might not give much additional info... — Petr, Jun 19 '15 at 14:24
What is the type of `some_position`? A literal `int`? An `int` variable? A `std::size_t` variable? — TartanLlama, Jun 19 '15 at 14:27

NathanOliver · Answer 1 · 2015-06-19T14:36:53.803

1

The Problem you are having is that operator T* and operator[] both will allow buf[some_position] to be valid. In your code T* is an int* and int* has an [] overload.

You can see that with this live example

I think this might be an issue with you VS as running the code on Coliru and ideone with some modifications to get it to run both give working code.

edited Jun 19 '15 at 14:36

answered Jun 19 '15 at 14:21

NathanOliver

171,901
28
288
402

Btw, g++ lists only 2 candidates, and OP's compiler (VS2005) says four. Is it just that OP has outdated Visual Studio? – Petr Jun 19 '15 at 14:23
Yeah I am going to code with a VS problem. [Coliru](http://coliru.stacked-crooked.com/a/c938150606e84349) doesn't mind it. – NathanOliver Jun 19 '15 at 14:28
That's only part of the issue, as there needs to be ambiguous implicit conversions for the call to be ambiguous. – TartanLlama Jun 19 '15 at 14:33
Thanks for your answer. I've updated my question, i forgot the assignment, so it wasn't about a const / not-const, but what TartanLlama said. – Virus721 Jun 19 '15 at 14:38

score 1 · Accepted Answer · answered Jun 19 '15 at 14:30

1

The issue is that you have implicit conversions going in both directions.

Disregarding const, your two candidates are:

T & buffer::operator [] ( size_t );
int& operator[] (int*, int);

For the call buf[pos] where pos is an int (such as a literal), the call is ambiguous. The first candidate could be selected by converting pos to a std::size_t and the second could be selected by converting buf to int*.

You could disambiguate this call by explicitly casting the argument to a std::size_t, or modifying the operator to take an int instead.

answered Jun 19 '15 at 14:30

TartanLlama

63,752
13
157
193

Thanks, i didn't know about `int& operator[] (int*, int);` I guess it is a non-member operator casting the object using its `operator T *` ? – Virus721 Jun 19 '15 at 14:34
It's just the built-in operation for indexing an integer pointer as an array. The compiler checks that because of your implicit conversion operator to `T*`. – TartanLlama Jun 19 '15 at 14:36
I assume that i should avoid coupling these two overloads and use a named method to access the buffer instead, shouldn't it ? Like std::vector – Virus721 Jun 19 '15 at 14:39
That's certainly a good option if it fits with your use-cases. – TartanLlama Jun 19 '15 at 14:40

C++ compilation error, ambigous operator overloads

2 Answers2