下标运算符重载和对指针错误的引用

Question

I'm struggling with a strange error for a while now and I need your help :D

I have an 'Image' class declared like

template <typename P>
class Image {
// ...
public: // Operators 
    Color<P>*& operator [] ( unsigned int const& i );
// ...
}

Since it's the first time I overload this operator, I'm not really sure of it (especially the *& part...)

The troube appears when I tried something like :

/// Sample Code
Image<P> img; img[0][0] = Color<double>(1.0, 1.0, 1.0);

I get

./src/Graphics/PNGWriter.cc:12:3: error: no match for ‘operator[]’ in ‘img[0]’
./src/Graphics/Image.cc:24:12: note: candidate is: Color<P>*& Image<P>::operator[](const unsigned int&) [with P = double] <near match>

Obviously the obscucate a bit more the solution to me...

Any idea ?

Thanks !

Answer 1

All "no match for '...'" errors mean the arguments in the call aren't compatible to the declared arguments for any functions or operators, so you should look to the argument declarations. Make argument to operator[] an unsigned int. Similarly, passing the returned pointer by reference is quite odd; not necessarily bad, but highly suspect. In general, non-compound types should be passed by value.

Answer 2

First of all, there is no need to pass so many things by reference. Taking the unsigned int by value would almost certainly not slow things down; same about the reference to pointer.

This could be the problem, but I doubt it; I don't see why 0 wouldn't convert to a unsigned int const& . Another possibility is that the image is actually const , but you failed to mention that in your paste. Subscript operators generally come in pairs; you need to declare another one as Color<P> const* operator [] ( unsigned int i ) const; .

Note that img[0] is what is being complained about, so the = Color<double>(1.0, 1.0, 1.0); is probably not an issue (unless there's some SFINAE error I'm missing). Finally, make sure you haven't specialised Image for double .

EDIT: By the way, returning Color<P>*& has a severe disadvantage; someone could do img[0] = 0 , which was probably not the way the class was meant to be used.