C++ and preprocessor macro gotcha

Question

Can you figure out what is wrong with the statement below?

GCC error states: "'type name' declared as function returning array".

#define MACRO(a) (a)[1]

class index {
public:
    typedef int index_type[2];
    const index_type& operator[](int i) const;
};

int k = 0;
int i = MACRO(index()[k]);

btw: I know what is wrong, I thought it was amusing thing to share. many thanks to litb, his explanation of previous gotcha helped to solve this error pretty quickly

Answer 1

When you apply your macro, it expands to:

class index 
{
    // ...
    typedef int index_type[2];
    const index_type& operator[](int i)const;
    // ...
};

int k = 0;
int i = (index()[k])[1];

Now the problem (assuming that index::operator[] is public, and it is non-obvious from your code snippet that it is) is that the result of index::operator[] is returned by reference, and you are constructing the index() object as a temporary, and so, assuming your index::operator[] is implemented how I'm guessing you've implemented it (returning a reference to a member object), the result of index::operator[] will be invalid immediately after it is returned (as the temporary is destructed), and so you have undefined behavior.

Answer 2

My guess would be there's an ambiguity in your syntax. The compiler might be looking at the expanded macro:

 int i = (index()[k])[1];

And thinking that index is actually a non-member function declaration that returns an array, rather than construction of a temporary object of type index.

But that's just a guess... if you know the answer already, please enlighten us :)

Answer 3

In the expanded line:

int i = (index()[k])[1];

(index()[k]) is interpreted as a cast expression, declaring a function that returns an array of length k of index. At least, that's what it looks like is happening. How gcc is manages to validly interpret the [1] as an expression, I'm not sure.