将表达式折叠为[]运算符的参数

Question

I am trying to use a fold expression as the argument of the [] operator. Unfortunately, only the first element is correct.

template <class ...U> T& operator[](U ...indices){
    size_t i=0, k[get_n_dimensions()];
    (... , void(k[i++]=indices));
    // use k[0], k[1], ...
    // only k[0] is correct
}

However, if I use the same syntax for the argument of a function, it works fine.

template <class ...U> T get(U ...indices) const {
    size_t i=0, k[get_n_dimensions()];
    (... , void(k[i++]=indices));
    // k[0], k[1], ... filled correctly
}

What is the reason? What would be the solution?

Answer 1

What is the reason?

The array subscript operator ( operator[] ) must have exactly one argument. The first snippet you have shown is invalid for any sizeof...(U) != 1 .

A function template like get or another operator like operator() do not have a similar limitation.

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What would be the solution?

Do not use operator[] .

Answer 2

C++ does not have multi-dimensional indexing, and that's "hard-wired" into the syntax - you can't add it by overloading.

The reason that your code compiles at all is that p[a,b] is equivalent to p[(a,b)] - that is, it uses the regular comma operator which will compute a and ignore the result, then produce the value of b as the index into p .

Thus, your template is never used "variadically", but is essentially the same as

template <class U> T& operator[](U indices)

You need to name the function, or overload an operator that can take more than one parameter, such as operator() .

Answer 3

As others have mentioned, the subscripting operator operator[] must accept only a single parameter.

See [over.sub] ( emphasis mine )

operator[] shall be a non-static member function with exactly one parameter . It implements the subscripting syntax

However, that one parameter doesn't have to be an integer.

You could still do what you want if you are able to move your values into a std::index_sequence like so:

template<size_t... indices>
T& operator[](std::index_sequence<indices...>){
    size_t i=0, k[get_n_dimensions()];
    (... , void(k[i++]=indices));
    // ...
}

You'd call it like so

my_instance[std::make_index_sequence<3>{}]; // passes 0, 1, 2 to the function

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