Incomprehensible function signature - Return reference to an array of N objects

Question

I\'ve come across the following signature

double(&rotate_vec(double(&val)[4]))[4];

In the comments it \"claims\" to accept and

Answer 1

double ( &rotate_vec( double (&val)[4] ) )[4]

A function named rotate_vec

double ( &rotate_vec( double (&val)[4] ) )[4]

...that takes as an argument, a reference to an array of four doubles

double ( &rotate_vec( double (&val)[4] ) )[4]

...and returns a reference to an array of four doubles.

Answer 2

With C++03 the best you can do to simplify the original

double(&rotate_vec(double(&val)[4]))[4];

is to use a typedef, to wit:

typedef double Four_vec[4];
Four_vec& rotate_vec( Four_vec& val );

In C++11 you can write

auto rotate_vec( double (&val)[4] )
    -> double (&)[4];

although I'd use a typedef or C++11 using to clarify.

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Regarding

“We can't return an array from a function, just pointers, or can we ?”

you can't return a raw array by value, but you can return a pointer or reference, or you can wrap it in a struct, like C++11 std::array.