Can std::begin work with array parameters and if so, how?

Question

I have trouble using std::begin() and std::end() (from the iterator library) with c-style array parameters.

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Answer 1

Yes, std::begin and std::end can work with parameters that are C style arrays.

The trick is in passing a parameter that's a C style array. When you specify a 1D array as a normal parameter to a normal function, its type is silently adjusted from "array of T" to "pointer to T". When you call that function, what gets passed isn't the array (as an array), but a pointer to the first element of the array.

It is, however, possible to pass an array by reference to a function template:

template 
void function(T (&array)[N]) {
   // function body here
}

In this case, where you're passing an actual array (albeit, by reference) rather than a pointer, you can use std::begin and std::end perfectly well. For example:

template 
T sum(T (&array)[N]) { 
    return std::accumulate(std::begin(array), std::end(array), T());
}

Now passing an array is trivial, such as:

int array[] = {1, 2, 3, 4};

auto total = sum(array);

std::begin and std::end themselves are implemented similarly to sum--the array is passed by reference, so they can look something like this:

template 
T *begin(T (&array)[N]) { 
    return array; 
}

template 
T *end(T (&array)[N]) {
    return array + N;
}

Note that although these were added to the standard more recently, they don't require any particularly tricky use of templates, so the implementation above should work fine with a plain old C++98 compiler (and, if memory serves, even with pre-standard compilers such as VC++ 6).