Question

I sort of assumed that range based for loops would support C-style strings

void print_C_str(const char* str)
{
    for(char c : str)
    {
        cout << c;
    }
}

However this is not the case, the standard [stmt.ranged] (6.5.4) says that range-based-for works in one of 3 possibilities:

1. The range is an array
2. The range is a class with a callable begin and end method
3. There is ADL reachable in an associated namespace (plus the std namespace)

When I add begin and end functions for const char* in the global namespace I still get errors (from both VS12 and GCC 4.7).

Is there a way to get range-based-for loops to work with C style strings?

I tried adding an overload to namespace std and this worked but to my understanding it’s illegal to add overloads to namespace std (is this correct?)

Answer 1

If you write a trivial iterator for null-terminated strings, you can do this by calling a function on the pointer that returns a special range, instead of treating the pointer itself as the range.

template <typename Char>
struct null_terminated_range_iterator {
public:
    // make an end iterator
    null_terminated_range_iterator() : ptr(nullptr) {}
    // make a non-end iterator (well, unless you pass nullptr ;)
    null_terminated_range_iterator(Char* ptr) : ptr(ptr) {}

    // blah blah trivial iterator stuff that delegates to the ptr

    bool operator==(null_terminated_range_iterator const& that) const {
        // iterators are equal if they point to the same location
        return ptr == that.ptr
            // or if they are both end iterators
            || is_end() && that.is_end();
    }

private:
    bool is_end() {
        // end iterators can be created by the default ctor
        return !ptr
            // or by advancing until a null character
            || !*ptr;
    }

    Char* ptr;
}

template <typename Char>
using null_terminated_range = boost::iterator_range<null_terminated_range_iterator<Char>>;
// ... or any other class that aggregates two iterators
// to provide them as begin() and end()

// turn a pointer into a null-terminated range
template <typename Char>
null_terminated_range<Char> null_terminated_string(Char* str) {
    return null_terminated_range<Char>(str, {});
}

And usage looks like this:

for(char c : null_terminated_string(str))
{
    cout << c;
}

I don’t think this loses any expressiveness. Actually, I think this one is clearer.