Prefer some function over ADL

Question

I want to use range-based for to iterate over the unicode code points in a UTF8 encoded std::string. I have defined my own begin and end in the global namespace but the begin and end in the std namespace are being preferred (i.e. those found by ADL). Is there any way to prefer my own functions?

Example:

const char* begin(const std::string& s) {
    std::cout << "BEGIN";
    return s.data();
}

const char* end(const std::string& s) {
    std::cout << "END";
    return s.data() + s.length();
}

int main() {
    std::string s = "asdf";

    for (char c : s)
        std::cout << c;
}

I want it to print BEGINENDasdf (or ENDBEGINasdf) but it prints asdf.

Is there no other way than to do a manual for using a qualified name?

Answer 1

Wrap std::string in your own type. By making it a template you can customise any existing container and add your own range logic to it. It's not even that different from your first attempt.

#include <string>
#include <iostream>

template <typename S>
struct custom_container {
    S &s_;

    custom_container (S &s) : s_(s) {}

    auto begin() -> decltype(s_.begin()) {
        std::cout << "BEGIN";
        return s_.begin();
    }

    auto end() -> decltype(s_.end()) {
        std::cout << "END";
        return s_.end();
    }
};

template <typename S>
custom_container make_container (S &s) {
     return custom_container <S> (s);
}


int main () {
    std::string t = "asdf";
    auto s = make_container(t);

    for (char c : s) {
        std::cout << c;
    }
}

Outputs

BEGINENDasdf