Question

I am working with 2 libraries. One takes in and returns std::strings while the other uses std::vector<unsigned char>s.

It would be good if I could steal the underlying arrays from std::string and std::vector<unsigned char> and be able to move them into each other without the excessive copying.

ATM I use something like:

const unsigned char* raw_memory =
    reinterpret_cast<const unsigned char*>(string_value.c_str()),
std::vector<unsigned char>(raw_memory, raw_memory + string_value.size();

And the other way:

std::string(
    reinterpret_cast<const char*>(&vector_value[0]),
    vector_value.size());

It’d be far better to be able to define a:

std::string move_into(std::vector<unsigned char>&&);
std::vector<unsigned char> move_into(std::string&&);

Answer 1

This is not possible.

The vector and string class do not provide way to steal from anything else than vector or string respectively. They are not meant to exchange content.

The problem issue is that vector and string may have widely different underlying representations. Typically in gcc for example, string use the oldish COW (Copy On Write) “optimization”, which is widely different from the typical vector representation (usually just a triple of pointers/size_t attributes).

If you are dealing with raw bytes, blame the library that decided to put them into string, and refactor it if you can.

Otherwise: copy. The reinterpret_cast should not be necessary because char and unsigned char have implicit casts between them (and now char is often unsigned by default).