Trying to write string class that can do move semantics from an std::string

Question

I am writing my own string class for really just for learning and cementing some knowledge. I have everything working except I want to have a constructor that uses move semantics with an std::string.

Within my constructor I need to copy and null out the std::string data pointers and other things, it needs to be left in an empty but valid state, without deleting the data the string points to, how do I do this?

So far I have this

class String
{
private:
char* mpData;
unsigned int mLength;
public:
String( std::string&& str)
    :mpData(nullptr), mLength(0)
    {
    // need to copy the memory pointer from std::string to this->mpData

    // need to null out the std::string memory pointer
    //str.clear();  // can't use clear because it deletes the memory
    }
~String()
{
delete[] mpData;
mLength = 0;
}

Answer 1

There is no way to do this. The implementation of std::string is implementation-defined. Every implementation is different.

Further, there is no guarantee that the string will be contained in a dynamically allocated array. Some std::string implementations perform a small string optimization, where small strings are stored inside of the std::string object itself.