move semantics

#include <cstdio>
#include <cstdlib>
#include <cstring>

struct X {
private:
    size_t m_len;
    char  *m_data;
public:
    X() : m_len(0),m_data(nullptr) {
        printf("default constructor\n");
    }

    X(size_t len) : m_len(len),m_data(new char[len]) {
        printf("init constructor\n");
        memset(m_data,0,m_len);
    }

    // copy constructor must make a copy of the object
    // because it has a reference to something
    // that belongs to the caller
    X(const X &r)
    {
        printf("copy constructor\n");
        m_len = r.m_len;
        m_data = new char[m_len];
        memcpy(m_data,r.m_data,m_len);
    }

    // move constructor doesn't need to copy 
    // because it is given a reference to 
    // a temporary object that would be discarded anyway
    X(X &&r)
    {
        printf("move constructor\n");
        m_len     = r.m_len;
        m_data    = r.m_data;
        r.m_data  = nullptr;
        r.m_len   = 0;
    }

    void print()
    {
        printf("%llu\n",m_len);
    }
};

// a function that returns a temporary value
X makeX(size_t len) 
{
    X x(len);
    return x;
}

int    main(int argc,char *argv[])
{
    X a(makeX(10)); // move constructor, because temporary object can be discarded
    X b(a);         // copy constructor, because lvalue is persistent
    X c;

    a.print();
    b.print();
    c.print();

    return 0;
}