Custom swap function NOT called in stl sorting algorithm

Question

I have created the following class (in a header file):

#ifndef HASPTR_H
#define HASPTR_H
#include <iostream>
#include <string>

class HasPtr
{
public:
    HasPtr(const std::string &);
    HasPtr(const HasPtr &);
    HasPtr & operator=(const HasPtr &);
    friend bool operator<(const HasPtr &, const HasPtr &);
    friend void swap(HasPtr &, HasPtr &);
    ~HasPtr();
private:
    int m_x;
    std::string *m_strPtr;
};

inline bool operator<(const HasPtr &lhs, const HasPtr &rhs)
{
    return (*lhs.m_strPtr < *rhs.m_strPtr);
}

inline void swap(HasPtr &lhs, HasPtr &rhs)
{
    using std::swap;
    swap(lhs.m_x, rhs.m_x);
    swap(lhs.m_strPtr, rhs.m_strPtr);
    std::cout << "Swap executed\n";
}
#endif // !HASPTR_H

Although I have provided a custom swap function for HasPtr objects, std::swap is called by the stl sorting algorithm instead of the above swap function, after running the following code:

#include "stdafx.h"
#include <algorithm>
#include "HasPtr.h"
#include <vector>

int main()
{
    HasPtr hp1("zzzzzzzzzzzzzz"), hp2("aaaaaaa"), hp3("cccccccccc");
    std::vector<HasPtr> hpVec{ hp1, hp3, hp2 };
    std::sort(hpVec.begin(), hpVec.end());
    return 0;
}

What is wrong? Thanks in advance