C++访问任意类的私有成员(来自转载)

#include <iostream>
#include <type_traits>

using namespace std;


/** Yet another concat implementation. */
#define YA_CAT_IMPL(x, y) x##y
/** Yet another concat. */
#define YA_CAT(x, y) YA_CAT_IMPL(x, y)
/**
* Init private class member hijacker.
* @param class_ Class name.
* @param member Private member to hijack.
* @param __VA_ARGS__ Member type.
*/
#define HIJACKER(class_, member, ...) \
    namespace dirty_hacks { namespace hijack { \
    template <class> struct tag_##member; \
    inline auto get(tag_##member<class_>) -> __VA_ARGS__ class_::*; \
    template <> struct tag_##member<class_> { \
        tag_##member() {} \
        template <class T, class = std::enable_if_t<std::is_base_of<class_, T>::value>> \
        tag_##member(tag_##member<T>) {} \
    }; \
    template <__VA_ARGS__ class_::* Ptr> struct YA_CAT(hijack_##member##_, __LINE__) { \
        friend auto get(tag_##member<class_>) -> __VA_ARGS__ class_::* { return Ptr; } \
    }; \
    template struct YA_CAT(hijack_##member##_, __LINE__)<&class_::member>; \
    }}
/**
* Hijack private class member.
* @param ptr Pointer to class instance.
* @param member Private member to hijack.
*/
#define HIJACK(ptr, member) \
    ((ptr)->*dirty_hacks::hijack::get( \
        dirty_hacks::hijack::tag_##member<std::remove_cv_t<std::remove_pointer_t<decltype(ptr)>>>() \
    ))


class TestClass
{
public:
    TestClass() :val(1),str("Hello World"){}

    const int & GetVal()const{ return val; }

    const string & GetString()const{ return str; }

private:
    int val;
    string str;
};

HIJACKER(TestClass, val, int);
HIJACKER(TestClass, str, string);

int main(void)
{
    TestClass d;

    HIJACK(&d, val) = 233;
    HIJACK(&d, str) = "Hello Beijing";

    int i = d.GetVal();

    string str = d.GetString();

    return 0;
}