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清除C++访问资格最干净的方法是什么?

private c++

Potatoswatter · 技术社区 · 15 年前

但是,如前所述,这会导致每个翻译单元复制一些小对象和访问器函数,从而导致令人讨厌的级联。有没有一个规范的方法来做到这一点,比如说推进最佳实践?

抱歉的坏幽默,但这不是无缘无故的,我们不会希望这个代码是“安全的工作”!

/* This hack installs a static initializer, so to avoid the ordering fiasco,
make one fresh copy per translation unit, via anonymous namespace. */
namespace {

template< typename T, T value, T &dest >
struct class_rape {
    class_rape() { dest = value; } // you've been raped in the class!
    static class_rape r;
};
template< typename T, T value, T &dest >
class_rape< T, value, dest > class_rape< T, value, dest >::r;


// Usage (cvt_[w]filebuf is a specialization of GCC basic_filebuf)

typedef bool cvt_filebuf::*cvt_fb_reading_t;
typedef bool cvt_wfilebuf::*cvt_wfb_reading_t;

/* Access these variables, or functions accessing them (applies recursively),
only in anonymous namespace or in non-header file, per one-definition rule. */
cvt_fb_reading_t cvt_filebuf_reading;
cvt_wfb_reading_t cvt_wfilebuf_reading;

template struct class_rape
    < cvt_fb_reading_t, &cvt_filebuf::_M_reading, cvt_filebuf_reading >;
template struct class_rape
    < cvt_wfb_reading_t, &cvt_wfilebuf::_M_reading, cvt_wfilebuf_reading >;

}

http://pastie.org/1188625 .

更新

我在下面的回答中解决了重复问题。因此,现在我对一个确定性的、定义良好的解决方案感兴趣,它不涉及编辑任何目标代码,并且允许同时对模板的多个专门化进行黑客攻击。(给定的hack需要为每个目标模板专门化提供一个新的实例化。)

3 回复 | 直到 11 年前

sbi 15 年前

非法访问:

class ClassIWantToViolate
{
    // Internal State
    public:
        template<typename T> void violate() {} // Do nothing
};

namespace { struct Attack {}; }

template<>
void ClassIWantToViolate::violate<Attack>()
{
     // Access to internal state here.

     // This is your own version of violate based on a local specialization
     // Thus it is unique but still has access to internal state of the class.
}

dgnorton 15 年前

我可能会选择不那么聪明的方法。从外部访问类的私有成员是一件非常罕见的事情,至少对我来说是这样(从来没有这样做过)。在我的实现中,我能在几分钟内完成这个。。。

删除的所有实例和来自邪恶的流.h

然后此代码编译:

#include <fstream>
#include "evil_fstream.h"

using namespace std;

typedef evil::basic_filebuf<char, char_traits<char> > evil_filebuf_t;

int main() {

   std::basic_filebuf<char, char_traits<char> > fb;

   evil_filebuf_t* efb = (evil_filebuf_t*)&fb;

   efb->_Pcvt; // access a private member

   return 1;
}

Potatoswatter 15 年前

我可以避免静态初始化顺序失败和重复问题,通过使变量真正全局化,而仅仅是

typedef bool cvt_filebuf::*cvt_fb_reading_t;
typedef bool cvt_wfilebuf::*cvt_wfb_reading_t;

/* Place accessible variables in global, non-anonymous namespace. */
cvt_fb_reading_t cvt_filebuf_reading;
cvt_wfb_reading_t cvt_wfilebuf_reading;

/* This hack installs a static initializer, so to avoid the ordering fiasco,
make one fresh copy per translation unit, via anonymous namespace. */
namespace {

template< typename T, T value, T &dest >
class class_rape { // change access qualification of hack to guarantee ODR, LOL
    class_rape() { dest = value; } // you've been raped in the class!
    static class_rape r;
};
template< typename T, T value, T &dest >
class_rape< T, value, dest > class_rape< T, value, dest >::r;

template struct class_rape
    < cvt_fb_reading_t, &cvt_filebuf::_M_reading, cvt_filebuf_reading >;
template struct class_rape
    < cvt_wfb_reading_t, &cvt_wfilebuf::_M_reading, cvt_wfilebuf_reading >;

}

/* Accessor functions go here, also outside anonymous namespace. */