知识总结

与书本所说不同,现代c++和其编译器在一段时间的发展后,本书3.6节的任何内容基本都不可以认为是对的了.
下面提到的几点都在实验代码有印证

指向data member的指针

1. 没有指向任何datamember的指针
   对于没有指向任何datamember的指针,现在只能赋值NULL,nullptr,或者同类型的指针.(在赋值NULL或nullptr时,指针的值为全1)
2. 个人猜测是完善了类型系统,导致现在已经不需要去给成员变量的指针偏移1来区分没有指向任何data member的指针和指向第一个元素的指针了. 另外说一句,咱c++别的地方也没有去加偏移来区分类似的东西啊,这里早年的处理就感觉很不美.

多继承情况下的data member pointer使用问题

• 现在子类指针在指向<父类class data member pointer的指向>时,总会调用正确的值.编译器会自己处理类型不同时的偏移问题.

指向members的指针的效率问题

在现代编译器下,使用

1. 类指针+指向data members的指针
2. 直接存取
3. 指向已经绑定的member

在优化开启时这三种时的效率已经相差无几,2,3几乎完全一样,在多重虚继承时,1可能有一些效率牺牲.但是在笔者的简单测试里也是一样的了.

实验环境

linux下
gcc 11.2.0+clang 14
c++20(c++2a)

测试代码

#include 
#include 
#include 
using namespace std;  // bad
class Point3d
{
public:
    static Point3d origin;
    float x, y, z;
};

// pointer to data member in multiple inheritance
class B1
{
public:
    int v = 1;
};
class B2
{
public:
    int v = 2;
};
class D : public B1, public B2
{
public:
    int v = 3;
};
void findMemberByClassMemberPointer( int D::*dataMemberPoiner, D* d )
{
    printf( "member value= %d\n", d->*dataMemberPoiner );
    //how to use data member pointer
};
int main( int argc, const char** argv )
{
    // pointer to data member in single data
    float Point3d::*px = &Point3d::x;
    float Point3d::*py = &Point3d::y;
    float Point3d::*pz = &Point3d::z;
    float Point3d::*pnull = nullptr;
    printf( "float Point3d::*pnull = %p\n", pnull );
    printf( "&Point3d::x = %p\n", px );
    printf( "&Point3d::y = %p\n", py );
    printf( "&Point3d::z = %p\n", pz );
    if ( px == NULL || px == nullptr ) {
        printf( "px in compiler's angle is nullptr\n" );
    }
    {
        printf( "px is 0x0 but px in compiler's angle is not nullptr\n" );
    }
    // pointer to data member in multiple inheritance
    auto d = new D;
    auto drivenClassMemberPointer = &D::v;
    findMemberByClassMemberPointer( drivenClassMemberPointer, d );
    auto base2ClassMemberPointer = &B2::v;
    findMemberByClassMemberPointer( base2ClassMemberPointer, d );

    return 0;
}

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输出

float Point3d::*pnull = 0xffffffffffffffff
&Point3d::x = (nil)
&Point3d::y = 0x4
&Point3d:😒 = 0x8
px is 0x0 but px in compiler’s angle is not nullptr
member value= 3
member value= 2