接上一篇 VC和GCC内成员函数指针实现的研究(二)

虚继承

终于到最后的虚继承了。

测试代码如下:

#include <ctime>
#include <vector>
#include <algorithm>
#include <cstdio>
#include <stdint.h>
#include <cstddef>
#include <string>

class foo_a
{
public:
    virtual ~foo_a(){}
    virtual void info() {
        printf("%s:%d\n", __FUNCTION__, __LINE__);
    }

    void print(){
        printf("%s:%d\n", __FUNCTION__, __LINE__);
    }
};

class foo_l
{
public:
    int l;
    foo_l(): l(1){}

    virtual void info() {
        printf("%s:%d\n", __FUNCTION__, __LINE__);
    }
};

class foo_r: virtual public foo_a
{
public:
    int r;
    foo_r(): r(2){}
};

class foo_c: virtual public foo_l, public foo_r
{
public:
    virtual void info() {
        printf("%d,%d. %s:%d\n", l, r, __FUNCTION__, __LINE__);
    }
};


int main(int argc, char* argv[]) {
    void (foo_a::*vptr1)() = &foo_a::info;
    void (foo_l::*vptr2)() = &foo_l::info;
    void (foo_a::*ptr)() = &foo_a::print;

    foo_c c;
    foo_r r;
    foo_a a;

    printf("pword size = %d\n", (int)(sizeof(size_t)));
    printf("address of &foo_a::info = 0x%llxH\n", &foo_a::info);
    printf("pointer to &foo_a::info = 0x%llxH\n", vptr1);
    printf("sizeof vptr = %d\n", (int)(sizeof(vptr1)));

    (a.*vptr1)();
    (r.*vptr1)();
    (c.*vptr1)();
    (c.*vptr2)();
    c.info();

    (c.*ptr)();

    return 0;
}

VC虚继承成员函数指针实现

因为是兼容虚继承和非虚继承的，所以赋值的部分的汇编是一样的。这里就不贴了。关键在于执行期它是怎么找到虚基类的。请往下看：

接上一篇 VC和GCC内成员函数指针实现的研究(一)

接下来是多重继承，用到的测试代码如下所示:

#include <ctime>
#include <vector>
#include <algorithm>
#include <cstdio>
#include <stdint.h>
#include <cstddef>
#include <string>

class foo_a
{
public:
    virtual ~foo_a(){}
    virtual void info() {
        printf("%s:%d\n", __FUNCTION__, __LINE__);
    }
};

class foo_b
{
public:
    virtual ~foo_b(){}

    // 这个函数用于试foo_b的结构与foo_a不同
    // 以防止VC对foo_a和foo_b的`vcall'{8}'的代码合并
    virtual void inspect() { 
        printf("%s:%d\n", __FUNCTION__, __LINE__);
    } 

    virtual void info() {
        printf("%s:%d\n", __FUNCTION__, __LINE__);
    }

    void print() {
        printf("%s:%d\n", __FUNCTION__, __LINE__);
    }
};


class foo_c: public foo_a, public foo_b
{
public:
    virtual void info() {
        printf("%s:%d\n", __FUNCTION__, __LINE__);
    }
};


int main(int argc, char* argv[]) {
    void (foo_a::*vptr1)() = &foo_a::info;
    void (foo_b::*vptr2)() = &foo_b::info;
    void (foo_b::*ptr)() = &foo_b::print;

    foo_c c;
    foo_b b;
    foo_a a;

    
    printf("word size = %d\n", (int)(sizeof(size_t)));
    printf("address of &foo_a::info = 0x%llxH\n", &foo_a::info);
    printf("pointer to &foo_a::info = 0x%llxH\n", vptr1);
    printf("address of &foo_b::info = 0x%llxH\n", &foo_b::info);
    printf("pointer to &foo_b::info = 0x%llxH\n", vptr2);
    printf("sizeof vptr = %d\n", (int)(sizeof(vptr1)));

    (a.*vptr1)();
    (b.*vptr2)();
    (c.*vptr1)();
    (c.*vptr2)();

    (c.*ptr)();

    return 0;
}

VC多重继承成员函数指针实现

最近在《C++对象模型》一书里说到的virtual的成员函数指针，低于128的被cfront编译器认为是虚表偏移量（支持子类对父类函数的覆盖）。VC只是提了下单继承、多继承和虚继承的实现方案不同，GCC没有提及，所以就专门稍微深入分析研究下他们的实现机制。