`vcall'{0,{flat}}' 在下面的例子中是什么意思? [英] What does `vcall'{0,{flat}}' signify in below example?

问题描述

我不确定问题是否已经解决.我正在检查堆栈溢出功能之一，并得到了这个疑问.

I am not sure if the question is already addressed. I was checking the one of the Stack overflow function and got this doubt.

让我们先检查代码:

#include <string>
#include <map>
#include <iostream.h>

class MyClass
{
public:

    virtual  int Func()
    {
        return 0;
    }

    int Func2()
    {
        return 0;
    }

};

class MyClass2 :  public MyClass
{
public:

    int Func( )
    {
        return 1;
    }

    int Func2()
    {
        return 1;
    }

};

class Processor
{
 private:
      typedef int (MyClass::*MemFuncGetter)();
      static std::map<std::string, MemFuncGetter> descrToFuncMap;

 public:
        static void Initialize();
        void Process(MyClass* m, const std::string&);

};

std::map<std::string, Processor::MemFuncGetter> Processor::descrToFuncMap;
void Processor::Initialize()
{

     descrToFuncMap["Func"]=&MyClass::Func;
     descrToFuncMap["Func2"]=&MyClass::Func2;
};

void Processor::Process(MyClass* ms, const std::string& key)
{
    std::map<std::string, MemFuncGetter>::iterator found = descrToFuncMap.find(key);
     if(found != descrToFuncMap.end())
     {
        MemFuncGetter memFunc = found->second;
        int dResult = (ms->*memFunc)();
        cout << "Result is : "<< dResult <<endl;
      }
 }


int main(int argc, char* argv[])
{
    Processor::Initialize();
    Processor p;

    MyClass *pMC2 = new MyClass2;
    p.Process(pMC2, "Func");
    p.Process(pMC2, "Func2");

    delete pMC2;
    pMC2 = NULL;

    return 0;
}

在这个例子中，结果如预期:

In this example, the result is as expected:

Result is : 1
Result is : 0

但是当我使用 VC 6 调试器进行调试并观察 Processor::Process 中 memFunc 的值时，发现以下值:

But when I debugged using VC 6 debugger and observed the value of memFunc in Processor::Process and found following values:

在 p.Process(pMC2, "Func");打电话

In p.Process(pMC2, "Func"); call

memFunc 0x004011bd [thunk]:`vcall'{0,{flat}}' 

在 p.Process(pMC2, "Func2");打电话

In p.Process(pMC2, "Func2"); call

memFunc 0x0040118b MyClass::Func2(void)

我不明白 "[thunk]:`vcall'{0,{flat}}" 中的 thunk 和 flat ?任何人都可以帮助我了解这里的内部结构吗?

I didn't understand thunk and flat in "[thunk]:`vcall'{0,{flat}}" ? Can anyone help me to understand the internals here ?

推荐答案

维基百科上对 thunk 的含义有非常详尽的解释

There is a pretty thorough explanation of what thunk means over on WikiPedia

http://en.wikipedia.org/wiki/Thunk

thunk 的要点是在运行时访问 C++ 虚拟表的机制.它被设置为为对象的运行时类型调用适当的虚函数.

The gist of is thunk is the mechanism by which the C++ virtual table is accessed at runtime. It is setup to call the appropriate virtual function for the runtime type of the object.

至于 vcall{0,{flat}} 的含义，我不是 100% 确定.我猜测是它报告了thunk访问方法的值.

As to what the vcall{0,{flat}} means I am not 100% sure. My guess is that it's reporting the values by which the thunk is accessing the method.

• 0:vtable 中的偏移量
• {flat}:继承层次是扁平的，不是多重的

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