Home/ Questions/Q 156551
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-11T10:20:51+00:00

I am not sure if the question is already addressed. I was checking the

  • 0

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

Lets check the code first:

#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; }

In this example, the result is as expected: 

Result is : 1 Result is : 0

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

In p.Process(pMC2, ‘Func’); call

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

In p.Process(pMC2, ‘Func2’); call

memFunc 0x0040118b MyClass::Func2(void)

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

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

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

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

    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.

    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: Offset in the vtable
    • {flat}: inheritance hierarchy is flat not multiple
    • 0