Hello. I'm trying to use this pointer in a constructor to store an
address of the this object in a global map like
this:

map<uintptr_t, void *> my_external_data;


MyClass::MyClass() {
    auto object_address = reinterpret_cast<uintptr_t>(this);
    my_external_data.insert(make_pair(object_address, new ...));
}

PointerType * MyClass::getExternalPointer() {
   auto object_address = reinterpret_cast<uintptr_t>(this);
   return static_cast<PointerType
*>(my_external_data[object_address]);
}

The problem is that when I print the this pointer in the constructor
and in the getExternalPointer() functions,
they don't point to the same address. Shouldn't the this pointer in
the constructor be the same as in
getExternalPointer? And if not, is there a workaround to achieve the
same result? Thanks in advance.


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On 2/10/2011 8:24 AM, german diago wrote:
>
> Hello. I'm trying to use this pointer in a constructor to store an
> address of the this object in a global map like
> this:
>
> map<uintptr_t, void *>  my_external_data;
>
>
> MyClass::MyClass() {
>      auto object_address = reinterpret_cast<uintptr_t>(this);
>      my_external_data.insert(make_pair(object_address, new ...));
> }
>
> PointerType * MyClass::getExternalPointer() {
>     auto object_address = reinterpret_cast<uintptr_t>(this);
>     return static_cast<PointerType
> *>(my_external_data[object_address]);
> }
>
> The problem is that when I print the this pointer in the constructor
> and in the getExternalPointer() functions,
> they don't point to the same address. Shouldn't the this pointer in
> the constructor be the same as in
> getExternalPointer? And if not, is there a workaround to achieve the
> same result? Thanks in advance.
>

I took your code and filled in the blanks.  It worked fine for me using
MSVC10.  The "this" pointers matched and when I indexed the map using
the address of the MyClass object, it returned the correct address of
the other object.  Which compiler are you using?

#include <map>
#include <utility>
#include <iostream>
using std::map;
using std::make_pair;
using std::cout;

typedef int * PointerType;

map<uintptr_t, void *> my_external_data;

class MyClass {
public:
     MyClass (void);
     PointerType * getExternalPointer (void);
};


MyClass::MyClass() {
     auto object_address = reinterpret_cast<uintptr_t>(this);
     my_external_data.insert(make_pair(object_address, new int(5)));
     cout << this << '\n';
}

PointerType * MyClass::getExternalPointer() {
     auto object_address = reinterpret_cast<uintptr_t>(this);
     cout << this << '\n';
     return static_cast<PointerType *>(my_external_data[object_address]);
}

int main (int argc, char * argv[])
{
     MyClass test_obj;

     // They're the same.
     cout << my_external_data[reinterpret_cast<uintptr_t>(&test_obj)]
     << '\n' << test_obj.getExternalPointer() << '\n';

     return 0;
}

Output:

0024F963
0024F963
002C0990
002C0990


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