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Re: this pointer question. Strange behaviour?

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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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this pointer question. Strange behaviour? german diago <germandiago@gmail.com> - 2011-02-10 07:24 -0600
  Re: this pointer question. Strange behaviour?  Suncho <stupid@fake.email> - 2011-02-10 10:30 -0600

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