C++ – Converting a pointer into an integer

32-bit64-bitc++castinggcc

I am trying to adapt an existing code to a 64 bit machine. The main problem is that in one function, the previous coder uses a void* argument that is converted into suitable type in the function itself. A short example:

void function(MESSAGE_ID id, void* param)
{
    if(id == FOO) {
        int real_param = (int)param;
        // ...
    }
}

Of course, on a 64 bit machine, I get the error:

error: cast from 'void*' to 'int' loses precision

I would like to correct this so that it still works on a 32 bit machine and as cleanly as possible. Any idea ?

Best Solution

I'd say this is the modern C++ way.

#include <cstdint>
void *p;
auto i = reinterpret_cast<std::uintptr_t>(p);

EDIT:

The correct type to the the Integer

so the right way to store a pointer as an integer is to use the uintptr_t or intptr_t types. (See also in cppreference integer types for C99).

these types are defined in <stdint.h> for C99 and in the namespace std for C++11 in <cstdint> (see integer types for C++).

C++11 (and onwards) Version

#include <cstdint>
std::uintptr_t i;

C++03 Version

extern "C" {
#include <stdint.h>
}

uintptr_t i;

C99 Version

#include <stdint.h>
uintptr_t i;

The correct casting operator

In C there is only one cast and using the C cast in C++ is frowned upon (so don't use it in C++). In C++ there is different casts. reinterpret_cast is the correct cast for this conversion (See also here).

C++11 Version

auto i = reinterpret_cast<std::uintptr_t>(p);

C++03 Version

uintptr_t i = reinterpret_cast<uintptr_t>(p);

C Version

uintptr_t i = (uintptr_t)p; // C Version

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