How to Safely Convert Pointers to Integers for 64-Bit Compatibility?

Converting Pointers to Integers for 64-Bit Compatibility

An existing codebase, originally designed for a 32-bit machine, employs a function with a void* argument that's subsequently converted to an appropriate type within the function:

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

When adapting this code to a 64-bit environment, the compiler flags an error:

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

To address this, a modification is required that maintains compatibility with 32-bit machines as well.

Solution

For a modern C approach, reinterpret_cast serves as the ideal conversion mechanism. The code transforms as follows:

#include <cstdint>

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

Correct Integer Type for Pointer Storage

The recommended data type for storing pointers as integers is uintptr_t or intptr_t. These types reside in the header for C99 and the std namespace for C 11.

Appropriate Casting Operator

In C , reinterpret_cast is the preferred casting mechanism for this conversion. It replaces the C-style cast operator, which is no longer favored in C .

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