从 char* 到 uintptr_t 的转换

Question

I have the following code and get a compiling error:

#include <cstddef>
#include <cstdint>

#define CHECK_ALIGN(ptr, alignment)                       \
  do{                                                     \
    constexpr size_t status                               \
       = reinterpret_cast<uintptr_t>(ptr) % alignment;    \
    static_assert(status == 0, "ptr must be aligned");    \
  }while(0)  

int main(int argc, char** argv) {
  char c;
  int i;
  long l ;
  float f;
  CHECK_ALIGN(&c, sizeof(c));
  CHECK_ALIGN(&i, sizeof(i));
  CHECK_ALIGN(&l, sizeof(l));
  CHECK_ALIGN(&f, sizeof(f));
  return 0;
}

error: conversion from pointer type 'char*' to arithmetic type 'uintptr_t' {aka 'long unsigned int'} in a constant expression

What is the proper type to convert to for those pointer type to do some arithmetic?

Answer 1

status cannot be constexpr because the value of ptr is not known at compile time. Also, static_assert() needs to be replaced with assert() for the same reason.

#include <cstddef>
#include <cstdint>
#include <cassert>

#define CHECK_ALIGN(ptr, alignment)                       \
  do{                                                     \
    const size_t status                                   \
       = reinterpret_cast<uintptr_t>(ptr) % alignment;    \
    assert(status == 0);                                  \
  } while(0)  

int main(int argc, char** argv) {
  char c;
  int i;
  long l ;
  float f;
  CHECK_ALIGN(&c, sizeof(c));
  CHECK_ALIGN(&i, sizeof(i));
  CHECK_ALIGN(&l, sizeof(l));
  CHECK_ALIGN(&f, sizeof(f));
  return 0;
}

Godbolt: https://godbolt.org/z/GrWdE3sGK

Alternatively, you could use constexpr expressions like this:

#include <cstddef>
#include <cstdint>

#define CHECK_ALIGN(value, alignment) \
    static_assert(alignof((value)) % alignment == 0, #value " must be aligned");    

int main(int argc, char** argv) {
  char c;
  int i;
  long l ;
  float f;
  CHECK_ALIGN(c, sizeof(c));
  CHECK_ALIGN(i, sizeof(i));
  CHECK_ALIGN(l, sizeof(l));
  CHECK_ALIGN(f, sizeof(f));
  return 0;
}

Godbolt: https://godbolt.org/z/fMfY3P9bd