conversion from char* to uintptr

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?

CodePudding user response：

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