如何进行与平台相关的检查，并且没有-Wtype-limits警告？

Question

I have code like this which is meant to target various compilers and platforms:

long value = obtainLongValue();
if(value <= 0)
   handleError();
if((unsigned long)value >= (size_t)-1)
   handleError();
size_t valueAsSizeT = value;
char* buffer = new char[value + 1];

Thechnically size_t can be smaller that unsigned long or the other way around or they can be of the same size.

On some targets both size_t and unsigned long are 64-bit and the code actually checks something and compiles cleanly. Yet on other targets size_t is 64-bit and unsigned long is 32-bit and for such platforms the check is meaningless - long is never long enough to overflow size_t - and so gcc emits a warning when used with -WExtra :

warning: comparison is always false due to limited range of data type [-Wtype-limits]

gcc is technically correct on those targets, but I need this check in place just in case.

How do I keep the check and have this code compiled cleanly with gcc?

Answer 1

Use template and specialization:

template <bool = sizeof(unsigned long) <= sizeof(std::size_t)>
constexpr bool CheckLongLessThanMaxSizeT(unsigned long value)
{
    return true;
}

template <>
constexpr bool CheckLongLessThanMaxSizeT<false>(unsigned long value)
{
    return value <= static_cast<std::size_t>(-1);
}