Is there a signed `sizeof` alternative in Qt

Question

Sizes of Qt containers return (signed) int. And we know in Qt, a lot more is using int instead of the unsigned type size_t in order to do arithmetic operations without the need for casting. See Why does Qt use a signed int type for its container classes & Why QVector.size() returns int

As the language keyword sizeof returns the size_t type, is there a Qt alternative for this?

Answer 1

You can make your own, safe version:

template<std::size_t s>
constexpr int safeIntCast()
{
    static_assert(s <= std::numeric_limits<int>::max(), "Type too large for ssizeof()!");
    return static_cast<int>(s);
}

#define ssizeof(x) safeIntCast<sizeof(x)>()

/// Usage

static_assert(ssizeof(int) == 4);
static_assert(ssizeof(1.0) == 8);

static_assert(ssizeof(std::declval<int>()) == 4);
static_assert(ssizeof(int[1000000000000]) > 4); // Conversion problem is caught!

Demo

It has the same semantics as sizeof but yields an int while ensuring that the conversion does not overflow.

Answer 2

The correct answer is that this is impossible. std::size_t can be std::uintmax_t, meaning that even std::intmax_t, the largest standards compliant type, cannot hold all values of std::size_t.

In most cases, you won't have to worry about it. An int (or a long long) will hold the size of your objects in most cases, so you can just cast to it: static_cast<long long>(sizeof(T)). If you are doing pointer arithmetic, consider using std::ptrdiff_t, or not using sizeof and doing end() - begin().