如何以类型安全的方式分配uint32_t :: max和uint64_t的最小值？

Question

I want to assign minimum of difference of two uint64_t numbers and maximum numerical limit of uint32_t in type safe manner. Is there a way to do it?

uint32_t max32 = std::numeric_limits<uint32_t>::max();
uint64_t a, b;

uint32_t minValue = std::min(max32, b - a);
                               ^^^^^^ (Parameter mismatch warning)

Basically. if the difference between a and b is greater than maximum value of uint32_t, I want to use the max value else I want to use the difference. I need to use minValue in an API that only accepts uint32_t.

Answer 1

You can use:

minValue = static_cast<uint32_t>(
               std::min(static_cast<uint64_t>(max32),
                        a < b ? b - a : a - b));

The static_cast creates a 64-bit unsigned value from max32 to match the type of the diff of a and b .

Note that when you subtract a larger unsigned number from a smaller one, the result will wrap around - often to some very large number. For example, (uint8_t)2 - (uint8_t)3 will yield 255. That's why I use the ? / : logic to work out which way around to perform the subtraction.