Most efficient standard-compliant way of reinterpreting int as float

Question

Assume I have guarantees that float is IEEE 754 binary32. Given a bit pattern that corresponds to a valid float, stored in std::uint32_t, how does

Answer 1

memcpy is always safe but does involve a copy

casting may lead to problems

union - seems to be allowed in C99 and C11, not sure about C++

Take a look at:

What is the strict aliasing rule?

and

Is type-punning through a union unspecified in C99, and has it become specified in C11?