Can you propose at least 1 scenario where there is a substantial difference between

union {
T var_1;
U var_2;
}

and

var_2 = reinterpret_cast<U> (var_1)

?

The more i think about this, the more they look like the same thing to me, at least from a practical viewpoint.

One difference that I found is that while the union size is big as the biggest data type in terms of size, the reinterpret_cast as described in this post can lead to a truncation, so the plain old C-style union is even safer than a newer C++ casting.

Can you outline the differences between this 2 ?

c++ unions reinterpret-cast
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