One way to simulate unions is using variants. Variants are type-safe unions. They are available since C++17. An instance of std::variant at any given time either holds a value of one of its alternative types, or in the case of error - no value. The latter state is hard to achieve.

Example (wrong):

union SuperFloat
{
    float f;
    int i;
};

Example (right):

using std::variant SuperFloat;

See the reference to StackOverflow for a discussion when to use variants over unions.