C++ Coding Standards: 101 Rules, Guidelines, and Best Practices

97. Don’t use unions to reinterpret representation

Summary

A deceit is still a lie: Unions can be abused into obtaining a “cast without a cast” by writing one member and reading another. This is more insidious and even less predictable than reinterpret_cast (see Item 92).

Discussion

Don’t read a field of a union unless it was the field that was last written. Reading a different field of a union than the field that was last written has undefined behavior, and is even worse than doing a reinterpret_cast (see Item 92); at least with the latter the compiler has the fighting chance to warn and repel an “impossible reinterpretation” such as pointer to char. When abusing a union, no reinterpretation of bits will ever yield a compile-time error or ...