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 a reliable result.

Consider this code that is intended to deposit a value of one type (char*) and extract the bits of that value as a different type (long):

union {


 long intValue_;


 char* pointerValue_;


};





pointerValue_ = somePointer;


long int gotcha = intValue_;

This has two problems:

• It assumes too much: It assumes that sizeof(long) and sizeof(char*) are equal, and that the bit representations are identical. These things are not true on all implementations (see Item 91).

• It obscures your intent for both human readers and compilers: Playing union games makes it harder for compilers to catch genuine type errors, and for humans to spot logical errors, than even the infamous reinterpret_cast (see Item 92).