One classic is to represent a value of “unknown” type, as in the core of a simplistic virtual machine: typedef enum { INTEGER, STRING, REAL, POINTER } Type; typedef struct { Type type; union { int integer; char *string; float real; void *pointer; } x; } Value; Using this you can write code that handles … Read more
This is undefined behaviour. u.i and u.ch are located at the same memory address. So, the result of writing into one and reading from the other depends on the compiler, platform, architecture, and sometimes even compiler’s optimization level. Therefore the output for u.i may not always be 515. Example For example gcc on my machine … Read more
Tony Park gave an answer which is pretty close to the truth. The C++ committee basically didn’t think it was worth the effort to make unions a strong part of C++, similarly to the treatment of arrays as legacy stuff we had to inherit from C but didn’t really want. Unions have problems: if we … Read more
Is there any better solution? No, the solution that you showed is the best (and the only) one. unions are pretty simplistic – they do not “track” what you’ve assigned to what. All they do is let you reuse the same memory range for all their members. They do not provide anything else beyond that, … Read more
Assuming your objects are of the same type, you can use either Union or Concat. Note that, like the SQL UNION keyword, the Union operation will ensure that duplicates are eliminated, whereas Concat (like UNION ALL) will simply add the second list to the end of the first. IEnumerable<T> first = …; IEnumerable<T> second = … Read more
Yes, you can use an index signature: interface MyType { [key: string]: string | boolean | number; } var obj: MyType = { key1: “apple”, key2: true, key3: 123 };
A union always takes up as much space as the largest member. It doesn’t matter what is currently in use. union { short x; int y; long long z; } An instance of the above union will always take at least a long long for storage. Side note: As noted by Stefano, the actual space … Read more
Anonymous unions are a GNU extension, not part of any standard version of the C language. You can use -std=gnu99 or something like that for c99+GNU extensions, but it’s best to write proper C and not rely on extensions which provide nothing but syntactic sugar… Edit: Anonymous unions were added in C11, so they are … Read more
Union typing is only needed when you have a statically typed language, as you need to declare that an object can return one of multiple types (in your case an int or str, or in the other example str or NoneType). Python deals in objects only, so there is never a need to even consider … Read more
The behavior of type punning with union changed from C89 to C99. The behavior in C99 is the same as C11. As Wug noted in his answer, type punning is allowed in C99 / C11. An unspecified value that could be a trap is read when the union members are of different size. The footnote … Read more