unions

I'm looking for some union examples, not to understand how union works, hopefully I do, but to see which kind of hack people do with union. So feel free to share your union hack (with some explanation of course :) ) 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 "values" without knowing their exact type, for instance implement a stack and so

This question already has an answer here: Purpose of Unions in C and C++ 15 answers In the comments of this answer it is said that it would be undefined behavior to split up an integer into their bytes using a union like follows. The code given at that place is similar though not identical to this, please give a note if have I changed undefined-behavior-relevant aspects of the code. union addr { uint8_t addr8[4]; uint32_t addr32; }; Up to now I thought this would be a fine approach to do things like addr = {127, 0, 0, 1}; and get the corresponding uint32_t in return. (I acknowledge that this

I'm updating a struct of mine and I was wanting to add a std::string member to it. The original struct looks like this: struct Value { uint64_t lastUpdated; union { uint64_t ui; int64_t i; float f; bool b; }; }; Just adding a std::string member to the union, of course, causes a compile error, because one would normally need to add the non-trivial constructors of the object. In the case of std::string (text from informit.com) Since std::string defines all of the six special member functions, U will have an implicitly deleted default constructor, copy constructor, copy assignment operator, move

I have two structures, with values that should compute a pondered average, like this simplified version: typedef struct { int v_move, v_read, v_suck, v_flush, v_nop, v_call; } values; typedef struct { int qtt_move, qtt_read, qtt_suck, qtd_flush, qtd_nop, qtt_call; } quantities; And then I use them to calculate: average = v_move*qtt_move + v_read*qtt_read + v_suck*qtt_suck + v_flush*qtd_flush + v_nop*qtd_nop + v_call*qtt_call; Every now and them I need to include another variable. Now, for instance, I need to include v_clean and qtt_clean . I can't change the structures to arrays: typedef

here is very simplified code of problem I have: enum node_type { t_int, t_double }; struct int_node { int value; }; struct double_node { double value; }; struct node { enum node_type type; union { struct int_node int_n; struct double_node double_n; }; }; int main(void) { struct int_node i; i.value = 10; struct node n; n.type = t_int; n. int_n = i; return 0; } And what I don't undestand is this: $ cc us.c $ cc -std=c99 us.c us.c:18:4: warning: declaration does not declare anything us.c: In function ‘main’: us.c:26:4: error: ‘struct node’ has no member named ‘int_n’ Using GCC without -std option

Background Discussions on the mostly un-or-implementation-defined nature of type-punning via a union typically quote the following bits, here via @ecatmur ( https://stackoverflow.com/a/31557852/2757035 ), on an exemption for standard-layout struct s having a "common initial sequence" of member types: C11 ( 6.5.2.3 Structure and union members ; Semantics ): [...] if a union contains several structures that share a common initial sequence (see below), and if the union object currently contains one of these structures, it is permitted to inspect the common initial part of any of them anywhere