unions

I have an API with a publicly facing struct A and an internal struct B and need to be able to convert a struct B into a struct A. Is the following code legal and well defined behavior in C99 (and VS 2010/C89) and C++03/C++11? If it is, please explain what makes it well-defined. If it's not, what is the most efficient and cross-platform means for converting between the two structs? struct A { uint32_t x; uint32_t y; uint32_t z; }; struct B { uint32_t x; uint32_t y; uint32_t z; uint64_t c; }; union U { struct A a; struct B b; }; int main(int argc, char* argv[]) { U u; u.b.x = 1; u.b.y = 2; u.b.z

Can a union in C++ have a member function? How do union with data members and member functions exist if an object is created? If I suppose yes, then are they feasible any where. If yes then where? 9.5/1 A union can have member functions (including constructors and destructors), but not virtual (10.3) functions. A union shall not have base classes. A union shall not be used as a base class. An object of a class with a non-trivial constructor (12.1), a non-trivial copy constructor (12.8), a non-trivial destructor (12.4), or a non-trivial copy assignment operator (13.5.3, 12.8) cannot be a member

I'm dealing with an union in C++, and I would like have a function template which access to the active union member depending on a template parameter. Code is something like (doSomething is just an example): union Union { int16_t i16; int32_t i32; }; enum class ActiveMember { I16 , I32 } template <ActiveMember M> void doSomething(Union a, const Union b) { selectMemeber(a, M) = selectMember(b, M); // this would be exactly (not equivalent) the same // that a.X = b.X depending on T. } To accomplish this I only found bad hacks like specialization, or a not homogeneous way to access and assign. I'm

If I declare a Union as: union TestUnion { struct { unsigned int Num; unsigned char Name[5]; }TestStruct; unsigned char Total[7]; }; Now, How can I know that whether Total[7] is used or TestStruct is used? I am using C! I was revisiting unions and structures and this question came to my mind. " sizeof " can't be used as both are of same size i.e. 7 bytes. (And Here comes another question) When I filled only "Total" with a Character 'a' and Tried sizeof(TestUnionInstance) , it returned 12 (Size of Char is 1 byte, Right?). So I isolated the structure from it and found that Size of Structure is

The standard defines that Unions cannot be used as Base class, but is there any specific reasoning for this? As far as I understand Unions can have constructors, destructors, also member variables, and methods to operate on those varibales. In short a Union can encapsulate a datatype and state which might be accessed through member functions. Thus it in most common terms qualifies for being a class and if it can act as a class then why is it restricted from acting as a base class? Edit: Though the answers try to explain the reasoning I still do not understand how Union as a Derived class is

The idea behind this question is to understand the deeper concepts of using union and using it in different way so as to save memory.. My question to all is-- let's say there is a structure struct strt { float f; char c; int a; } and the same structure represented in union union unin { float f; char c; int a; } If I allocate values to structure members one after another and then print them, it gets printed. But in case of union it doesn't happen, some overwriting is being done.. So I need to find out a method which can store the values of f,c,a using union and then I can print the same. (Apply

I am writing a C program. I want a variable that I can access as a char but I can also access the specific bits of. I was thinking I could use a union like this... typedef union { unsigned char status; bit bits[8]; }DeviceStatus; but the compiler doesn't like this. Apparently you can't use bits in a structure. So what can I do instead? Sure, but you actually want to use a struct to define the bits like this typedef union { struct { unsigned char bit1 : 1; unsigned char bit2 : 1; unsigned char bit3 : 1; unsigned char bit4 : 1; unsigned char bit5 : 1; unsigned char bit6 : 1; unsigned char bit7 :

I have the following struct: struct type1 { struct type2 *node; union element { struct type3 *e; int val; }; }; When initialising a pointer *f that points to an instance of type1 and doing something like: f.element->e or even just f.element , I get: error: request for member ‘element’ in something not a structure or union What am I overseeing here? element is the name of the union, not the name of a member of type1 . You must give union element a name: struct type1 { struct type2 *node; union element { struct type3 *e; int val; } x; }; then you can access it as: struct type1 *f; f->x.e If f is