unions

If I have a class: class Odp { int i; int b; union { long f; struct { WCHAR* pwszFoo; HRESULT hr; }; }; } Union means that, of all values listed, it can only take on one of those values at a time? How does that work in terms of accessing these variables? How would I access hr directly? If I set hr , what happens if I try to access f ? This is a very fraught area in the C++ standard - basically a union instance, per the standard can only be treated at any one time as if it contained one "active" member - the last one written to it. So: union U { int a; char c; }; then: U u; u.a = 1; int n = u.a

I have a typedef boost::variant<int, float, double, long, bool, std::string, boost::posix_time::ptime> variant which I use to store different types of values in a struct. Only one of the specific type is ever going to be stored in that struct, however I have a vector of these structs which I need to go through and get the actual type out of the variant. Now when I need to do the conversion the types out of this variant I do this: variant second = mystruct.variant; if (second.which() == 5) //string { std::string val = boost::get<std::string>(second); modvalue->AddNodeAttribute(key, val); } else

Consider this variable declaration: union { struct { float x, y, z, padding; } components; __m128 sse; } _data; My idea is to assign the value through x , y , z fields, perform SSE2 computations and read the result through x , y , z . I have slight doubts as to whether it is legal, though. My concern is alignment: MSDN says that __m128 variables are automatically aligned to 16 byte boundary, and I wonder if my union can break this behavior. Are there any other pitfalls to consider here? The union's alignment should be fine, but in the case of Windows you may be able to access the 32 bit

I am trying to understand how unions were extended by C++11. One thing that changed is the ability to use now non-static data members with non-trivial special member functions. From cppreference.com If a union contains a non-static data member with a non-trivial special member function (default constructor, copy/move constructor, copy/move assignment, or destructor), that function is deleted by default in the union and needs to be defined explicitly by the programmer. At most one data member can have a default member initializer. I am trying the following code: struct X { ~X() {}; }; union U {

This question already has an answer here: Can a union be initialized in the declaration? 3 answers If it's a struct then it can be done *p = {var1, var2..}; But seems this doesn't work with union : union Ptrlist { Ptrlist *next; State *s; }; Ptrlist *l; l = allocate_space(); *l = {NULL}; Only to get: expected expression before ‘{’ token Crashworks In C99, you can use a d esignated union initializer : union { char birthday[9]; int age; float weight; } people = { .age = 14 }; In C++, unions can have constructors . In C89, you have to do it explicitly. typedef union { int x; float y; void *z; }

Hi I wanted know the reasons of the following code void main() { class test { public: test(){} int k; }; class test1 { public: test1(){} int k; }; union Test { test t1; test1 t2; }; } For the Above code it gives error " error C2620: union 'Test' : member 't1' has user-defined constructor or non-trivial default constructor " class test { public: //test(){} int k; }; class test1 { public: //test()1{}; int k; }; union Test { test t1; test1 t2; }; For the Above, No Errors. I wanted to know the Reasons. Thank you in Advance. :) According to the C++ standard (§9.5.1, cited as well in other answers):

What if I have this: union{ vector<int> intVec ; vector<float> floatVec ; vector<double> doubleVec ; } ; Of course, I'll be using just one of the 3 vectors. But... what happens when all the 3 vectors are contructed?? Would the consructors of the 3 vectors interfere with each other?? (since the 3 of them are in the same memory address) Thanks. Current C++ standard does not allow non-POD types inside unions. You will get this compiler error from gcc : error: member ‘std::vector<int, std::allocator<int> > <anonymous union>::i’ with constructor not allowed in union error: member ‘std::vector<int,