unions

问题 I have defined to following structures to emulate a C++ union (which will eventually be used for C++ Interop): [StructLayout(LayoutKind.Sequential)] internal struct STRUCT1 { public Guid guid; public String str1; public String str2; } [StructLayout(LayoutKind.Sequential)] internal struct STRUCT2 { public Guid guid; public String str1; public String str2; public Int32 i1; } [StructLayout(LayoutKind.Explicit)] internal struct MASTER_STRUCT_UNION { [FieldOffset(0)] public STRUCT1 Struct1;

问题 I am trying to better understand a rather surprising discovery regarding unions and the common initial sequence rule. The common initial sequence rule says (class.mem 23):  In a standard-layout union with an active member of struct type T1, it is permitted to read a non-static data member m of another union member of struct type T2 provided m is part of the common initial sequence of T1 and T2; the behavior is as if the corresponding member of T1 were nominated. So, given: struct A { int a;

问题 Consider the following code snippet: union { int a; float b; }; a = /* ... */; b = a; // is this UB? b = b + something; Is the assignment of one union member to another valid? 回答1: Unfortunately I believe the answer to this question is that this operation on unions is under specified in C++, although self assignment is perfectly ok. Self assignment is well defined behavior, if we look at the draft C++ standard section 1.9 Program execution paragraph 15 has the following examples: void f(int,

问题 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

问题 Whenever I see examples of union, they are always different types. For example, from MSDN: // declaring_a_union.cpp union DATATYPE // Declare union type { char ch; int i; long l; float f; double d; } var1; // Optional declaration of union variable int main() { } What happens if I have a union (in this case anonymous, but that shouldn't matter) like this: union { float m_1stVar; float m_1stVarAlternateName; }; Regardless of whether this is good practice or not, will this cause any issues? 回答1:

问题 I'm trying to convert this C code to C#, is there a C# equivalent to the C union typedef? struct sockaddr_in { short sin_family; u_short sin_port; struct in_addr sin_addr; char sin_zero[8]; }; typedef struct in_addr { union { struct { u_char s_b1,s_b2,s_b3,s_b4; } S_un_b; struct { u_short s_w1,s_w2; } S_un_w; u_long S_addr; } S_un; } IN_ADDR, *PIN_ADDR, FAR *LPIN_ADDR; Thanks. 回答1: You may check out the following page. This being said, in .NET you have classes that allows you to work directly

问题 As the (Working Draft of) C++ Standard says: 9.5.1 [class.union] In a union, at most one of the non-static data members can be active at any time , that is, the value of at most one of the non-static data members can be stored in a union at any time. [...] The size of a union is sufficient to contain the largest of its non-static data members. Each non-static data member is allocated as if it were the sole member of a struct. All non-static data members of a union object have the same address

问题 This question already has answers here : Why does C++ disallow anonymous structs? (6 answers) Closed 5 years ago . How would you go about doing this in standard C++11/14 ? Because if I'm not mistaken this isn't standard compliant code with the anonymous structs. I wish to access the members the same way as you would with this. template <typename some_type> struct vec { union { struct { some_type x, y, z; }; struct { some_type r, g, b; }; some_type elements[3]; }; }; 回答1: Yes, neither C++11

问题 I am looking for an alternative to C-style union. boost::variant is one such option. Is there anything in std C++ ? union { int i; double d; } 回答1: As several commenters said: No, there is no Boost Variant-alike in standard C++. Maybe in a few years there will be, but why wait--use Boost Variant today! Edit (four years later, 2016): In C++17 there will be std::variant . Similar but not identical to boost::variant . So when your compiler supports C++17, you will have a solution in the standard

问题 I am working on turbo C on windows where char takes one byte.Now my problem is with the below union. union a { unsigned char c:2; }b; void main() { printf("%d",sizeof(b)); \\or even sizeof(union a) } This program is printing output as 2 where as union should be taking only 1 byte. Why is it so? for struct it is fine giving 1 byte but this union is working inappropriately. And one more thing how to access these bit fields. scanf("%d",&b.c); //even scanf("%x",b.c); is not working because we