Regarding this code: #include <string> int main() { union u { u() { i = 0; } ~u() {} int i; std::string s1; std::string s2; } u; new (&u) std::string{}; } [intro.object]/2 s...

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;...

I want to create a Discriminated Union Type, where it isn't required to pass the discriminator value. Here's my current code: interface Single<T> { multiple?: false // this is optional, be...

I came to C from high-level Scala language and came up with a question. In Scala we usually handle error/exceptional conditions using Either which looks like the following: sealed abstract class E...

My story starts off the same as this person's here: Unions in C++11: default constructor seems to be deleted The resolution here (now about three years old) is a bit unsatisfactory, because the "...

Is it legal to have fields with the same name across different anonymous unions inside one union? union Foo { union { int bar; }; union { int bar; }; }; This code fails to compile by GC...

In the c++ standard, in [basic.lval]/11.6 says: If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined...

This is the relationship I am talking about: struct A{ int i = 1; }; struct B{ union{A a;}; }; void main(){ B b; }; In this constellation, my compiler (vs2015) complains about the default c...

Look at this code: struct A { short s; int i; }; struct B { short s; int i; }; union U { A a; B b; }; int fn() { U u; u.a.i = 1; return u.b.i; } Is it guaranteed that fn() returns 1? ...

C++11 gaves us the ability to create anonymous unions with non-trivial members. That can be very useful sometimes - for example, if I want to create Holder class for some non-trivial object without...

When I read ISO/IEC 9899:1999 (see:6.5.2.3), I saw an example like this (emphasis mine) : The following is not a valid fragment (because the union type is not visible within function f): struct...

While reading through GCC's implementation of std::optional I noticed something interesting. I know boost::optional is implemented as follows: template <typename T> class optional { // ... ...

The following is not undefined behavior in modern C: union foo { int i; float f; }; union foo bar; bar.f = 1.0f; printf("%08x\n", bar.i); and prints the hex representation of 1.0f. However th...

This question followes this one Let's consider this example code: struct sso { union{ struct { char* ptr; char size_r[8]; } large_str; char short_str[16]; }; bool is_short_str() const...

The following example is given in the C11 standard, 6.5.2.3 The following is not a valid fragment (because the union type is not visible within function f): struct t1 { int m; }; struct t2 { i...

Not sure if there is a term for this, "choice" seems to work. I'm working in C++, and I have a bunch of unions I need to create where the union represents a choice of one of the members of the unio...

From a response to some question yesterday, I learned that it is nonportable and unsafe to write into one union member and read the value from another member of a different type, assuming underlyin...

Important clarification: some commenters seem to think that I am copying from a union. Look carefully at the memcpy, it copies from the address of a plain old uint32_t, which is not contained withi...

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 T...

GCC complains about this code even though I compile with -std=c++11 flag, and my gcc version supposedly supports Unrestricted unions (>4.6). union { struct { float4 I,J,K,T; }; struct { ...

Quote from C99 standard: 6.5.2.3 5 One special guarantee is made in order to simplify the use of unions: if a union contains several structures that share a common initial sequence (see below), an...

I can't seem to create a union where a member is or contains a glm::vec3 object (an object for representing a coordinate, containing 3 floats in this case). (source code for glm::vec) It's used in...

I recently read a description of default constructors for unions: Default Constructor There is a following rule: Blockquote Deleted implicitly-declared default constructor: [...] T is a unio...

I implemented a tagged union using a class containing an anonymous union and a tag: class LogFile { public: LogFile(std::ostream& stdStream); LogFile(std::ofstream fileStream); LogFile(Log...

C++11 allows an anonymous union to be defined in a function, and its members can be accessed as variables of the function. If I examine the pointers to the different members, they are the same, but...