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Why inherited struct members are inaccessible in union?


By : Scott C
Date : September 15 2020, 02:00 AM
wish helps you Okay, after a bit of head-scratching, I think I can explain this correctly. Well, to start things off, this is Illegal As Heck (tm).
A struct defined within a union confuses the compiler a bit. If a struct def is followed by a name, a new variable of type of such struct would normally be created, but C++ explicitly disallows anonymous structs. If you omit that name, it could mean two things - either an anonymous struct type is created and then immediately dropped (which meets with a -Wpedantic warning), or, as seems to be happening with GCC, it considers it a creation of an anonymous member of an anonymous type, which is a non-standard extension:
code :
union alias {
    struct { int one; };     // anonymous member of an anonymous type
    struct a { int two };    // no member, just def of "struct a"
    struct { int three; } b; // named (b) member of type anonymous struct
};
struct s { int three; };

union alias {
    struct A { int one; };
    struct B { int two; };
    struct C : s { };

    A a;
    B b;
    C c;
};

int main() {
    alias a;
    a.a.one = 1;   // OK
    a.b.two = 2;   // OK
    a.c.three = 3; // OK
}


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How to set parent members when creating inherited struct in C++


By : user2925945
Date : March 29 2020, 07:55 AM
may help you . What you intend to do is achievable using initialization lists and can be done as follows:
code :
struct A
{
    A(int _a):a(_a){}
};

    struct B : public A
    {
        B(int _a, int _b):A(a), b(_b){}
    } ;

Why are inaccessible members considered inherited?


By : Hector He
Date : March 29 2020, 07:55 AM
this one helps. In your case, you're talking about const, which is implicitly static. Static members effectively aren't inherited anyway. Having just checked the spec, that implies that static members are inherited - but as they don't represent state and can't be part of polymorphism, it's at least a "different" kind of inheritance. I think of static members as merely being "available via simple name" to derived classes, assuming they're accessible at all - in other words, it's to do with name resolution more than real inheritance.
If this were a private instance variable, then it would be part of the state of any instance of an instance of any type derived from A, so it would be inherited state. If you think about an object's state and behaviour being inherited, it makes sense in my view.

Why does named union inside struct overwriting other struct members?


By : KSRay
Date : March 29 2020, 07:55 AM
fixed the issue. Will look into that further When you name the union there is no longer a data member in the class corresponding to d so instance.ZZZ::d = 300; doesn't mean anything. (With VS2012 I get "error C2039: 'ZZZ' : is not a member of 'TestStruct'")
To have a member of a union type you'll have to give it a name.
code :
struct TestStruct
{   
    int a;
    int b;

    union ZZZ // Note here!!
    {
        char c[2];
        int d;
    } z;
};

instance.a = 100;
instance.b = 200;
instance.z.d = 300;

C++ Call default constructor for Struct and its inherited members


By : Fatih CEVIK
Date : March 29 2020, 07:55 AM
I wish this help you I'd like to be able to initialize an object with the following syntax in C++14: , As of C++17, the syntax you wish for is valid:
code :
const auto data3 = DataThree{1, 2, 3, 4, 5, 6, 7};

How to add union members in an inherited class/struct


By : user2422354
Date : March 29 2020, 07:55 AM
This might help you Your unions in Foo and in Bar are an anonymous ones. This is syntactic sugar that allows to access their members as if they were in the enclosing class:
code :
struct ComplexNumber : public Vec2
{
    float &real=x, &imag=y;

    ComplexNumber(float realPart, float imagPart) : Vec2 (realPart, imagPart)
    {
    }
    ...
 }; 
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