class MyClass {
public:
MyClass() {}
MyClass(...) { /* other constructor */ }
};
union MyUnion {
int numbers[4];
MyClass instance;
};
union MyUnion {
int numbers[4];
struct {
MyClass instance;
};
};
C++: Union, struct and default constructor
Author
Quick question regarding unions. I understand why a type I put in a union has to have a trivial default constructor. So my class definition is:
Now when I declare a union like so:
my compiler (the one in Visual Studio 2005 SP1) complains that MyClass has a non-trivial default constructor. However, this compiles without problem:
Can somebody enlighten me why this does work?
The rule is that POD types can have no user-defined constructors (which are regarded as "non-trivial"). I'm actually surprised the second example works; a POD type can only contain other POD types as data members. Logically, the anonymous struct should also be non-POD. Although I'm not certain what the standard mandates here (or even if anonymous structs inside a union are technically legal).
If we make it conformant ...
... then g++ will not compile this.
union MyUnion { int numbers[4]; struct { MyClass instance; } x; // << anonymous structs not allowed, hence declare an instance};... then g++ will not compile this.
The anonymous struct isn't legal C++ - it's a compiler extension. Allowing you to add a non-pod class to a union looks like a bug in that extension.
Does the constructor actually get run?
Does the constructor actually get run?
This topic is closed to new replies.
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