C++ Compiler behavior regerding struct constructors

From: Karl M (Karl.M_at_email.com)
Date: 12/19/04 

Date: Sun, 19 Dec 2004 08:09:55 -0500

Hi everyone,
I just notice some strange behaviors on the MS C++ compiler regarding struct
default constructor, see the example bellow:
struct MyStruct
{
    int a[5];
};
class MyClass
{
public:
    MyClass();
    ~MyClass();
protected:
    MyStruct m_MyStruct;
};
//#1:
MyClass::MyClass() //The default ctor for the class does not init the struct
with NULL(maybe there is no default ctor for struct yet?)
{
}
//#2 if I define the class ctor like this:
MyClass::MyClass() : m_MyStruct() //The default ctor for the class does init
the struct with NULL(call m_MyStruct() default ctor, now there is one!)
{
}
//#3 or like this:
MyClass::MyClass()
{
    m_MyStruct = MyStruct(); // dose init the struct with NULL using a local
stack var init with NULL
}
//Now if I define the struct like this:
struct MyStruct
{
    MyStruct()
    {
        int i;
        for(i=0;i<5;i++)
            a[i] = NULL;
    }
    int a[5];
};
//#1 will init the struct with NULL because uses the defined struct ctor but
generate almost the same bin (asm) code as case #2 and #3 (the init stack
var) without the struct ctor defined!
I guess I don't understand when the compiler knows about the struct default
constructor and when dose not (and expect for one defined).

Thank you
Karl M

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