Question

In this contrived example, I have a static initialization function that is run at construction time.

I’d like to know if this a legal way to initialize a_, b_, and c_. Or, is it too early in the construction process to use them this way?

DWORD SomeInitializationFunction( HANDLE*, DWORD*, DWORD* );

class MyClass
{
public:
    MyClass() : m_( MyClass::Create( &a_, &b_, &c_ ), &::CloseHandle )
    {
    };

private:

    static HANDLE Create( DWORD* a, DWORD* b, DWORD* c )
    {
        DWORD a1, b1;
        HANDLE h;

        *c = ::SomeInitializationFunction( &h, &a1, &b1 );
        if( *c == 0 )
        {
            *a = a1;
            *b = b1;
            return h;
        }
        return NULL;
    };

    boost::shared_ptr< void > m_;
    DWORD a_;
    DWORD b_;
    DWORD c_;
};

Thanks,
PaulH

Answer 1

Members in a class are initialized in the order they are defined in the class. Thus if you have

class A
    int t
    int u
    int v
A() : v(0), u(1), t(2) {}

Then despite the order in which you write the constructor arguments, first the value of t would be set, then the value of u and finally the value of v.

So if you change the order of definition of your class to:

class MyClass
{
public:
    MyClass() : m_( MyClass::Create( &a_, &b_, &c_ ), &::CloseHandle )
    {
    };

private:

    static HANDLE Create( DWORD* a, DWORD* b, DWORD* c )
    {
        DWORD a1, b1;
        HANDLE h;

        *c = ::SomeInitializationFunction( &h, &a1, &b1 );
        if( *c == 0 )
        {
            *a = a1;
            *b = b1;
            return h;
        }
        return NULL;
    };


    DWORD a_;
    DWORD b_;
    DWORD c_;
    boost::shared_ptr< void > m_;
};

then your constructor should be okay.

I’d suggest putting in a comment saying that there is a dependency in the initialization of the members of your class so that people reading the code would know not to fiddle around.