Constructors

[xxvms]

Hi there
I have question does this make sense?

thanks :)

class Alpha
{
private:
    int data;
public:
    Alpha() : data (0)                                              // no argument constructor
    {}
    Alpha(int d) : data(0) // <- here assigning 0 to data before data will be assigned with d ???                                          // one argument constructor
    { data = d; }

[kbw]

Yes, but in the second one, you could do:

Alpha(int d) : data (d)
{}

[xxvms]

Hi KBW

I am working on example from book where they didn't initialize data it was my modification,
and I was wondering if that is sensible approach or would as you suggest be better to use this format

Alpha (int d) : data(d)
{}

[Moschops]

There's no benefit in assigning zero to the value and then immediately assigning another value. Just assign it to the value you want, in the initialisation list.

[AbstractionAnon]

Your second constructor is doing double initialization (actually an initialization and an assignment). There is no point in doing so. kbw's example is preferred.

A further simplification would be:

class Alpha
{   int data;
public:
    Alpha(int d = 0) : data (d)      // defaults to 0 if no argument
    {}
};

[kbw]

For variables of simple built-in types (that we call Plain Old Data Types of PODS), there isn't much difference as it's straightforward for the optimizer to remove d = 0.

However, if the member is another object, you can see that it gets initialized, then assigned. This can be expensive. So I would recommend that you initialize members in the initializer list where possible.

For example:

class A
{
    std::string s;
public:
    A(const std::string& str) { s = str; } // default string constructor then assignment is used
};

[xxvms]

thank you all :)

that is very informative! noted in my manual for future use.