How do C++ class members get initialized if I don&

Question

Suppose I have a class with private memebers ptr, name, pname, rname, crname and age. What happens if I don't initialize them myself? Here is an example:

class Example {
    private:
        int *ptr;
        string name;
        string *pname;
        string &rname;
        const string &crname;
        int age;

    public:
        Example() {}
};

And then I do:

int main() {
    Example ex;
}

How are the members initialized in ex? What happens with pointers? Do string and int get 0-intialized with default constructors string() and int()? What about the reference member? Also what about const references?

What else should I know about?

Does anyone know a tutorial that covers these cases? Maybe in some books? I have access in university's library to a lot of C++ books.

I'd like to learn it so I can write better (bug free) programs. Any feedback would help!

Answer 1

First, let me explain what a mem-initializer-list is. A mem-initializer-list is a comma-separated list of mem-initializers, where each mem-initializer is a member name followed by (, followed by an expression-list, followed by a ). The expression-list is how the member is constructed. For example, in

static const char s_str[] = "bodacydo";
class Example
{
private:
    int *ptr;
    string name;
    string *pname;
    string &rname;
    const string &crname;
    int age;

public:
    Example()
        : name(s_str, s_str + 8), rname(name), crname(name), age(-4)
    {
    }
};

the mem-initializer-list of the user-supplied, no-arguments constructor is name(s_str, s_str + 8), rname(name), crname(name), age(-4). This mem-initializer-list means that the name member is initialized by the std::string constructor that takes two input iterators, the rname member is initialized with a reference to name, the crname member is initialized with a const-reference to name, and the age member is initialized with the value -4.

Each constructor has its own mem-initializer-list, and members can only be initialized in a prescribed order (basically the order in which the members are declared in the class). Thus, the members of Example can only be initialized in the order: ptr, name, pname, rname, crname, and age.

When you do not specify a mem-initializer of a member, the C++ standard says:

If the entity is a nonstatic data member ... of class type ..., the entity is default-initialized (8.5). ... Otherwise, the entity is not initialized.

Here, because name is a nonstatic data member of class type, it is default-initialized if no initializer for name was specified in the mem-initializer-list. All other members of Example do not have class type, so they are not initialized.

When the standard says that they are not initialized, this means that they can have any value. Thus, because the above code did not initialize pname, it could be anything.

Note that you still have to follow other rules, such as the rule that references must always be initialized. It is a compiler error to not initialize references.