Thread: don't understand "strange" constructor

I found this constructor in a C++ book. Unfortunatly the authors didn't think it's worth any explanation or I'm too blind to find it

Code:

class iStack {
  public:
    iStack(int capacity): _stack(capacity), _top(0) {}


[...snip...]

  private:
    int _top;
    vector<int> _stack;
}

I don't understand the blue part. What does it do? I would expect it to do nothing because there is nothing between { and } and there is no further implementation for the constructor, but _stack(capacity) looks like a possible constructor for the vector
Does it initialize _stack and _top ? If so, why not beween the { and } ?

That allows you to initialize a variable with out doing var = value; It is shorthand.

so, this is the same?

Code:

iStack(int capacity) { _stack(capacity); _top(0) };

Seems even shorter to me. Anyway, thanks

Yes, it's pretty much the same thing although using the method you have shown (called an initializer list), you can do things that you could not do any other way such as initialize a const data member.

Code:

class something
{
    const int data;
public:
    something(int val) : data(val) {}  // Ok, can initialize const member "data"
    something(int val)
    {
        data = val; // Error, can't initialize const member "data" like this
    }
};

Code:

constructor(): var(value) {} // is permitted
constructor(): { var(value); }   is not permitted

The second would make the compiler look for a function not a variable.

It's usually faster to use intializer list's then standard direct assignment.

also, if your class is inherited, you can call the parent's constructor in the initializer list.

there are many advantages to using an initializer list...

Originally posted by ...
also, if your class is inherited, you can call the parent's constructor in the initializer list.

there are many advantages to using an initializer list...

As a matter of fact, it is the only place where you can call the parent's constructor. Also, anything that does not have a default constructor must be initialized in the initializer list.