Question

I’ve just been having a look at the boost asio framework and the examples. In the source code for Daytime.3 – An asynchronous TCP daytime server, the code defines a constructor for the tcp_server class as follows:

tcp_server(boost::asio::io_service& io_service) : acceptor_(io_service, tcp::endpoint(tcp::v4(), 50500))

I’m confused as to how the constructor is initializing the acceptor_ instance variable which is defined later on in the private section? I wanted to write equivalent code for this initialization within the body of the constructor (just for learning), but I can’t figure out how this ivar is being initialized.

Many thanks in advance for any help.

Answer 1

Uhm.. why shouldn’t it be able to do so? The member variables are visible through-out the whole class, no matter where they are defined:

class Foo{
public:
  Foo(int i) : _i(i) {}

private:
  int _i;
};

If you meant how that while initialization thing works after the colon :, search for “initializer list”/”ctor initializer” if you want the standard wording.

---

Edit: Consider this class:

class Foo{
public:
  Foo(int i, float f) : _i(i), _f(f) {}

private:
  int _i;
  float _f;
};

And now consider this function and other class:

int random(){
  return 4;
}

class Bar{
public:
  Bar() : _f(random(), 3.14159f) {}

private:
  const Foo _f;
};

You can initialize a member in the initializer list however you want, assuming the member has a fitting constructor. Now, for const member, you can only initialize them that way, because inside the ctor body, it can’t be assigned any more. You could only do that with non-const and non-reference variables:

class Bar{
public:
  Bar(){
    _f = Foo(random(), 3.14159f); // invoke copy assignment operator
  }

private:
  Foo _f; // non-const
};

If this isn’t any help, I guess you really don’t express what you don’t understand.