Constructor? or something else?

Question

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What is this weird colon-member syntax in the constructor?

So I was looking through some samples in a Direct X 10 book, when I came across this

PeaksAndValleys::PeaksAndValleys()
: mNumRows(0), mNumCols(0), mNumVertices(0), mNumFaces(0),
  md3dDevice(0), mVB(0), mIB(0)
{
}

I understand this is probably supposed to be a constructor......but i've never seen one like this? can someone explain? specifically why is there a single : after it's declared?

Not directly answering your question, but you might invest in a good C++ book. You can find a list of good books here: http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list — Robᵩ, May 09 '11 at 15:03

score 4 · Answer 1 · answered May 09 '11 at 14:57

4

Constructor Initialization list

answered May 09 '11 at 14:57

Prasoon Saurav

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score 2 · Accepted Answer · answered May 09 '11 at 15:12

This is called an initialization list, and is used for initializing class members. It is useful both as a shorthand and it is required for initializing members that do not have a default constructor. For example:

#include <iostream>
using namespace std;

class Foo {
public:
    Foo(int x) { cout << "Foo(" << x << ")" << endl; }
};

class Bar : public Foo {
    Foo member;
public:
    Bar() { /* error */ }
};

This gives an error because member cannot be default constructed, as Foo does not have a default constructor. Change it to

Bar(): member(42) {}

and now it works.

This syntax is also useful for initializing const members of a class, as while they might be default-constructible, you cannot overwrite them in the constructor body.

class Baz {
    const member;
public:
    Baz(int x): member(x) {}
};

The same idea also applies to references, as they also must be initialized directly. Finally, it is used for specifying arguments for the base class constructor.

class Xyzzy : public Foo {
public:
    Xyzzy(int y): Foo(y+3) {}
};

score 1 · Answer 3 · answered May 09 '11 at 14:59

That's just a syntax for setting the class fields. In most cases, it's functionally identical to the version you're expecting where each field is set in the constructor body. It lets you pass arguments to constructors where you otherwise might not be able to. However, there are certain other circumstances where you need this syntax (assigning member references, etc.). You'll also sometimes see constructor chaining, where you call the base class constructor in the same way instead of just setting member variables.

class Base { 
public:
    Base(int n) {}
};

class Derived : public Base {
public:
    Derived(int n) : Base(n) {}
};

score 1 · Answer 4 · answered May 09 '11 at 14:59

It's called a constructor initializer.

ClassName::ClassName(Type someArgument) : memberName(initialValue),
    otherMember(someArgument)
{
    //constructor logic
}

It sets memberName to initialValue and otherMember to someArgument. Several members are separated by a ,.

score 1 · Answer 5 · answered May 09 '11 at 15:07

I just want to add:

You it whenever you can! It's a lot faster that using classical method. You can use it to initialize constants, members, call parent constructors, etc.

Everything you specify in this list, will be created before you create class instance!

score 0 · Answer 6 · answered May 09 '11 at 14:57

The : syntax allows you to specify arguments for the constructors of class members.

As an example, this class has a member named md3dDevice whose constructor takes an integer (or maybe a pointer) as an argument. Using the : operator you can set that constructor's argument.

Constructor? or something else?

6 Answers6