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?

Answer 1

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) {}
};