Initializing a member class of an object using a non-default constructor in C++

Question

I have a specific situation where I've got an object that I want to use the boost random number generators on, and it has lead to a greater question which I cannot seem to answer. Here is the example code of what I'm trying to produce.

First, my header:

Class MyObject {

 protected:
    double some variable;
    boost::random::mt19937 rgenerator;
    boost::uniform_real<double> dist_0_1;
    boost::variate_generator< boost::mt19937&, boost::uniform_real<double> > rand01
}

Now what I want to do is:

Class MyObject {

 protected:
    double some variable;

    boost::random::mt19937 rgenerator(std::time(0)); //initialize to a "random" seed
    boost::uniform_real<double> dist_0_1(0,1); //set the distribution to 0-1
    boost::variate_generator< boost::mt19937&, boost::uniform_real<double> > rand01(rgenerator, dist_0_1);//tell it to use the above two objects
}

But this doesn't work because it is in a header. I thought I could use the constructor of MyObject to somehow call the constructors on the various sub-objects (distribution, generator, but I can't figure out how. By the time the constructor of MyObject is called, the sub-objects' default constructors have already been called, and I haven't found that they have member methods to reset these properties... besides which, that isn't the point where I am confused. Now maybe there are too many things going on and I'm confusing issues, but as far as I can tell, my problem reduces to this following, childish example:

Class Tree {

    Tree();
    Tree(int);

    protected: 

        fruit apples(int);
}

Tree::Tree() {
    apples(0); //won't work because we can't call the constructor again?
}

Tree::Tree(int fruit_num) {
    apples(fruit_num); //won't work because we can't call the constructor again?
}

Class Fruit {

    public:
        Fruit();
        Fruit(int);

    protected:
        int number_of_fruit;

}

Fruit::Fruit() {

    number_of_fruit = 0;
}

Fruit::Fruit(int number) {

    number_of_fruit = number;

}

I'm sure this is second nature to everyone else out there, but I can't find an article that talks about the best practice for initializing member objects of an object to a non-default constructor value.

Answer 1

What you want is an initializer list. For example:

Tree::Tree(int fruit_num) 
    : apples(fruit_num) // Initializes "apple" with "fruit_num"
{
}

You simply add a colon (:) after the constructor parameters and before the opening brace {. You can separate different member constructors with commas (,). Example:

Tree::Tree(int fruit1, int fruit2) : apples(fruit1), bananas(fruit2) {
}

Answer 2

The answer to this question is relatively straightforwards. You use initializer lists.

Here's an example:

class MyClass {
private:
     SomeOtherType sot;
public:
    MyClass() : sot(parametersForConstructorOfSOT) {}
};

You can extend this to multiple member objects, of course; it can also be used to call the constructor of a parent class to initialize private entries in a parent class if you need to do something like that.

Answer 3

You're so close! Just use the initialiser list syntax:

Tree::Tree() : apples(0) {
    // ...
}

Tree::Tree(int fruit_num) : apples(fruit_num) {
    // ...
}