Thread: What is this?

I found the following code snippet in a book, and it solved my initial problem, but now I'm kinda wondering...what does it do?

PHP Code:

struct command_struct
{
    string name;
    bool msg;
    
    //initializes command_struct
    // (1)
    command_struct(string x, bool a) : name(x), msg(a){}
};

command_struct cmdArray[] =
{
    // (2)
    command_struct("say", true),
    command_struct("NULL", false)
}; 


[list=1][*]What does the line right after comment (1) do?[*]What does the line right after comment (2) do?[/list=1]I've tried looking it up, but I'm not having much luck seeing as I'm not real sure what to look under. To me, the line after (2) looks a lot like a parameterized constructor, but the line after (1) has me very stumped. Any ideas?

>What does the line right after comment (1) do?

It's a constructor. Classes & structs in C++ are the same, except that with structs, by default, everything is public, but with classes everything is private.


>What does the line right after comment (2) do?

It's initailising an array of two structs, by calling the constructors.

In C++, we have a choice of syntax for initializing variables in the constuctor. The one in your snippet is called the "preamble", but it does the same thing as this:

command_struct(string x, bool a) {
name = x, msg = a;
}

Okay, thanks. That's what it was acting like it was doing, but I just wanted to make sure. I'd never seen this 'preamble' form before. Learning things every day...

Hello,

In C++, we have a choice of syntax for initializing variables in the constuctor. The one in your snippet is called the "preamble", but it does the same thing as this:

command_struct(string x, bool a) {
name = x, msg = a;
}

Quite wrong. This code will call the default constructor for name then use the assignment operator to assign x to name. It is always better to initialize members in the initializer list. It doesn't matter for built-in types but I suggest doing it for them too to be consistent.

Excellent point.

>It is always better to initialize members in the initializer list.
It's not always better, but it's never worse. The initialization list should be preferred.

-Prelude