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[MG-Zero]

MG-Zero

Confused about this syntax >_>

I've been confused about this one for quite some time now.  None of the textbooks I've used have ever explained what it means.  The line in question is:  MyClass(int i) : m_i(i) {}

Code: [Select]

class MyClass {
public:
   MyClass(){}
   MyClass(int i) : m_i(i) {}

private:
   int m_i;
};


i love big weenies and i cannot lie

[Hero of Vortex]

Hero of Vortex

Re: Confused about this syntax >_>

I'm not entirely sure what this specific case means, but I know something similar that might help. When you have a child class and you want to call the parent class' constructor and pass it some variables, you'd do something like

Code: [Select]

childConstructor(int variable) : parentConstructor(variable)
{
       // Child constructor code
}
I guess here it's the same thing, only m_i is not actually related or something. Again, I'm not sure, I just noticed that it looked similar. I hope that helps.

No one located in this area of the internet remembers me.

[MG-Zero]

MG-Zero

Re: Confused about this syntax >_>

Now, see that makes sense, but what I don't get is that m_i is a variable, yet it appears to have a parameter passed to it.

i love big weenies and i cannot lie

[4Sword]

4Sword

Re: Confused about this syntax >_>

The easiest way to clear confusion is to test the code yourself. Thinking about all it could do also helps, anyway running this:

Code: [Select]

#include <iostream>

using std::cin;
using std::cout;
using std::endl;

class MyClass
{
  public:
    MyClass();
    MyClass(int i) : m_i(i) {}
    void show_mi(){ cout << m_i; }
  private:
    int m_i;
};

int main(){
  MyClass Test(3);
  Test.show_mi();
  system("PAUSE");
  return 0;
};
I figured out that it sets the variable.

It's probably also listed somewhere in here: http://www.research.att.com/~bs/C++.html

[Infinitus]

Infinitus

Re: Confused about this syntax >_>

Its pretty much just a shorthand way of setting fields. Nothing particularly intresting about it.

[TheDarkJay]

TheDarkJay

Re: Confused about this syntax >_>

It invokes the constructor, not the assignment. Whilst this means nothing really for variables like integers, this can be all the difference in the world for classes.

It's like doing: int ten(10);
instead of int ten = 10;

[MG-Zero]

MG-Zero

Re: Confused about this syntax >_>

Oh....I..don't quite see the point of that..but at least that clears it up.  Thanks =)

i love big weenies and i cannot lie

[TheDarkJay]

TheDarkJay

Re: Confused about this syntax >_>

Well it looks a bit neater according to some people, especially if otherwise your constructor would be empty, and like I said for objects can be vital.

Also it's the only way to assign value to constant variables in a class (i.e if instead of int you have a const int).