What Does :x(a) Mean in a C++ Constructor?

[yungman]

I am copying some program from youtube videos. This is the program I copied:

#include <iostream>
using namespace std;
class Base
{ int x;
  public:  Base() {} Base(int a) :x(a) {  }
           int get(){return x;}
           void set(int a){x=a;}
};
int main()
{    Base b1; Base b2(10);
    b2.get();    b2.set(20);    return 0;
}

I don't know what is :x(a) in line 5 .

I really don't even understand line 5, Base() {} Base(int a) :x(a) { } what is this?

Never learn this, don't even know the name to search online. Please help

Thanks

 

[DaveC426913]

Have you tried Googling the line itself?
"public: Base() {} Base(int a) :x(a) { }"

I suspect that it is shorthand for some sort of empty class but my C++ is very rusty.

public: 
    Base()
    {
    
    } Base(int a) :x(a)
      {
      
      }

 

[phyzguy]

The x(a) after the colon is called an initializer list. It initializes the variable x to have the value a. Below are some links with an explanation.

https://en.cppreference.com/w/cpp/language/constructor

http://www.cs.technion.ac.il/users/yechiel/c++-faq/init-lists.html

 

[yungman]

The x(a) after the colon is called an initializer list. It initializes the variable x to have the value a. Below are some links with an explanation.

https://en.cppreference.com/w/cpp/language/constructor

http://www.cs.technion.ac.il/users/yechiel/c++-faq/init-lists.html

Thanks I got it, I also watch this video after I learn from you it's called initialization list. This is very good:


Thank you.

 

[Mark44]

I suspect that it is shorthand for some sort of empty class but my C++ is very rusty.

No, it's not. That line merely initializes the x member to the value that a represents. Just as @phyzguy said.

 

[jtbell]

The reason the empty braces are there is that a constructor has to have a function body, even if there are no executable statetments.

 

[yungman]

The reason the empty braces are there is that a constructor has to have a function body, even if there are no executable statetments.

Just want to say thank you for your thread on ThreeVector. I really learn a lot so far and I am still working on it. I take where you start and run with it to see what should and what should not be done and why.

Thanks