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Class Constructor Shorthand

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Class Constructor Shorthand

Jan 19th, 2006

I've been reading through a beginning chapter on classes and I have come upon this below:

class Critter
{
public:
Critter(const string& name = ""): m_Name(name) {}
...
};

I was wondering why they wrote the constructor this way. If anyone can point me to some info, I would be very happy.

If in doubt, reach into the trash can and remove the user guide.

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Re: Class Constructor Shorthand

Jan 19th, 2006

Constructors can initialise through assignment operator or initialisation lists. Benefit of using initialisation lists is the improved performance. Your code will run faster if you use initialization lists rather than assignment.

http://www.parashift.com/c++-faq-lit....html#faq-10.6

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Re: Class Constructor Shorthand

Jan 22nd, 2006

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Originally Posted by sunnypalsingh

Constructors can initialise through assignment operator or initialisation lists. Benefit of using initialisation lists is the improved performance. Your code will run faster if you use initialization lists rather than assignment.

http://www.parashift.com/c++-faq-lit....html#faq-10.6

I know what constructors do, that wasn't my question. My question was why did they write it this way.

Is it shorthand? If so, is there a long way of writing it?

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Re: Class Constructor Shorthand

Jan 22nd, 2006

I answered your question correctly

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Re: Class Constructor Shorthand

Jan 22nd, 2006

I believe you are reading too far into my question. I appreciate your help. Let me define my question more specifically.

One person defined a class constructor this way:

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 C++ Syntax (Toggle Plain Text) 
Critter::Critter(int hunger, int boredom):
     m_Hunger(hunger),
     m_Boredom(boredom)
{}
Critter::Critter(int hunger, int boredom):
     m_Hunger(hunger),
     m_Boredom(boredom)
{}

Another person defined a class constructor this way:

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 C++ Syntax (Toggle Plain Text) 
Box::Box(int ht, int wd, int dp)
{
    height = ht;
    width = wd;
    depth = dp;
}
Box::Box(int ht, int wd, int dp)
{
    height = ht;
    width = wd;
    depth = dp;
}

Besides the fact the two constructors are from different classes. What is the difference of writing it the first way apposed of writing it the second way?

I noticed that the first initializes the members after a colon and the second initializes the members inside the curley braces. So what is the difference?

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Re: Class Constructor Shorthand

Jan 22nd, 2006

Oh man you didn't read the link....
You can do both ways...first one through initilisation lists and second one is through assignment operator.
First one is better...for more information read my first post.

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Re: Class Constructor Shorthand

Jan 22nd, 2006

Oh so my first post was using an initializing list. I was under the impression that it was the same as an assignment constructor, just written in "shorthand" I thought. So when you use an assignment constructor it passes a temporary object that isn't efficient. Yet an initializing list does it all in one instance. Cool

One last question about the initializing list.

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Fred::Fred() : x_(whatever) {}
Fred::Fred() : x_(whatever) {}

When you create your Fred object, will anything that is passed to whatever be assigned to x_ using the initializing list?

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Re: Class Constructor Shorthand

Jan 22nd, 2006

>will anything that is passed to whatever be assigned to x_ using the initializing list?
Assuming whatever is a compatible initializer for x_, yes.

I'm here to prove you wrong.

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Re: Class Constructor Shorthand

Jan 22nd, 2006

Another small thing to add that I read in the book C++ in Action .
Say you have a const member declared inside the C++ class.

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 C++ Syntax (Toggle Plain Text) 
class Planet
{
public:
         Planet( int iWeight);
private:
         const int weight;
}
class Planet
{
public:
         Planet( int iWeight);
private:
         const int weight;
}

since the weight member cant be assigned a value in a standard contructor like this

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Planet::Planet( int iWeight)
{
       weight = iWeight ; // wrong
}
Planet::Planet( int iWeight)
{
       weight = iWeight ; // wrong
}

The only way you can do that is using an initialization list.

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 C++ Syntax (Toggle Plain Text) 
Planet::Planet( int iWeight):weight(iWeight) // correct
{
}
Planet::Planet( int iWeight):weight(iWeight) // correct
{
}

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Re: Class Constructor Shorthand

#10

Jan 22nd, 2006

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Originally Posted by WolfPack

Another small thing to add that I read in the book C++ in Action .
Say you have a const member declared inside the C++ class.
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C++ Syntax (Toggle Plain Text)
class Planet
{
public:
         Planet( int iWeight);
private:
         const int weight;
}
class Planet { public: Planet( int iWeight); private: const int weight; }
since the weight member cant be assigned a value in a standard contructor like this
Help with Code Tags

C++ Syntax (Toggle Plain Text)
Planet::Planet( int iWeight)
{
       weight = iWeight ; // wrong
}
Planet::Planet( int iWeight) { weight = iWeight ; // wrong }
The only way you can do that is using an initialization list.
Help with Code Tags

C++ Syntax (Toggle Plain Text)
Planet::Planet( int iWeight):weight(iWeight) // correct
{
}
Planet::Planet( int iWeight):weight(iWeight) // correct { }

Since weight is of Private Access you can't directly initialize it.

So an Initialization list has the ability to indirectly access the data member weight and maintain encapsulation, right?

If in doubt, reach into the trash can and remove the user guide.