How does this constructor work

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Hi,

I have come across a code when I was reading a C++ primer which looks the following:

class MyIntArray {
  MyIntArray(int size = DefaultSize){}
 private:
 const static DefaultSize = 12;
};

My doubts are the followng:
->Its mentioned in the book that it is a default constructor,but how can it have an argument?
->How does that code in parenthesis work?

Thaks in advance.

Posted 19-May-11 3:43am

hakz.code

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4 solutions

Solution 2

To add to what Manfred said, it is a default constructor: it is just a default constructor with a optional argument. If you omit the argument, it is a default constructor. It you don't, it is a parametrised constructor.

Any parameter in C++ can be optional, provided there are no non-optional parameters to the right of it. All you have to do it provide the default value as part of the function declaration.
MSDN[^] has a reasonable explanation.

Posted 19-May-11 4:08am

OriginalGriff

Solution 1

When the parameter size is omitted it will be set to DefaultSize which is twelfe (12) in this case. The code block of the constructor which you depicted is empty though which would mean it does nothing (AFAICS).

Cheers!

-MRB

Posted 19-May-11 3:54am

Manfred Rudolf Bihy

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Albert Holguin · Accepted Answer · 2011-05-19T04:16:00

Solution 3

IBM:
A default constructor is a constructor that either has no parameters, or if it has parameters, all the parameters have default values.

http://publib.boulder.ibm.com/infocenter/comphelp/v8v101/index.jsp?topic=%2Fcom.ibm.xlcpp8a.doc%2Flanguage%2Fref%2Fcplr376.htm[^]

Posted 19-May-11 4:16am

Albert Holguin

Comments

hakz.code 20-May-11 0:44am

Thank you for the link,it cleared all my doubts about this constructor.

Albert Holguin 20-May-11 9:47am

you're welcome :)

Stefan_Lang · Accepted Answer · 2011-05-19T04:57:00

While your questions have been thoroughly answered above, I'd like to point out that if this code is a literal transcript of that primer you are reading, you'd best discard it at once and get some decent book or other online source:

1. You can't define a const value right inside the class definition. If that definition is inside a header and that header may be included from multiple locations, as is normally the case. Defined like that, you'll get linker errors.

2. Not only the const, but also the constructor is defined private. I doubt that was the intention.

3. I would expect the constructor to do some trivial initialization, using the argument. Instead it does nothing, raising questions about the validity and purpose of that code example.

4. I'd expect a decent primer to explain what default arguments are if it uses them in example code. Judging by your question it doesn't.

It's not that I'm expecting a full-fledged introduction into C++ from a simple primer, but it seems wrong to provide examples that wouldn't even compile & link. How can you get a good impression of the functionality and capabilities of C++ if the explanations are plain wrong?