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From: rmartin@clear.com (Bob Martin)
Newsgroups: comp.lang.c++
Subject: Re: Initializing a Member That is a Class
Message-ID: <1991Feb4.154627.15167@clear.com>
Date: 4 Feb 91 15:46:27 GMT
References: <1991Jan31.194631.3447@persoft.com>
Distribution: na
Organization: Clear Communications, Inc.
Lines: 44

In article <1991Jan31.194631.3447@persoft.com> eda@persoft.com (Ed Almasy) writes:
>This has been puzzling me for some time, and I'm hoping that someone
>here can explain it. 
>
>Assuming:
>    A is a class, with a constructor that requires 3 arguments
>    B is a class, containing a member of type A
>    C is a struct, containing a member of type A
>
>My question is:  When I create data items of type B or C, where do the
>constructor arguments for the member that is of type A come from?  Do I
>have to put them in my declaration for B or C, or is there a way to
>specify them at run time?  If so, what is the proper syntax?
>

Refer to ARM 12.1 concerning constructors and 12.6.2 concerning member
initialization.

You can proceed in one of two ways.  Either create a "default constructor" for
class A.  (You must do this since the compiler wont do it for you if you
have declared other constructors for A).  Then when B or C are constructed
the default constructor will be used for A.

Or you can provide the Constructors of B and C with member initialization
lists which will construct A for you.  You can specify all 3 parameters
in the member intialization list.

	class B
	{
		A theA;
	};

	// B constructor.
	B::B(a,b,c) : theA(a,b,c) // <- member intialization list
	{
		... code for B construction
	}


-- 
+-Robert C. Martin-----+:RRR:::CCC:M:::::M:| Nobody is responsible for |
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