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From: leo@atcmp.nl (Leo  Willems)
Newsgroups: comp.lang.c++
Subject: Re: Constructors and new
Message-ID: <523@atcmpe.atcmp.nl>
Date: 1 Jun 89 22:39:24 GMT
References: <DLD.89May31104058@F.GP.CS.CMU.EDU>
Organization: AT Computing, Nijmegen, The Netherlands
Lines: 84

>
>Dave Geary asks why this program:
>
>
>>class testclass
>>{
>>  public:
>>
>>  testclass()   { if(this == 0) puts("Used new..."); else puts("Did not use new..."); }
>> ~testclass()   { puts("I'm a destructor..."); }
>>};
>>
>>main()
>>{
>>  testclass   var;
>>  new testclass;
>>}
>
>produces this output:
>
>>Did not use new...
>>Did not use new...
>>I'm a destructor...

In the paper `the evolution of C++ 1985 to 1987' Bjarne Stroustrup gives
a more detailed example of the way how to use the -this- pointer
in a constructor when allocating memory yourself. (versus par. 5.5.7 in 
the C++ P.L.)
Somewhere (I can't find it  right now) it is mentioned that the
assignment to the -this- pointer in the constructor should be made on
every possible execution path.
The following code does not suffer from the problem above:

extern char*malloc(int);
extern void free(char*);

class testclass
{
private:
	int used_new;
public:
	testclass();
	~testclass(){
		if (used_new){
			free((char*)this);
			this = 0;
				// this assignment must be done
		}
		puts("I'm a destructor...");
	}
};

testclass::testclass()
{
		if (this == 0){
			 puts("Used new...");
			 this = (testclass*) malloc(sizeof(testclass));
				// this assignment must be done
			 used_new = 1;	//assigment after allocation!
		 } else {
			used_new = 0;
		 	puts("Did not use new...");
		 	this = this;
				// this assignment must be done
		 }
}

main()
{
	testclass   var;
	new testclass;
}


This solution leaves me with a question too.

Shouldn't the destructor also assign on every possible path to the
-this- pointer?

Leo Willems				Internet: leo@atcmp.nl
AT Computing				UUCP:     mcvax!hp4nl!kunivv1!atcmpe!leo
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