Memory allocation related question

[Mladen Adamovic]

Why is memory safe that function allocate inner object and return its
result? That inner object is destroyed when function come to the
"return" statement. Is default copy contructor doing some job? Can
somebody explain me what exactly is happening in the following code when
it invoke boo myres=calculate Something(12);

#include <iostream>
#include <cstdlib>
#include <math.h>

using namespace std;

class boo {
public:
int intValue;
double doubleValue;
};

boo calculateSometh ing(int arg) {
boo result;
result.intValue =arg;
result.doubleVa lue=arg*sqrt(ar g);
return result;
}
int main(int argc, char *argv[])
{
cout << "Hello, world!" << endl;
boo myres=calculate Something(12);
cout << myres.intValue << endl;
cout << myres.doubleVal ue << endl;
return EXIT_SUCCESS;
}

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Mladen Adamovic
http://home.blic.net/adamm
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[Victor Bazarov]

Mladen Adamovic wrote:

Why is memory safe that function allocate inner object and return its
result?
In your example below, the object is returned by value. A _temporary_
object is created [somewhere] and used when function returns.
That inner object is destroyed when function come to the
"return" statement.
Yes.
Is default copy contructor doing some job?
Yes. The default one or the one that you declare/define.
Can
somebody explain me what exactly is happening in the following code when
it invoke boo myres=calculate Something(12);
What book on C++ are you reading that doesn't explain copy-construction
upon returning from a function?
#include <iostream>
#include <cstdlib>
#include <math.h>

using namespace std;

class boo {
public:
int intValue;
double doubleValue;
};

boo calculateSometh ing(int arg) {
boo result;
result.intValue =arg;
result.doubleVa lue=arg*sqrt(ar g);
return result;
}
int main(int argc, char *argv[])
{
cout << "Hello, world!" << endl;
boo myres=calculate Something(12);
(a) Memory for 'myres' is allocated.
(b) 'calculateSomet hing(12)' is called.
(c) Return value from 'calculateSomet hing' is a temporary object
(d) A temporary 'boo' object is constructed from the returned temporary
(d.1) The temporary returned from 'calculateSomet hing' is destroyed
(e) 'myres' is copy-initialised from a temporary constructed in (d)
(e.1) The temporary used to construct 'myres' is destroyed

The compiler is allowed to optimize away step (d) and construct 'myres'
as if the declaration statement were written:

boo myres(calculate Something(12));

Moreover, the compiler is allowed to forgo creation of a temporary when
'calculateSomet hing' is returning and construct the return value in the
'myres' object _directly_.
cout << myres.intValue << endl;
cout << myres.doubleVal ue << endl;
return EXIT_SUCCESS;
}

V
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