Constructor return value?

[Bonj]

OK I know a constructor can't have a return value but is it feasible such that I have a class whereby
//in MyClass1.
class MyClass

MyClass1(){}
~MyClass1(){}
MyClass1(int & Result) {Result = InitMyClass()}

int InitMyClass()


//in main.cp

int main(

int result
MyClass theobject(resul t)
printf("The result of initializing the class is %d", result)

[Carl Daniel [VC++ MVP]]

Re: Constructor return value?

Bonj wrote:[color=blue]
> OK I know a constructor can't have a return value but is it feasible
> such that I have a class whereby: //in MyClass1.h
> class MyClass1
> {
> MyClass1(){};
> ~MyClass1(){};
> MyClass1(int & Result) {Result = InitMyClass()};
>
> int InitMyClass();
> }
>
> //in main.cpp
>
> int main()
> {
> int result;
> MyClass theobject(resul t);
> printf("The result of initializing the class is %d", result);
> }[/color]

Of course it's feasible, and perfectly legal.

The Standard C++ way of indicating failure in a constructor is to throw an
exception, but there might be reasons sometimes that you don't want to do
that (the only one that comes to mind is a severe performance constraint -
exception throwing is expensive).

-cd

[Simon Trew]

Re: Constructor return value?

There is another reason not to throw out of a constructor: you don't know
how well or badly the object is constructed, so you can't clean up. This is
most important if the constructor allocates resources (e.g. memory)
dynamically.

e.g.

struct C
{
bool *b1, b2;
C() { b1 = b(); b2 = b();}
bool *b() { if (rand() < 0.5) throw true; else return new b; }
};

int main()
{
C* c;
try
{
c = new C();
}
catch (...)
{
// now, what might c point to? What might c->b1 or c->b2 point to?
}
return;
}

"Carl Daniel [VC++ MVP]" <cpdaniel_remov e_this_and_nosp am@mvps.org.nos pam>
wrote in message news:%23j08xc%2 37DHA.2432@TK2M SFTNGP10.phx.gb l...[color=blue]
> Bonj wrote:[color=green]
> > OK I know a constructor can't have a return value but is it feasible
> > such that I have a class whereby: //in MyClass1.h
> > class MyClass1
> > {
> > MyClass1(){};
> > ~MyClass1(){};
> > MyClass1(int & Result) {Result = InitMyClass()};
> >
> > int InitMyClass();
> > }
> >
> > //in main.cpp
> >
> > int main()
> > {
> > int result;
> > MyClass theobject(resul t);
> > printf("The result of initializing the class is %d", result);
> > }[/color]
>
> Of course it's feasible, and perfectly legal.
>
> The Standard C++ way of indicating failure in a constructor is to throw an
> exception, but there might be reasons sometimes that you don't want to do
> that (the only one that comes to mind is a severe performance constraint -
> exception throwing is expensive).
>
> -cd
>
>[/color]

[Carl Daniel [VC++ MVP]]

Re: Constructor return value?

This situation should be handled by following the RAII principle: Resource
Acquisition Is Initialization.

struct C
{
boost::scoped_p tr<bool> b1, b2;
C() : b1(b()), b2(b()) {}
bool *b() { if (rand() < 0.5) throw true; else return new b; }
};

Now if an exception is thrown you're guaranteed that if b1 was initialized,
it'll be destructed.

-cd

Simon Trew wrote:[color=blue]
> There is another reason not to throw out of a constructor: you don't
> know how well or badly the object is constructed, so you can't clean
> up. This is most important if the constructor allocates resources
> (e.g. memory) dynamically.
>
> e.g.
>
> struct C
> {
> bool *b1, b2;
> C() { b1 = b(); b2 = b();}
> bool *b() { if (rand() < 0.5) throw true; else return new b; }
> };
>
> int main()
> {
> C* c;
> try
> {
> c = new C();
> }
> catch (...)
> {
> // now, what might c point to? What might c->b1 or c->b2
> point to? }
> return;
> }
>
> "Carl Daniel [VC++ MVP]"
> <cpdaniel_remov e_this_and_nosp am@mvps.org.nos pam> wrote in message
> news:%23j08xc%2 37DHA.2432@TK2M SFTNGP10.phx.gb l...[color=green]
>> Bonj wrote:[color=darkred]
>>> OK I know a constructor can't have a return value but is it feasible
>>> such that I have a class whereby: //in MyClass1.h
>>> class MyClass1
>>> {
>>> MyClass1(){};
>>> ~MyClass1(){};
>>> MyClass1(int & Result) {Result = InitMyClass()};
>>>
>>> int InitMyClass();
>>> }
>>>
>>> //in main.cpp
>>>
>>> int main()
>>> {
>>> int result;
>>> MyClass theobject(resul t);
>>> printf("The result of initializing the class is %d", result);
>>> }[/color]
>>
>> Of course it's feasible, and perfectly legal.
>>
>> The Standard C++ way of indicating failure in a constructor is to
>> throw an exception, but there might be reasons sometimes that you
>> don't want to do that (the only one that comes to mind is a severe
>> performance constraint - exception throwing is expensive).
>>
>> -cd[/color][/color]

[Simon Trew]

Re: Constructor return value?

Absolutely, but (unless I am thinking fuzzily as usual) that only moves it
down a peg: we can only construct members that themselves obey the RAII
principle. (If boost::scoped_p tr's constructor threw after allocating some
resource then we'd be back to square one.)

S.

"Carl Daniel [VC++ MVP]" <cpdaniel_remov e_this_and_nosp am@mvps.org.nos pam>
wrote in message news:OzfhZ%23J8 DHA.632@TK2MSFT NGP12.phx.gbl.. .[color=blue]
> This situation should be handled by following the RAII principle:[/color]
Resource[color=blue]
> Acquisition Is Initialization.
>
> struct C
> {
> boost::scoped_p tr<bool> b1, b2;
> C() : b1(b()), b2(b()) {}
> bool *b() { if (rand() < 0.5) throw true; else return new b; }
> };
>
> Now if an exception is thrown you're guaranteed that if b1 was[/color]
initialized,[color=blue]
> it'll be destructed.
>
> -cd
>
> Simon Trew wrote:[color=green]
> > There is another reason not to throw out of a constructor: you don't
> > know how well or badly the object is constructed, so you can't clean
> > up. This is most important if the constructor allocates resources
> > (e.g. memory) dynamically.
> >
> > e.g.
> >
> > struct C
> > {
> > bool *b1, b2;
> > C() { b1 = b(); b2 = b();}
> > bool *b() { if (rand() < 0.5) throw true; else return new b; }
> > };
> >
> > int main()
> > {
> > C* c;
> > try
> > {
> > c = new C();
> > }
> > catch (...)
> > {
> > // now, what might c point to? What might c->b1 or c->b2
> > point to? }
> > return;
> > }
> >
> > "Carl Daniel [VC++ MVP]"
> > <cpdaniel_remov e_this_and_nosp am@mvps.org.nos pam> wrote in message
> > news:%23j08xc%2 37DHA.2432@TK2M SFTNGP10.phx.gb l...[color=darkred]
> >> Bonj wrote:
> >>> OK I know a constructor can't have a return value but is it feasible
> >>> such that I have a class whereby: //in MyClass1.h
> >>> class MyClass1
> >>> {
> >>> MyClass1(){};
> >>> ~MyClass1(){};
> >>> MyClass1(int & Result) {Result = InitMyClass()};
> >>>
> >>> int InitMyClass();
> >>> }
> >>>
> >>> //in main.cpp
> >>>
> >>> int main()
> >>> {
> >>> int result;
> >>> MyClass theobject(resul t);
> >>> printf("The result of initializing the class is %d", result);
> >>> }
> >>
> >> Of course it's feasible, and perfectly legal.
> >>
> >> The Standard C++ way of indicating failure in a constructor is to
> >> throw an exception, but there might be reasons sometimes that you
> >> don't want to do that (the only one that comes to mind is a severe
> >> performance constraint - exception throwing is expensive).
> >>
> >> -cd[/color][/color]
>
>[/color]

[Carl Daniel [VC++ MVP]]

Re: Constructor return value?

Simon Trew wrote:[color=blue]
> Absolutely, but (unless I am thinking fuzzily as usual) that only
> moves it down a peg: we can only construct members that themselves
> obey the RAII principle. (If boost::scoped_p tr's constructor threw
> after allocating some resource then we'd be back to square one.)[/color]

Sure. But properly designed RAII containers (like boost::scoped_p tr) will
behave correctly. In the example given, the only thing that can throw is
the new expression, and if that throws boost::scoped_p tr's constructor will
never be entered. If new doesn't throw and scoped_ptr's constructor is
entered, it must either properly recover from any exception or (in reality)
promise not to throw.

-cd

[Doug Harrison [MVP]]

Re: Constructor return value?

Simon Trew wrote:
[color=blue]
>Absolutely, but (unless I am thinking fuzzily as usual) that only moves it
>down a peg: we can only construct members that themselves obey the RAII
>principle.[/color]

Well, yes, you gotta start somewhere. :) But once you have a set of nice,
exception-safe, low-level classes such as std::string, std::vector, various
smart pointers, etc, you can build your own classes in terms of them and not
worry about them leaking.
[color=blue]
>(If boost::scoped_p tr's constructor threw after allocating some
>resource then we'd be back to square one.)[/color]

That's a problem only if you have a mix of exception-safe and unsafe
members. I can't remember the last time I used a raw pointer as a class
member, and that pointer owned the data it pointed to.

--
Doug Harrison
Microsoft MVP - Visual C++

[Simon Trew]

Re: Constructor return value?

Yep, I was not suggesting that boost::scoped_p tr behaved anything other than
properly.

"Carl Daniel [VC++ MVP]" <cpdaniel_remov e_this_and_nosp am@mvps.org.nos pam>
wrote in message news:%23ug7PDM8 DHA.2952@TK2MSF TNGP09.phx.gbl. ..[color=blue]
> Sure. But properly designed RAII containers (like boost::scoped_p tr) will
> behave correctly.[/color]