Assign return value to reference

[Knut Stolze]

Hi,

I'm trying to figure out if the following code is correct or could cause problems in some way...

class MyClass {
...
};

MyClass doSomething() {
...
MyClass obj;
....
return obj;
}

int main() {
MyClass &ret = doSomething();
...
return 0;
}

So function doSomething() returns an object, and "ret" is referring to that object. The question is now whether "ret" refers to a valid object or not?

--
Knut Stolze
Data Warehousing for DB2 z/OS
IBM Germany Research & Development

[anon]

Re: Assign return value to reference

Knut Stolze wrote:

Hi,
>
I'm trying to figure out if the following code is correct or could cause problems in some way...
>
class MyClass {
...
};
>
MyClass doSomething() {
...
MyClass obj;
....
return obj;
}
>
int main() {
MyClass &ret = doSomething();
...
return 0;
}
>
So function doSomething() returns an object, and "ret" is referring to that object. The question is now whether "ret" refers to a valid object or not?
>

Your example doesnt compile. However, I tried to compile this:
class MyClass {
};
MyClass doSomething() {
MyClass obj;
return obj;
}
int main() {
MyClass &ret = doSomething();
return 0;
}

with gcc without any additional options, and I got :
o1.cpp: In function ‘int main()’:
o1.cpp:10: error: invalid initialization of non-const reference of type
‘MyClass&’ from a temporary of type ‘MyClass’


btw don't forget to take a look at this:

http://www.parashift.com/c++-faq-lite/how-to-post.html#faq-5.8

[Leandro Melo]

Re: Assign return value to reference

On 19 ago, 09:13, Knut Stolze <sto...@de.ibm. comwrote:

Hi,
>
I'm trying to figure out if the following code is correct or could cause problems in some way...
>
class MyClass {
    ...
>
};
>
MyClass doSomething() {
    ...
    MyClass obj;
    ....
    return obj;
>
}
>
int main() {
    MyClass &ret = doSomething();
    ...
    return 0;
>
}
>
So function doSomething() returns an object, and "ret" is referring to that object.  The question is now whether "ret" refers to a valid object ornot?

Hi.

It does not. In fact, it should give you a compiler error (or at least
a warning) because the return of doSomething is temporary. Using a
const-ref in this case is allowed because the life-time of the copy
will be extended.

int main()
{
const MyClass &ret = doSomething(); //Notice the const.
...
return 0;
}

--
Leandro T. C. Melo

[Knut Stolze]

Re: Assign return value to reference

Leandro Melo wrote:

On 19 ago, 09:13, Knut Stolze <sto...@de.ibm. comwrote:

>Hi,
>>
>I'm trying to figure out if the following code is correct or could cause
>problems in some way...
>>
>class MyClass {
>...
>>
>};
>>
>MyClass doSomething() {
>...
>MyClass obj;
>....
>return obj;
>>
>}
>>
>int main() {
>MyClass &ret = doSomething();
>...
>return 0;
>>
>}
>>
>So function doSomething() returns an object, and "ret" is referring to
>that object. The question is now whether "ret" refers to a valid object
>or not?

>
Hi.
>
It does not. In fact, it should give you a compiler error (or at least
a warning) because the return of doSomething is temporary. Using a
const-ref in this case is allowed because the life-time of the copy
will be extended.
>
int main()
{
const MyClass &ret = doSomething(); //Notice the const.
...
return 0;
}

Sorry, my fault. I should have written a short program to demonstrate the issue.

Where can I find the details (in the C++ standard) on the life-time extension of the temporary copy?

--
Knut Stolze
Data Warehousing for DB2 z/OS
IBM Germany Research & Development

[Juha Nieminen]

Re: Assign return value to reference

Leandro Melo wrote:

const MyClass &ret = doSomething(); //Notice the const.

AFAIK, in practice there's absolutely no difference between that line
and this one:

const MyClass ret = doSomething();

I could even guess most compilers will probably generate the exact
same machine code from both (although some compilers might not, ie. they
might allocate the object at different parts depending on the situation,
but the effect will nevertheless be exactly the same).

[Leandro Melo]

Re: Assign return value to reference

On 19 ago, 12:17, Juha Nieminen <nos...@thanks. invalidwrote:

Leandro Melo wrote:

     const MyClass &ret = doSomething(); //Notice the const.

>
  AFAIK, in practice there's absolutely no difference between that line
and this one:
>
     const MyClass ret = doSomething();
>
  I could even guess most compilers will probably generate the exact
same machine code from both (although some compilers might not, ie. they
might allocate the object at different parts depending on the situation,
but the effect will nevertheless be exactly the same).

So could I ;) For this particular case it will probably not make a
difference from a practical point of view. But I wanted to show that
const refs can be used to extend the life-time of temporary objects,
which can be useful in other situations.

--
Leandro T. C. Melo

[Thomas J. Gritzan]

Re: Assign return value to reference

Juha Nieminen schrieb:

Leandro Melo wrote:

> const MyClass &ret = doSomething(); //Notice the const.

>
AFAIK, in practice there's absolutely no difference between that line
and this one:
>
const MyClass ret = doSomething();
>
I could even guess most compilers will probably generate the exact
same machine code from both (although some compilers might not, ie. they
might allocate the object at different parts depending on the situation,
but the effect will nevertheless be exactly the same).

It might generate the same machine code in this case, but if you don't
use a reference and you have a different type for the local variable
than the return type, the return value gets sliced.

Example:

class Base {};
class MyClass : public Base {};
MyClass doSomething() {
return MyClass();
}
int main() {
const Base base1 = doSomething();
const Base &base2 = doSomething();
return 0;
}

base1 actually *is* a Base, base2 references an object of type MyClass.

--
Thomas