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Paul Rosen
Guest
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 Posted: Wed Oct 26, 2005 11:15 pm Post subject: trick to access derived members in base constructor |
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I have something like this:
[Note: This code is truncated as much as possible to illustrate the
problem.]
typedef std::vector<std::string> VStr;
class MyBase
{
public:
explicit MyBase(VStr& vstr) { /* initialize vstr */ };
};
class MyDerived
{
public:
explicit MyDerived(VStr& vstr)
: MyBase(vstr)
{};
};
Now, that all works fine. I've recently needed to create another derived
class, though, that needs the vector as a member instead. I'd like to do
this:
class MyDerived2
{
public:
MyDerived2()
: MyBase(m_vstr)
{};
private:
VStr m_vstr;
};
But, of course, m_vstr isn't initialized at the time that MyBase's ctor
is called.
I'd rather not change MyBase because it is used in a number of places
already.
What I did to get around the problem is the following ugly hack. Can
anyone suggest anything that is thread safe and prettier?
static VStr s_vstrScratch;
class MyDerived2
{
public:
MyDerived2()
: MyBase(s_vstrScratch)
, m_vstr(s_vstrScratch)
{};
private:
VStr m_vstr;
};
Thanks!
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Alf P. Steinbach
Guest
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| Posted: Thu Oct 27, 2005 10:31 am Post subject: Re: trick to access derived members in base constructor |
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|
* Paul Rosen:
| Quote: | I have something like this:
[Note: This code is truncated as much as possible to illustrate the
problem.]
typedef std::vector<std::string> VStr;
class MyBase
{
public:
explicit MyBase(VStr& vstr) { /* initialize vstr */ };
};
class MyDerived
{
public:
explicit MyDerived(VStr& vstr)
: MyBase(vstr)
{};
};
Now, that all works fine.
|
Be aware that "works" is not the same as "sound design" or "robust
against maintenance" or even "understandable".
The code you currently have in the MyBase constructor should not be
there: it has nothing to do with the construction of a MyBase object.
It should probably be a freestanding helper function.
| Quote: | I've recently needed to create another derived
class, though, that needs the vector as a member instead. I'd like to do
this:
class MyDerived2
{
public:
MyDerived2()
: MyBase(m_vstr)
{};
private:
VStr m_vstr;
};
But, of course, m_vstr isn't initialized at the time that MyBase's ctor
is called.
|
Right, but equally true, m_vstr _is_ guaranteed initialized when
MyDerived2's constructor body is executed, and that's a good place to
call the freestanding function you should have had.
If you're not overly concerned with efficiency you can, alternatively,
make that a function that returns a VStr, and call it in MyDerived2's
constructor initializer list.
If that turns out to be magnitudes too slow, you can then change the
function's result type, and the type of m_vstr, to std::auto_ptr<VStr>
(where you discover how smart it was to only ever access that member via
a getter function, in the rest of the code), but personally I'd go for
the call in the constructor body instead.
| Quote: | I'd rather not change MyBase because it is used in a number of places
already.
|
That's why you should change MyBase immediately, or better, excise it
from your system, torch-burn it, and give the ashes to a homeopath (so
they'll be sufficiently diluted to never again rise like Terminator II).
| Quote: | What I did to get around the problem is the following ugly hack. Can
anyone suggest anything that is thread safe and prettier?
|
See above.
Hth.,
- Alf
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
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Ulrich Eckhardt
Guest
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| Posted: Thu Oct 27, 2005 10:34 am Post subject: Re: trick to access derived members in base constructor |
|
|
Paul Rosen wrote:
| Quote: | typedef std::vector<std::string> VStr;
class MyBase
{
public:
explicit MyBase(VStr& vstr) { /* initialize vstr */ };
};
[...]
static VStr s_vstrScratch;
class MyDerived2
{
public:
MyDerived2()
: MyBase(s_vstrScratch)
, m_vstr(s_vstrScratch)
{};
private:
VStr m_vstr;
};
|
Hmmm, if you can pass a scratch object, it seems like it is not really
needed, so I guess the whole design smells a bit fishy.
Other than that, you could use the fact that base-classes are initialized
in their order and put the vector in a baseclass. IIRC, Boost even has a
helper for that, but can't remember its name.
--
Questions ?
see C++-FAQ Lite: http://parashift.com/c++-faq-lite/ first !
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Daniel Krügler
Guest
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| Posted: Thu Oct 27, 2005 10:36 am Post subject: Re: trick to access derived members in base constructor |
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|
Paul Rosen wrote:
| Quote: | I have something like this:
[Note: This code is truncated as much as possible to illustrate the
problem.]
typedef std::vector<std::string> VStr;
class MyBase
{
public:
explicit MyBase(VStr& vstr) { /* initialize vstr */ };
};
class MyDerived
{
public:
explicit MyDerived(VStr& vstr)
: MyBase(vstr)
{};
};
|
I assume you meant:
class MyDerived : public MyBase
{
.....
};
| Quote: |
Now, that all works fine. I've recently needed to create another derived
class, though, that needs the vector as a member instead. I'd like to do
this:
class MyDerived2
{
public:
MyDerived2()
: MyBase(m_vstr)
{};
private:
VStr m_vstr;
};
|
I assume you meant:
class MyDerived2 : public MyBase
{
.....
};
| Quote: | I'd rather not change MyBase because it is used in a number of places
already.
What I did to get around the problem is the following ugly hack. Can
anyone suggest anything that is thread safe and prettier?
static VStr s_vstrScratch;
class MyDerived2
{
public:
MyDerived2()
: MyBase(s_vstrScratch)
, m_vstr(s_vstrScratch)
{};
private:
VStr m_vstr;
};
|
This workaround not only has thread-safety problems, but also is
sensitive to the order-of-initialization problem (consider the situation
that there somewhere exists a global MyDerived2 instance...)
You can do the following:
class ArrayWrapper {
protected:
VStr m_vstr;
};
class MyDerived2 : private ArrayWrapper, public MyBase
{
public:
MyDerived2() :
ArrayWrapper(),
MyBase(m_vstr) // It is ok now to access m_vstr in the base class!
{
}
};
Boost provides a helper class template, base_from_member, which
supports this special kind of initialization problem and can even be
used for multiple member situations.
Greetings from Bremen,
Daniel Krügler
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Yuri Khan
Guest
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| Posted: Thu Oct 27, 2005 10:38 am Post subject: Re: trick to access derived members in base constructor |
|
|
Paul Rosen wrote:
| Quote: | I have something like this:
typedef std::vector<std::string> VStr;
class MyBase
{
public:
explicit MyBase(VStr& vstr) { /* initialize vstr */ };
};
Now, that all works fine. I've recently needed to create another derived
class, though, that needs the vector as a member instead. I'd like to do
this:
class MyDerived2 : public MyBase
{
public:
MyDerived2()
: MyBase(m_vstr)
{};
private:
VStr m_vstr;
};
But, of course, m_vstr isn't initialized at the time that MyBase's ctor
is called.
|
As you've pointed out, the m_vstr does not yet exist when
MyBase::MyBase runs. You may store a pointer or reference for future
use, but may not access the object itself, period.
You may, however, cause the m_vstr to be constructed earlier, by moving
it to another base class and putting that earlier in the list of bases:
class MyBase3
{
protected:
VStr m_vstr;
};
class MyDerived3 : private MyBase3, public MyBase // order matters!
{
public:
MyDerived3() :
// first, MyBase3 is default-constructed,
// and as part of it, m_vstr is default-constructed.
// Then, we construct MyBase, passing it our m_vstr:
MyBase(m_vstr)
{}
};
But generally speaking, you probably have a design error here. Why
would you want to modify something passed into your constructor?
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Paul Rosen
Guest
|
| Posted: Fri Oct 28, 2005 8:27 am Post subject: Re: trick to access derived members in base constructor |
|
|
Daniel Krügler wrote:
| Quote: | Boost provides a helper class template, base_from_member, which
supports this special kind of initialization problem and can even be
used for multiple member situations.
|
A number of people suggested this, and thanks! The first line of the
documentation says:
"The class template boost::base_from_member provides a workaround for a
class that needs to initialize a base class with a member."
That pretty much nails the problem. Thanks all!
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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