Re: std::vector question related with gsl_vector

From:

Mark P <usenet@fall2005REMOVE.fastmailCAPS.fm>

Newsgroups:

comp.lang.c++

Date:

Sat, 31 Mar 2007 08:00:52 GMT

Message-ID:

<UyoPh.3440$YL5.1490@newssvr29.news.prodigy.net>

zl2k wrote:

hi, all

I need to use gsl_vector pointer with std::vector but not sure how to
free the memory when I don't need it. Here is a piece of the code.

gsl_vector is not standard C++ which makes it outside the scope of this
group.

===================
std::vector<gsl_vector * > v;
gsl_vector * a = gsl_vector_alloc(3);
gsLvector_set(a, 0, 7);
v.push_back(a);

gsl_vector_free(a); // problem! after a is free, the v[0] is null

Not likely. v holds a pointer value and whatever gsl_vector_free does,
I seriously doubt that it's going to change the *contents* of vector v.
More likely the pointer value stored in v[0] is invalid and should not
be dereferenced, but that's not the same as being NULL.

std::cout<<gsl_vector_get(v[0], 0)<<std::endl;
===================

How can I free the memory when I don't need them? If I say

v.clear();

will the memory allocated by the gsl_vector still hold by the
unreachable vectors?

v.clear() will empty out the vector v. If that vector holds pointers,
it will do _nothing_ insofar as the objects pointed to by those pointers
are concerned.

Or should I do

for (int i = 0; i < v.size(); i++){
gsl_vector_free(v[i]);
}

Not knowing anything about the gsl classes I can't be sure, but if
that's the proper way to deallocate the resource obtained by
gsl_vector_alloc, then yes, this is probably what you need to do.

v.clear();

This is not necessary in general. The vector will clear itself when it
is destructed. Only if you want to clear v sometime before it goes out
of scope do you need to do so explicitly.

to free the memory? But iterating on each elements of the vector is
tedious if the std::vector is long.

Why is it tedious? It looks like two lines of code and, as far as I can
tell, it takes at least as much work to create them in the first place.

Of course there are good reasons not to do this iteration, namely that
it's probably not exception safe and in any event is an additional
burden upon the programmer. A more robust approach would be to store
some sort of smart pointer to gsl_vector in your std::vector.

Is there any easy way to deal with

the problem? Thanks for comments.

zl2k

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