Re: Dynamic allocation of memory problem

From:
James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Fri, 26 Jun 2009 00:50:45 -0700 (PDT)
Message-ID:
<d044ea20-09e6-45e2-9057-ab1c252978ca@j19g2000vbp.googlegroups.com>
On Jun 25, 1:13 pm, Richard Herring <junk@[127.0.0.1]> wrote:

    [...]

Count up how much time you have wasted already trying to make
your code work. Then look at this:

Instead of this:
double **Matrix = new double*[n];
  for(int i=0; i<n; i++)
     Matrix[i] = new double[n];

you'd write:

vector<vector<double> > Matrix(n, n);


Is this still true? I know that the C++03 standard guarantees
it, but I was under the impression that this was unintentional
(and the current draft seems to have been reworded to ban it).

The correct way of writing this is:

    std::vector< std::vector< double > >
                        matrix( n, std::vector< double >( n ) ) ;

This can be made easier with typedefs.

More reasonably, of course, you'd write a Matrix class, and
never really write anything like the above, except maybe in the
Matrix class. (Also, if you use a Matrix class, you can easily
switch between a vector of vectors, and a one dimensional vector
with calculation of the indices in the class, depending on what
works best for you.) Trying to use a matrix without defining a
class to manage it is his first mistake, regardless of the
structures used to implement it.

--
James Kanze (GABI Software) email:james.kanze@gmail.com
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