Re: Initializing arrays of objects

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Wed, 21 Oct 2009 00:53:19 -0700 (PDT)

Message-ID:

<bc4c885c-a958-4fd0-a5c5-c5f07f50cd73@m11g2000vbl.googlegroups.com>

On Oct 21, 1:07 am, "Cristiano" <cristiano...@NSquipo.it> wrote:

I'm trying to compile this code:

template <typename T> class matrix
{
private:
class row
{
public:
  T *data;
  row(int n) {data = new T[n];};
  ...
} *rdata;
int rows, cols;
public:
matrix(int r, int c) { rows = r; cols = c; rdata = new row[r](c); };
...

but I get:
    'Target of operator new()' : array initialization needs curly braces
and
    'new' : 'matrix<T>::row' no default constructor to initialize arrays of
objects.

Any way to fix the problem?

The obvious answer is to use std::vector:
    std::vector<row> rdata;
If you do, be sure to provide an appropriate copy constructor
and assignment operator, however. And you're likely to get a
lot more allocations than desired, because of temporary objects.

Alternatively, you can use the same technique used in
std::vector: separate allocation and initialization. Something
like:
    rdata = static_cast< row* >( operator new( sizeof(row) * r ));
    for ( size_t i = 0 ; i < r ; ++ i ) {
        new ( rdata + i ) row( c ) ;
    }
If you do this, you'll also have to use a similar technique in
the destructor, explicitly calling delete for each element, then
calling the operator delete function to free the memory.

You might want to consider a different approach:
    std::vector< T > data ;
    matrix( int r, int c ) : rows( r ), cols( c ), data( r * c ) {}
and then calculating the index in your operator[] or whatever.

--
James Kanze