Re: PolyLine

"Giovanni Dicanio" <>
Thu, 5 Nov 2009 14:59:24 +0100
"Luigino" <> ha scritto nel messaggio

(dc.)BOOL CDC::Polyline(const POINT * lpPoints, int nCount);

Actually I implemented like this:

dc.Polyline(reinterpret_cast<POINT*> (&singlepoints), singlepoints.size

If 'singlepoints' is an instance of a std::vector<POINT> (as you wrote in a
previous post), the reinterpret_cast is unnecessary and wrong.
&singlepoints is the address of an instance of std::vector, which is wrong
in your above context.
Instead, you need a pointer to the first element of the vector, i.e.
&singlepoints[0] (assuming the vector is not empty).
In fact, the first argument of CDC::Polyline is a 'const POINT *'.

I repeat:

  dc.Polyline( &singlepoints[0], singlepoints.size() );

is just fine.
Please read my previous post.

You can also verify with the following C++ FAQ:

I verified also with a simple code snippet:

Create a MFC doc/view project with MFC app wizard, and paste this code in
the view class OnDraw() method:

    typedef std::vector<POINT> Points;
    Points points;
    const int N = 5;
    POINT pt;
    int x1 = 10;
    int x2 = 30;
    int dx = (x2 - x1)/N;
    int x = x1;

    for (int i = 0; i < N; i++)
        pt.x = x;
        pt.y = 2*x;

        x += dx;

    pDC->Polyline( &points[0], points.size() );


Assuming that you add '#include <vector>' in "StdAfx.h", the above code
compiles fine and seems to me to run fine.



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