Why does GetPointCount of a GraphicsPath always return 0?-CodePudding

I am trying to learn the gdiplus windows API, and in particular how to use GraphicsPath to get points from different shapes. I noticed that I could never get anything to appear from microsoft's example code, so I tried to see how many points were actually in a GraphicsPath like this:

#include <windows.h>
#include <gdiplus.h>
#include <iostream>

//use gdiplus library when compiling
#pragma comment( lib, "gdiplus" )

using namespace Gdiplus;

VOID GetPointCountExample()
{
    // Create a path that has one ellipse and one line.
    GraphicsPath path;
    path.AddLine(220, 120, 300, 160);

    // Find out how many data points are stored in the path.
    int count = path.GetPointCount();
    std::cout << count << std::endl;
}

int main()
{
    GetPointCountExample();
}

This always returned a count of 0. Why is this?

I have tried compiling with mingw-64 and Visual Studio with the same result.

I also tried printing out the Gdiplus::Status returned from this:

GraphicsPath path;
int stat = path.StartFigure();

std::cout << stat << std::endl;

Which printed a status of 2, "InvalidParameter" even though StartFigure doesn't take parameters.

CodePudding user response：

Ahh read more documentation and found this: The GdiplusStartup function initializes Windows GDI . Call GdiplusStartup before making any other GDI calls https://docs.microsoft.com/en-us/windows/win32/api/Gdiplusinit/nf-gdiplusinit-gdiplusstartup

This code works:

#include <windows.h>
#include <gdiplus.h>
#include <iostream>

//use gdiplus library when compiling
#pragma comment( lib, "gdiplus" )

using namespace Gdiplus;

VOID GetPointCountExample()
{
    // Create a path that has one ellipse and one line.
    GraphicsPath path;
    path.AddLine(0, 0, 0, 1);

    // Find out how many data points are stored in the path.
    int count = path.GetPointCount();
    std::cout << count << std::endl;
}

int main()
{
    //Must call GdiplusStartup before making any GDI  calls
    //https://docs.microsoft.com/en-us/windows/win32/api/Gdiplusinit/nf-gdiplusinit-gdiplusstartup
    ULONG_PTR token;
    GdiplusStartupInput input;
    input.GdiplusVersion = 1;
    input.SuppressBackgroundThread = false;

    GdiplusStartup(&token, &input, NULL);

    GetPointCountExample();

    //Shutdown GDI  when finished using
    GdiplusShutdown(token);
}