如何从C#中使用std :: vector<> :: iterator作为参数调用非托管C ++函数?

如何从C#中使用std :: vector<> :: iterator作为参数调用非托管C ++函数?

问题描述:

在非托管C ++中,我有一个函数正在尝试从C#调用.此C ++函数如下:

In unmanaged C++ I have a function which I'm trying to call from C#. This C++ function is as follows:

typedef std::vector<Point> Points;
typedef std::back_insert_iterator<Points> OutputIterator;

namespace MYNAMESPACE{
    DLLEXPORT OutputIterator convexHull(Points::iterator first, Points::iterator last,  OutputIterator result);
}

从C ++调用时,该函数的用法如下:

When called from C++, the function is used as follows:

  Points points, result;

  points.push_back(Point(0,0));
  points.push_back(Point(10,0));
  points.push_back(Point(10,10));
  points.push_back(Point(6,5));
  points.push_back(Point(4,1));

  OutputIterator resultIterator = std::back_inserter(result);

  MYNAMESPACE::convexHull( points.begin(), points.end(), resultIterator);
  std::cout << result.size() << " points on the convex hull" << std::endl;

我已经开始编写C#代码,但是我不知道应该传递什么类型:

I've started writing the C# code, but I've no idea what types I should be passing:

[DllImport("unmanagedCode.dll", EntryPoint = "convexHull", CallingConvention = CallingConvention.StdCall)]
        public static extern ???<Point> convex_hull_2(???<Point> start, ???<Point> last, ???<Point> result);

C#中的Point结构只是:

The Point structure in C# is just:

struct Point{
    double x;
    double y;
}

是否要传递数组或Point of List?

Is it a case of passing an array or List of Point?

我有C ++的源代码,可以更改函数参数;是否会有其他类型的参数,这些参数会更容易从C#调用?

I have the source to the C++ and can make changes to the function parameters; would there be a different type of parameters which would be easier to call from C#?

通过P/Invoke传递C ++类型是行不通的.您不知道它们的布局,也不能保证它们不会改变. P/Invoke实际上仅是为了与C互操作.

Passing C++ types through P/Invoke is not going to work. You don't know their layout and nothing guarantees they won't change. P/Invoke is really only meant for inter-operating with C.

一种选择是使用C ++/CLI而不是C ++.这不会是可移植的(仅VC ++/Windows支持),但这可能是最简单的解决方案,具体取决于您的C ++代码的大小.

One option is to use C++/CLI instead of C++. This won't be portable (only supported with VC++/Windows), but it might be the easiest solution depending on how large your C++ code is already.

如果您想保持可移植性并使用C#的直接P/Invoke,最好的选择是稍微重构C ++ convexHull并提供一个可从C调用的新函数(并因此调用P/Invoke).

If you want to remain portable and use straight P/Invoke from C#, your best bet is to slightly refactor the C++ convexHull and provide a new function callable from C (and thus P/Invoke).

// C-safe struct.
struct Results
{
    Point *points;
    std::size_t num_points;
};

// Store the real results in a vector, but derive from the C-safe struct.
struct ResultsImpl : Results
{
    Points storage;
};

// convexHull has been refactored to take pointers
// instead of vector iterators.
OutputIterator convexHull(Point const *first, Point const *last,
    OutputIterator result);

// The exported function is callable from C.
// It returns a C-safe Results, not ResultsImpl.
extern "C" DLLEXPORT Results* convexHullC(Point const *points,
                                          std::size_t num_points)
{
    try
    {
        std::unique_ptr<ResultsImpl> r(new ResultImpl);

        // fill in r->storage.
        convexHull(points, points + num_points,
            std::back_inserter(r->storage));

        // fill in C-safe members.
        r->points = &r->storage[0];
        r->numPoints = &r->storage.size();

        return r.release();
    }
    catch(...)
    {
        // trap all exceptions!
        return 0;
    }
}

// needs to be called from C# to clean up the results.
extern "C" DLLEXPORT void freeConvexHullC(Results *r)
{
    try
    {
        delete (ResultsImpl*)r;
    }
    catch(...)
    {
        // trap all exceptions!
    }
}

然后从C#:

[StructLayout(LayoutKind.Sequential)]
struct Point
{
    double x;
    double y;
}

[StructLayout(LayoutKind.Sequential)]
struct Results
{
    IntPtr points;
    IntPtr num_points;
}

[DllImport("unmanagedCode")]
IntPtr convexHullC(Point[] points, IntPtr pointCount);

[DllImport("unmanagedCode")]
void freeConvexHullC(IntPtr results);

Point[] ConvexHull(Point[] points)
{
    IntPtr pr = convexHull(points, new IntPtr(points.Length));

    if(pr == IntPtr.Zero)
    {
        throw new Exception("native error!");
    }

    try
    {
        Results r = Marshal.PtrToStructure(pr, typeof(Results));

        points = new Point[checked((int)(long)r.num_points)];

        for(int i = 0; i < points.Length; ++i)
        {
            points[i] = Marshal.PtrToStructure(
                r.points + Marshal.Sizeof(typeof(Point)) * i,
                typeof(Point));
        }

        return points;
    }
    finally
    {
        freeConvexHull(pr);
    }
}

代码未经测试!