在非托管C ++中,我有一个函数,我试图从C#调用。这个C ++函数如下:
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,该函数使用如下:
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,但我不知道应该传递哪种类型:
[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:
struct Point{
double x;
double y;
}
是传递数组还是List of Point?
我有C ++的源代码,可以更改函数参数;是否会有更容易从C#调用的不同类型的参数?
答案 0 :(得分:6)
使用C ++ / CLI包装器类(因为您有C ++源代码,您可以将它与现有代码一起编译到一个DLL中)。
P / invoke不是为与C ++类交互而设计的,任何这样做的尝试都非常脆弱。 C ++模板可能更糟糕。不要试试。即使{{1}将该函数用于其他C ++代码也是一个糟糕的想法,诸如dllexport
和vector
之类的STL类不应该跨DLL边界传递。
使用vector::iterator
并让Visual C ++编译器处理细节。
答案 1 :(得分:1)
通过P / Invoke传递C ++类型不起作用。你不知道他们的布局,没有什么能保证他们不会改变。 P / Invoke实际上仅用于与C进行互操作。
一种选择是使用C ++ / CLI而不是C ++。这将不可移植(仅支持VC ++ / Windows),但它可能是最简单的解决方案,具体取决于您的C ++代码已经有多大。
如果你想保持可移植性并使用C#中的直接P / Invoke,最好的办法是稍微重构C ++ convexHull
并提供一个可以从C调用的新函数(从而调用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!