Question

在another question中，我学会了如何通过复制对象来公开将C ++对象返回给Python的函数。必须执行副本似乎不是最佳的。如何在不复制的情况下返回对象？即如何直接访问self.thisptr.getPeaks(data)中PyPeakDetection.getPeaks返回的峰值（在 peak_detection_.pyx 中定义）？

peak_detection.hpp

#ifndef PEAKDETECTION_H
#define PEAKDETECTION_H

#include <string>
#include <map>
#include <vector>

#include "peak.hpp"


class PeakDetection
{
    public:
        PeakDetection(std::map<std::string, std::string> config);
        std::vector<Peak> getPeaks(std::vector<float> &data);

    private:
        float _threshold;               
};

#endif

peak_detection.cpp

#include <iostream>
#include <string>

#include "peak.hpp"
#include "peak_detection.hpp"


using namespace std;


PeakDetection::PeakDetection(map<string, string> config)
{   
    _threshold = stof(config["_threshold"]);
}

vector<Peak> PeakDetection::getPeaks(vector<float> &data){

    Peak peak1 = Peak(10,1);
    Peak peak2 = Peak(20,2);

    vector<Peak> test;
    test.push_back(peak1);
    test.push_back(peak2);

    return test;
}

peak.hpp

#ifndef PEAK_H
#define PEAK_H

class Peak {
    public:
        float freq;
        float mag;

        Peak() : freq(), mag() {}
        Peak(float f, float m) : freq(f), mag(m) {}
};

#endif

peak_detection_.pyx

# distutils: language = c++
# distutils: sources = peak_detection.cpp

from libcpp.vector cimport vector
from libcpp.map cimport map
from libcpp.string cimport string

cdef extern from "peak.hpp":
    cdef cppclass Peak:
        Peak()
        Peak(Peak &)
        float freq, mag


cdef class PyPeak:
    cdef Peak *thisptr

    def __cinit__(self):
        self.thisptr = new Peak()

    def __dealloc__(self):
        del self.thisptr

    cdef copy(self, Peak &other):
        del self.thisptr
        self.thisptr = new Peak(other)

    def __repr__(self):
        return "<Peak: freq={0}, mag={1}>".format(self.freq, self.mag)

    property freq:
        def __get__(self): return self.thisptr.freq
        def __set__(self, freq): self.thisptr.freq = freq

    property mag:
        def __get__(self): return self.thisptr.mag
        def __set__(self, mag): self.thisptr.mag = mag


cdef extern from "peak_detection.hpp":
    cdef cppclass PeakDetection:
        PeakDetection(map[string,string])
        vector[Peak] getPeaks(vector[float])

cdef class PyPeakDetection:
    cdef PeakDetection *thisptr

    def __cinit__(self, map[string,string] config):
        self.thisptr = new PeakDetection(config)

    def __dealloc__(self):
        del self.thisptr

    def getPeaks(self, data):
        cdef Peak peak
        cdef PyPeak new_peak
        cdef vector[Peak] peaks = self.thisptr.getPeaks(data)

        retval = []

        for peak in peaks:
            new_peak = PyPeak()
            new_peak.copy(peak) # how can I avoid that copy?
            retval.append(new_peak)

        return retval

Answer 1

如果你有一个现代的C ++编译器并且可以使用rvalue引用，那么移动构造函数和std :: move它非常简单。我认为最简单的方法是为向量创建一个Cython包装器，然后使用移动构造函数来保存向量的内容。

显示的所有代码都在peak_detection_.pyx。

首先换行std::move。为简单起见，我只是包装了我们想要的一个案例（vector<Peak>）而不是搞乱模板。

cdef extern from "<utility>":
    vector[Peak]&& move(vector[Peak]&&) # just define for peak rather than anything else

其次，创建一个矢量包装类。这定义了像列表一样访问它所必需的Python函数。它还定义了一个调用移动赋值运算符的函数

cdef class PyPeakVector:
    cdef vector[Peak] vec

    cdef move_from(self, vector[Peak]&& move_this):
        self.vec = move(move_this)

    def __getitem__(self,idx):
        return PyPeak2(self,idx)

    def __len__(self):
        return self.vec.size()

然后定义包裹Peak的类。这与您的其他类略有不同，因为它不拥有它包装的Peak（向量确实）。否则，大多数功能保持不变

cdef class PyPeak2:
    cdef Peak* thisptr
    cdef PyPeakVector vector # keep this alive, since it owns the peak rather that PyPeak2

    def __cinit__(self,PyPeakVector vec,idx):
        self.vector = vec
        self.thisptr = &vec.vec[idx]

    # rest of functions as is

    # don't define a destructor since we don't own the Peak

def class PyPeakVector:
    cdef vector[Peak] vec

    cdef move_from(self, vector[Peak]&& move_this):
        self.vec = move(move_this)

    def __getitem__(self,idx):
        return PyPeak2(self,idx)

    def __len__(self):
        return self.vec.size()

最后，实施getPeaks()

cdef class PyPeakDetection:
    # ...    
    def getPeaks(self, data):
        cdef Peak peak
        cdef PyPeak new_peak
        cdef vector[Peak] peaks = self.thisptr.getPeaks(data)

        retval = PyPeakVector()
        retval.move_from(move(peaks))

        return retval

替代方法：

如果Peak非常重要，那么当您构建move时，您可以采用Peak上的PyPeak而不是矢量上的getPeaks方法。对于你在这里的情况，移动和复制将相当于“峰值”。

如果您无法使用C ++ 11功能，则需要稍微更改一下界面。而不是让你的C ++ PyPeakVector函数返回一个向量，它需要一个空向量引用（由input()拥有）作为输入参数并写入它。其余大部分包装都保持不变。

Answer 2

有两个项目可以完成与C ++代码到Python的连接，它们经受了时间Boost.Python和SWIG的测试。两者都通过向相关的C / C ++代码添加额外的标记并生成动态加载的python扩展库（.so文件）和相关的python模块来工作。

但是，根据您的使用情况，可能仍有一些额外的标记看起来像“复制”。但是，复制不应该那么广泛，它将全部暴露在C ++代码中，而不是在Cython / Pyrex中逐字显式地复制。

如何在不复制对象的情况下公开将C ++对象返回给Python的函数？

2 个答案: