将std :: vector转换为NumPy数组而不复制数据 [英] Convert a std::vector to a NumPy array without copying data

问题描述

我有一个C ++库，目前有一些方法里面返回 std :: vector 定义像

I have a C++ library which currently has some methods inside which return a std::vector defined like

public:
  const std::vector<uint32_t>& getValues() const;

我目前正在使用SWIG封装Python的整个库，这个工作进行得很好。

I'm currently working on wrapping the whole library for Python using SWIG and this is working well so far.

SWIG将此 getValues（）函数包装成函数，使其返回一个Python元组。问题是在我的Python端代码我想把它转换为NumPy数组。当然我可以这样做：

SWIG wraps this getValues() function fine such that it returns a Python tuple. The issue is in my Python-side code I want to convert this to a NumPy array. Of course I can do this by:

my_array = np.array(my_object.getValues(), dtype='uint32')

但是这会导致原始向量中的所有条目首先通过SWIG复制到Python元组，然后再由我进入numpy数组。因为这个向量可能非常大，我宁愿避免做这两个副本，并希望有一种方法让SWIG创建一个numpy.array包装内存中的原始向量数据。

but this causes all the entries in the original vector to be first copied into a Python tuple by SWIG and then again into a numpy array by me. Since this vector could be very large, I'd rather avoid making these two copies and would like for a way to have SWIG create a numpy.array wrapper around the original vector data in memory.

我已阅读 numpy.i 的文档，但是明确提到不支持输出数组，因为它们似乎是在C风格数组而不是C ++向量的假设下工作。

I've read the documentation for numpy.i but that explicitly mentions that output arrays are not supported since they seem to be working under the assumption of C-style arrays rather than C++ vectors.

numpy.array的底层数据结构是只是一个C风格的数组作为一个C ++ std :: vector，所以我希望它是可行的，然后访问内存中的相同的数据。

numpy.array's underlying data structure is just a C-style array as is a C++ std::vector so I would hope that it is feasible to have then access the same data in memory.

有任何方法让SWIG返回一个不复制原始数据的numpy.array？

Is there any way to make SWIG return a numpy.array which doesn't copy the original data?

推荐答案

显然是 C ++向量到（C）数组，请参见有关此问题的答案：如何将向量转换为数组C ++

Apparently is is trivial to "cast" a C++ vector to (C) array, see answer on this question: How to convert vector to array C++

接下来，您可以创建一个numpy数组，使用该C数组而不复制，请参阅这里讨论： https://groups.google.com/forum/?fromgroups=#!topic/cython-users/Ubh2nG0_R9g

Next you can create a numpy array which will use that C array without copying, see discussion here: https://groups.google.com/forum/?fromgroups=#!topic/cython-users/Ubh2nG0_R9g or google for PyArray_SimpleNewFromData.

我不希望SWIG自动为你执行所有这些操作，你应该为自己的函数 getValues 写一个包装器，比如 getValuesAsNumPyArray 。

I wouldn't expect SWIG to do all these for you automatically, instead you should probably write a wrapper for your function getValues yourself, something like getValuesAsNumPyArray.

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