C++类在Python中使用SWIG的奇怪行为

我在C++中有一个类，有两种方法(一个大项目的一部分(。这些方法执行非常相似的工作：第一个方法规范化一个向量并返回它，而第二个返回一个规范化的向量而不改变原始向量。

vector3.h：

class Vector3
{
public:
Vector3(double a = 0.0, double b = 0.0, double c = 0.0) 
: x(a), y(b), z(c) {}
Vector3(const Vector3& other) : x(other.x), y(other.y), z(other.z) {}
double length() const { return sqrt(x*x+y*y+z*z); }
Vector3& normalize()
{
float_type n = length();
n = float_type(1.0) / ( n ? n : 1.0 );
this->x *= n;
this->y *= n;
this->z *= n;
return *this;
}
Vector3 normalized() const
{
Vector3 v(*this);
v.normalize();
return v;
}
std::string toString()
{
std::ostringstream strm;
strm << '(' << x << ", " << y << ", " << z << ')' ;
return strm.str();
}
Vector3 operator+(const Vector3& other) const
{
return Vector3(x + other.x, y + other.y, z + other.z);
}
private:
double x,y,z;
};

我用SWIG(通过cmake(为这个类构建了Python绑定。

vector3.i：

%include <stl.i>
%include <carrays.i>
%include <cpointer.i>
%module vectortest
%{
#include "vector3.h"
%}
%include "vector3.h"

CMakeLists.txt：

cmake_minimum_required(VERSION 2.8)
add_executable("vtest" "vtest.cpp")
find_package(SWIG)
include(${SWIG_USE_FILE})
find_package(PythonLibs)
find_package(PythonInterp)
include_directories(${CMAKE_SOURCE_DIR})
include_directories(${PYTHON_INCLUDE_DIRS})
SET(CMAKE_SWIG_FLAGS "")
SET_SOURCE_FILES_PROPERTIES(SOURCE vector3.i PROPERTIES CPLUSPLUS ON)
SWIG_ADD_MODULE(vectortest_python python vector3.i )
SWIG_LINK_LIBRARIES(vectortest_python ${PYTHON_LIBRARIES} ${LIBS})
set_target_properties(_vectortest_python PROPERTIES PREFIX "_" OUTPUT_NAME "vectortest")
execute_process(COMMAND ${PYTHON_EXECUTABLE} -c "from distutils.sysconfig import get_python_lib; print (get_python_lib())" OUTPUT_VARIABLE PYTHON_SITE_PACKAGES OUTPUT_STRIP_TRAILING_WHITESPACE)
install(TARGETS ${SWIG_MODULE_vectortest_python_REAL_NAME} DESTINATION ${PYTHON_SITE_PACKAGES})
install(FILES ${CMAKE_BINARY_DIR}/vectortest.py DESTINATION ${PYTHON_SITE_PACKAGES})

C++的行为很好。

vtest.cpp：

#include <iostream>
#include "vector3.h"
int main()
{
Vector3 v1(1,0,0);
Vector3 v2(0,1,0);
std::cout << (v1+v2).toString() << std::endl;
std::cout << (v1+v2).normalized().toString() << std::endl;
std::cout << (v1+v2).normalize().toString() << std::endl;
return 0;
}

输出：

(1, 1, 0)
(0.707107, 0.707107, 0)
(0.707107, 0.707107, 0)

但是，它在Python中的行为非常奇怪：

vtest.py：

#!/usr/bin/python3
from vectortest import Vector3
v1 = Vector3(1,0,0)
v2 = Vector3(0,1,0)
print( (v1+v2).toString() )
print( (v1+v2).normalized().toString() )
print( (v1+v2).normalize().toString() )

输出：

(1, 1, 0)
(0.707107, 0.707107, 0)
(0, 0.707107, 0)

第二种方法(normalized()(按预期工作，但第一种方法(normalize()(则不然。是什么原因造成的？关于如何解决此问题的任何建议？