__str__()在打印用SWIG为Python包装的c++类时未调用

__str__() not called when printing C++ class wrapped for Python with SWIG

本文关键字：包装 c++ 调用 Python str 打印 SWIG      更新时间：2023-10-16

我正在尝试打印我使用SWIG为Python包装的c++类。我遵循文档和这个问题:如何在python中字符串化swig矩阵对象

扩展的__str__函数在那里，但是当我从Python打印对象时没有调用它。让我举一个最小的例子:

TestClass.h 

#include <iostream> 
class TestClass{
private:
    int my_int;
public:
    TestClass():
        my_int(0)
    {}
    friend std::ostream& operator<< (std::ostream& o, TestClass const& t){
        o<< "TestClass: " << t.my_int;
        return o;
    }
};

TestClass.cpp 

#include "TestClass.h"
int main(){
    using namespace std;
    TestClass t;
    cout << t << endl;
}

TestClass.i 

%define __STR__() 
const char* __str__() { 
  std::cout << *$self << std::endl;
  std::ostringstream out; 
  out << *$self; 
  return out.str().c_str(); 
}
const char* __unicode__() { 
  std::cout << "unicode: " << *$self << std::endl;
  std::ostringstream out; 
  out << *$self; 
  return out.str().c_str(); 
}
%enddef
%extend TestClass{
    __STR__()
};

CMakeLists.txt 

cmake_minimum_required (VERSION 2.6)
project (battery_lib_cpp)
set(CMAKE_VERBOSE_MAKEFILE 1)
FIND_PACKAGE(SWIG REQUIRED)
INCLUDE(${SWIG_USE_FILE})
SET(CMAKE_SWIG_FLAGS "-Wall")
#look for Pythonlibs
FIND_PACKAGE(PythonLibs)
INCLUDE_DIRECTORIES(${PYTHON_INCLUDE_PATH})
INCLUDE_DIRECTORIES(${CMAKE_CURRENT_SOURCE_DIR})
SET_SOURCE_FILES_PROPERTIES(TestClass.i PROPERTIES CPLUSPLUS ON)
SET_PROPERTY(SOURCE TestClass.i PROPERTY SWIG_FLAGS "-builtin")
SWIG_ADD_MODULE(TestClass python TestClass.i TestClass.cpp)

在构建(cmake .， make)之后，我得到以下内容:

matthias@rp3deb:~/dvl/swig_str_minimal$ python
Python 2.7.8 (default, Jul 26 2014, 15:25:14) 
[GCC 4.9.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import TestClass
>>> t = TestClass.TestClass()
>>> print t
<Swig Object of type 'TestClass *' at 0x7f1dffcb2b90>
>>> print t.__str__()
TestClass: 0
TestClass: 0
>>> print t.__str__
<built-in method __str__ of TestClass object at 0x7f1dffcb2b90>

__unicode__()的另一个测试

>>> import TestClass
>>> t = TestClass.TestClass()
>>> print str(t)
<Swig Object of type 'TestClass *' at 0x7f6584ae4b58>
>>> print t
<Swig Object of type 'TestClass *' at 0x7f6584ae4b58>
>>> print t.__unicode__()
unicode: TestClass: 0
TestClass: 0
>>> print t.__unicode__
<built-in method __unicode__ of TestClass object at 0x7f6584ae4b58>

预期的输出将是在"print t"隐式地调用__str__()。我错过了什么?

要让str(x)为使用swig -python -builtin生成的SWIG包装对象调用您自己的c++代码，您需要使用适当的插槽来注册您的函数。这是用SWIG中的%feature("python:slot", ...)完成的，例如:

%module test
%include <std_string.i>
%feature("python:slot", "tp_str", functype="reprfunc") foo::as_string;
%inline %{
struct foo {
  std::string as_string() const { return "Hello world"; }
};
%}

与SWIG 2.0和Python 2.7让我运行:

import test
print str(test.foo())
print repr(test.foo())

结果是:

Hello world
<Swig Object of type 'foo *' at 0xb727ac40>

插槽允许快速调度标准对象函数调用-这是-builtin为您带来的一大块。

您可以看到可用插槽及其类型的完整列表以及相应的SWIG 2.0文档。

基于Flexo的回答，这让我得到了我想要的功能(源文件和cmake不变)。

TestClass.i 

%module TestClass
%{
#include <sstream>
#include "TestClass.h"
%}
%include "TestClass.h"
//http://stackoverflow.com/questions/2548779/how-to-stringfy-a-swig-matrix-object-in-python
%define __STR__(class_name) 
%feature("python:slot", "tp_str", functype="reprfunc") class_name::py_to_string();
%extend class_name{
    const char* py_to_string() { 
      std::ostringstream out; 
      out << *$self; 
      return out.str().c_str(); 
    }
}
%enddef
__STR__(TestClass);

测试:

matthias@rp3deb:~/dvl/swig_str_minimal$ python -c "import TestClass; print TestClass.TestClass()"
TestClass: 0

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