在从C++dll到Python的指针的向量上返回指针

Question

我已经成功地将指向结构(包含wchar_t*)的指针从c++dll返回到Python中，如下所示：C++代码：

...
typedef struct myStruct{
wchar_t* id; 
wchar_t* content; 
wchar_t* message;
} myStruct;
DLLAPI myStruct* DLLApiGetStruct(){
myStruct* testStruct = new myStruct();
testStruct->id = _T("some id"); 
testStruct->content = _T("some content"); 
testStruct->message = _T("some message");
return testStruct;
}

Python代码：

class MyPyStruct(Structure):
_fields_ = [
("id", c_wchar_p),
("content", c_wchar_p),
("message", c_wchar_p)
]
...
...
myDLL = cdll.LoadLibrary('myDLL.dll')
myDLL.DLLApiGetStruct.restype = POINTER(MyPyStruct)
result = myDLL.DLLApiGetStruct().contents
print result.id, result.content, result. message# those are valid values

好的，这很好，问题是，现在我需要在指向这些结构的指针的向量上返回指针。我试过这个：

C++代码：

typedef std::vector<myStruct*> myVector;
...
DLLAPI myVector* DLLApiGetVector(){
myVector* testVektor = new myVector();
for(i=0; i< 5; i++){
myStruct* testStruct = new myStruct();
testStruct->id = _T("some id"); 
testStruct->content = _T("some content"); 
testStruct->message = _T("some message");
testVektor->push_back(testStruct);
}
return testVektor;// all values in it are valid
}

Python代码：

#我认为第一行和第二行不正确(这是重新键入的正确方法吗？)：

vectorOfPointersType = (POINTER(DeltaDataStruct) * 5)  #5 is number of structures in vector
myDLL.DLLApiGetVector.restype = POINTER(vectorOfPointersType)
vectorOfPointersOnMyStruct= myDLL.DLLApiGetVector.contents
for pointerOnMyStruct in vectorOfPointersOnMyStruct:
result = pointerOnMyStruct.contents
print result.id, result.content, result.message

最后一行的值无效——我想这是内存中的一些随机部分。这是我得到的错误：

UnicodeEncodeError: 'charmap' codec can't encode characters in position 0-11: character maps to <undefined>

Answer 1

vector与C兼容，但您需要将第一个元素的地址传递给C调用者(或ctypes)。也就是说，您必须抓住指向vector的指针才能稍后释放它。我认为从一开始就使用数组会更好。您可以向函数传递一个intout参数来接收数组的长度。由于您使用new进行分配，因此请记住，如果分配失败，请捕获bad_alloc异常。

就我个人而言，我会使用一个结构数组，而不是指针数组，这样数据就在一个连续的块中。这在ctypes中产生了更干净的界面。对于指针数组，您必须两次取消引用才能获得结构。

C++：

#include <new>
#include <cwchar>
typedef struct myStruct {
wchar_t *id;
wchar_t *content;
wchar_t *message;
} myStruct;
const wchar_t ID[] = L"some id";
const wchar_t CONTENT[] = L"some content";
const wchar_t MESSAGE[] = L"some message";
DLLAPI myStruct **DLLApiGetArray(int *size)
{
int i, n = 5;
myStruct **result;
try {
result = new myStruct *[n];
for(i = 0; i < n; i++) {
myStruct *tmp = new myStruct();
tmp->id = new wchar_t[wcslen(ID) + 1];
tmp->content = new wchar_t[wcslen(CONTENT) + 1];
tmp->message = new wchar_t[wcslen(MESSAGE) + 1];
wcscpy(tmp->id, ID);
wcscpy(tmp->content, CONTENT);
wcscpy(tmp->message, MESSAGE);
result[i] = tmp;
}
} catch (std::bad_alloc &ba) {
*size = -1; return NULL;
}
*size = n; return result;
}

Python:

from ctypes import *
class myStruct(Structure):
_fields_ = [
("id", c_wchar_p),
("content", c_wchar_p),
("message", c_wchar_p)
]
myDLL = cdll.myDLL
myDLL.DLLApiGetArray.restype = POINTER(POINTER(myStruct))
myDLL.DLLApiGetArray.argtypes = [POINTER(c_int)]
n = c_int()
p = myDLL.DLLApiGetArray(byref(n))
n = n.value

结果循环示例：

>>> for i in range(n):
...     print i, p[i][0].id
...
0 some id
1 some id
2 some id
3 some id
4 some id

---

FYI，将_T宏与显式wchar_t数组一起使用是不正确的。这适用于Microsoft的TCHAR类型，用于编译为ANSI与Unicode。使用L"wide character string literals"。