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Document directorout std::string_view limitations 

See #1567
This commit is contained in:
Olly Betts 2023-12-21 13:45:44 +13:00
parent 95020a70bf
commit 0d80357cea
2 changed files with 18 additions and 1 deletions
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14
Doc/Manual/Library.html
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@ -1582,6 +1582,20 @@ bar("Hello World"); # Pass string as std::string_view
</pre>
</div>
<p>
For target languages for which SWIG supports directors, <tt>directorout</tt>
typemaps are provided for <tt>std::string_view</tt>, but these require extra
care to use safely. The issue is that returning <tt>std::string_view</tt>
effectively returns a pointer to string data but doesn't own the pointed to
data. For target languages where there isn't a native narrow string
representation (e.g. C#, Java) a <tt>static std::string</tt> is used to cache
the data, which works but isn't thread/reentrant safe. For target languages
where there is a native narrow string representation SWIG will return a
<tt>std::string_view</tt> pointing to that data, so you need to store the
string to return somewhere which will persist for the lifetime the caller
needs (e.g. put it in a member variable) - you can't return a temporary target
language string. In both cases SWIG will issue a warning by default.
</p>
<H3><a name="Library_std_vector">12.4.3 std::vector</a></H3>

5
Lib/python/std_string_view.i
View File

@ -87,7 +87,10 @@ namespace std {
PyObject *bytes = NULL;
if (PyUnicode_Check($input)) {
p = SWIG_PyUnicode_AsUTF8AndSize($input, &len, &bytes);
// Py_XDECREF(bytes); // Avoid undefined behaviour ($input will be pointing to a temporary if bytes is not NULL), for now we just leak by not calling Py_XDECREF
// Avoid undefined behaviour (p will be pointing to a temporary
// if bytes is not NULL which happens when Py_LIMITED_API is defined
// and < 0x030A0000) and just leak by not calling Py_XDECREF.
// Py_XDECREF(bytes);
} else {
p = PyBytes_AsString($input);
if (p) len = PyBytes_Size($input);