RFC - Add documentation for strings and Unicode issues (#636)

* Add documentation for strings and Unicode issues

* More Unicode documentation on character literals and wide characters

docs/advanced/cast/index.rst

@@ -33,6 +33,7 @@ the last case of the above list.
    :maxdepth: 1
    :maxdepth: 1
 
 
    overview
    overview
+   strings
    stl
    stl
    functional
    functional
    chrono
    chrono

docs/advanced/cast/strings.rst

+Strings, bytes and Unicode conversions
+######################################
+
+.. note::
+
+    This section discusses string handling in terms of Python 3 strings. For Python 2.7, replace all occurrences of ``str`` with ``unicode`` and ``bytes`` with ``str``.  Python 2.7 users may find it best to use ``from __future__ import unicode_literals`` to avoid unintentionally using ``str`` instead of ``unicode``.
+
+Passing Python strings to C++
+=============================
+
+When a Python ``str`` is passed from Python to a C++ function that accepts ``std::string`` or ``char *`` as arguments, pybind11 will encode the Python string to UTF-8. All Python ``str`` can be encoded in UTF-8, so this operation does not fail.
+
+The C++ language is encoding agnostic. It is the responsibility of the programmer to track encodings. It's often easiest to simply `use UTF-8 everywhere <http://utf8everywhere.org/>`_.
+
+.. code-block:: c++
+
+    m.def("utf8_test",
+        [](const std::string &s) {
+            cout << "utf-8 is icing on the cake.\n";
+            cout << s;
+        }
+    );
+    m.def("utf8_charptr",
+        [](const char *s) {
+            cout << "My favorite food is\n";
+            cout << s;
+        }
+    );
+
+.. code-block:: python
+
+    >>> utf8_test('🎂')
+    utf-8 is icing on the cake.
+    🎂
+
+    >>> utf8_charptr('🍕')
+    My favorite food is
+    🍕
+
+.. note::
+
+    Some terminal emulators do not support UTF-8 or emoji fonts and may not display the example above correctly.
+
+The results are the same whether the C++ function accepts arguments by value or reference, and whether or not ``const`` is used.
+
+Passing bytes to C++
+--------------------
+
+A Python ``bytes`` object will be passed to C++ functions that accept ``std::string`` or ``char*`` *without* conversion. 
+
+
+Returning C++ strings to Python
+===============================
+
+When a C++ function returns a ``std::string`` or ``char*`` to a Python caller, **pybind11 will assume that the string is valid UTF-8** and will decode it to a native Python ``str``, using the same API as Python uses to perform ``bytes.decode('utf-8')``. If this implicit conversion fails, pybind11 will raise a ``UnicodeDecodeError``. 
+
+.. code-block:: c++
+
+    m.def("std_string_return",
+        []() {
+            return std::string("This string needs to be UTF-8 encoded");
+        }
+    );
+
+.. code-block:: python
+
+    >>> isinstance(example.std_string_return(), str)
+    True
+
+
+Because UTF-8 is inclusive of pure ASCII, there is never any issue with returning a pure ASCII string to Python. If there is any possibility that the string is not pure ASCII, it is necessary to ensure the encoding is valid UTF-8.
+
+.. warning::
+
+    Implicit conversion assumes that a returned ``char *`` is null-terminated. If there is no null terminator a buffer overrun will occur.
+
+Explicit conversions
+--------------------
+
+If some C++ code constructs a ``std::string`` that is not a UTF-8 string, one can perform a explicit conversion and return a ``py::str`` object. Explicit conversion has the same overhead as implicit conversion.
+
+.. code-block:: c++
+
+    // This uses the Python C API to convert Latin-1 to Unicode
+    m.def("str_output",
+        []() {
+            std::string s = "Send your r\xe9sum\xe9 to Alice in HR"; // Latin-1
+            py::str py_s = PyUnicode_DecodeLatin1(s.data(), s.length());
+            return py_s;
+        }
+    );
+
+.. code-block:: python
+
+    >>> str_output()
+    'Send your résumé to Alice in HR'
+
+The `Python C API <https://docs.python.org/3/c-api/unicode.html#built-in-codecs>`_ provides several built-in codecs.
+
+
+One could also use a third party encoding library such as libiconv to transcode to UTF-8.
+
+Return C++ strings without conversion
+-------------------------------------
+
+If the data in a C++ ``std::string`` does not represent text and should be returned to Python as ``bytes``, then one can return the data as a ``py::bytes`` object.
+
+.. code-block:: c++
+
+    m.def("return_bytes",
+        []() {
+            std::string s("\xba\xd0\xba\xd0");  // Not valid UTF-8
+            return py::bytes(s);  // Return the data without transcoding
+        }
+    );
+
+.. code-block:: python
+
+    >>> example.return_bytes()
+    b'\xba\xd0\xba\xd0'
+
+
+Note the asymmetry: pybind11 will convert ``bytes`` to ``std::string`` without encoding, but cannot convert ``std::string`` back to ``bytes`` implicitly.
+
+.. code-block:: c++
+
+    m.def("asymmetry",
+        [](std::string s) {  // Accepts str or bytes from Python
+            return s;  // Looks harmless, but implicitly converts to str
+        }
+    );    
+
+.. code-block:: python
+
+    >>> isinstance(example.asymmetry(b"have some bytes"), str)
+    True
+
+    >>> example.asymmetry(b"\xba\xd0\xba\xd0")  # invalid utf-8 as bytes
+    UnicodeDecodeError: 'utf-8' codec can't decode byte 0xba in position 0: invalid start byte
+
+
+Wide character strings
+======================
+
+When a Python ``str`` is passed to a C++ function expecting ``std::wstring``, ``wchar_t*``, ``std::u16string`` or ``std::u32string``, the ``str`` will be encoded to UTF-16 or UTF-32 depending on how the C++ compiler implements each type, in the platform's endian. When strings of these types are returned, they are assumed to contain valid UTF-16 or UTF-32, and will be decoded to Python ``str``.
+
+.. code-block:: c++
+
+    #define UNICODE
+    #include <windows.h>
+
+    m.def("set_window_text",
+        [](HWND hwnd, std::wstring s) {
+            // Call SetWindowText with null-terminated UTF-16 string
+            ::SetWindowText(hwnd, s.c_str());
+        }
+    );
+    m.def("get_window_text",
+        [](HWND hwnd) {
+            const int buffer_size = ::GetWindowTextLength(hwnd) + 1;
+            auto buffer = std::make_unique< wchar_t[] >(buffer_size);
+
+            ::GetWindowText(hwnd, buffer.data(), buffer_size);
+
+            std::wstring text(buffer.get());
+
+            // wstring will be converted to Python str
+            return text;
+        }
+    );
+
+.. warning::
+
+    Wide character strings may not work as described on Python 2.7 or Python 3.3 compiled with ``--enable-unicode=ucs2``.
+
+Strings in multibyte encodings such as Shift-JIS must transcoded to a UTF-8/16/32 before being returned to Python.
+
+
+Character literals
+==================
+
+C++ functions that accept character literals as input will receive the first character of a Python ``str`` as their input. If the string is longer than one Unicode character, trailing characters will be ignored.
+
+When a character literal is returned from C++ (such as a ``char`` or a ``wchar_t``), it will be converted to a ``str`` that represents the single character.
+
+.. code-block:: c++
+
+    m.def("pass_char", [](char c) { return c; });
+    m.def("pass_wchar", [](wchar_t w) { return w; });
+
+.. code-block:: python
+    
+    >>> example.pass_char('A')
+    'A'
+
+While C++ will cast integers to character types (``char c = 0x65;``), pybind11 does not convert Python integers to characters implicitly. The Python function ``chr()`` can be used to convert integers to characters.
+
+.. code-block:: python
+    
+    >>> example.pass_char(0x65)
+    TypeError
+
+    >>> example.pass_char(chr(0x65))
+    'A'
+
+If the desire is to work with an 8-bit integer, use ``int8_t`` or ``uint8_t`` as the argument type.
+
+Grapheme clusters
+-----------------
+
+A single grapheme may be represented by two or more Unicode characters. For example 'é' is usually represented as U+00E9 but can also be expressed as the combining character sequence U+0065 U+0301 (that is, the letter 'e' followed by a combining acute accent). The combining character will be lost if the two-character sequence is passed as an argument, even though it renders as a single grapheme.
+
+.. code-block:: python
+
+    >>> example.pass_wchar('é')
+    'é'
+
+    >>> combining_e_acute = 'e' + '\u0301'
+
+    >>> combining_e_acute
+    'é'
+
+    >>> combining_e_acute == 'é'
+    False
+
+    >>> example.pass_wchar(combining_e_acute)
+    'e'
+
+Normalizing combining characters before passing the character literal to C++ may resolve *some* of these issues:
+
+.. code-block:: python
+
+    >>> example.pass_wchar(unicodedata.normalize('NFC', combining_e_acute))
+    'é'
+
+In some languages (Thai for example), there are `graphemes that cannot be expressed as a single Unicode code point <http://unicode.org/reports/tr29/#Grapheme_Cluster_Boundaries>`_, so there is no way to capture them in a C++ character type.
+
+
+References
+==========
+
+* `The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!) <https://www.joelonsoftware.com/2003/10/08/the-absolute-minimum-every-software-developer-absolutely-positively-must-know-about-unicode-and-character-sets-no-excuses/>`_
+* `C++ - Using STL Strings at Win32 API Boundaries <https://msdn.microsoft.com/en-ca/magazine/mt238407.aspx>`_ 
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