How to efficiently read a UTF-8 file into a wstring on Windows?

Accessing Unicode Data through wstring on Windows

When handling Unicode text on Windows, the question arises of how to efficiently read a Unicode (UTF-8) file into wstring(s). Fortunately, C 11 provides a versatile solution through the std::codecvt_utf8 facet.

The codecvt_utf8 facet serves as a bridge between UTF-8 byte strings and UCS2 or UCS4 character strings, enabling read and write operations for both text and binary UTF-8 files. To leverage this facet, it's recommended to create a locale object that encapsulates the necessary UTF-8 facet. This locale object can then be used to imbue stream buffers, allowing for efficient UTF-8 file handling.

The following code snippet demonstrates how to read a UTF-8 file into a wstring using this technique:

<code class="cpp">#include <sstream>
#include <fstream>
#include <codecvt>

std::wstring readFile(const char* filename) {
    std::wifstream wif(filename);
    wif.imbue(std::locale(std::locale::empty(), new std::codecvt_utf8<wchar_t>));
    std::wstringstream wss;
    wss << wif.rdbuf();
    return wss.str();
}</code>

To use this function, simply pass the file name as an argument and assign the returned wstring to a variable:

<code class="cpp">std::wstring wstr = readFile("a.txt");</code>

Alternatively, you can set the global C locale to UTF-8 using the codecvt_utf8 facet before working with string streams. This ensures that calls to the std::locale default constructor will return the global C locale imbued with the desired codec:

<code class="cpp">std::locale::global(std::locale(std::locale::empty(), new std::codecvt_utf8<wchar_t>));</code>

With this global setting, there's no need to explicitly imbue stream buffers with the locale, simplifying the process of handling UTF-8 files in your C code.

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