Visual Studio C++ 2015 std::codecvt with char16_t 或 char32_t

问题描述

这段代码在VS2013下编译OK:

This code compiled OK under VS2013:

std::string Unicode::utf16_to_utf8(std::u16string utf16_string)
{
    std::wstring_convert<std::codecvt_utf8_utf16<char16_t>, char16_t> convert;
    return convert.to_bytes(utf16_string);
}

现在使用 VS2015 我得到:

Now with VS2015 I get:

1>unicode.obj : error LNK2001: unresolved external symbol "__declspec(dllimport) public: static class std::locale::id std::codecvt<char16_t,char,struct _Mbstatet>::id" (__imp_?id@?$codecvt@_SDU_Mbstatet@@@2V0localeA)

推荐答案

老问题，但供以后参考:这是 Visual Studio 2015 中的一个已知错误，如 此主题 MSDN Social.

Old question, but for future reference: this is a known bug in Visual Studio 2015, as explained in the latest post (January 7th 2016) in this thread of MSDN Social.

您示例的解决方法如下所示(为简单起见，我将您的方法实现为免费函数):

The workaround for your example looks like this (I implemented your method as a free function for simplicity):

#include <codecvt>
#include <locale>
#include <string>
#include <iostream>

#if _MSC_VER >= 1900

std::string utf16_to_utf8(std::u16string utf16_string)
{
    std::wstring_convert<std::codecvt_utf8_utf16<int16_t>, int16_t> convert;
    auto p = reinterpret_cast<const int16_t *>(utf16_string.data());
    return convert.to_bytes(p, p + utf16_string.size());
}

#else

std::string utf16_to_utf8(std::u16string utf16_string)
{
    std::wstring_convert<std::codecvt_utf8_utf16<char16_t>, char16_t> convert;
    return convert.to_bytes(utf16_string);
}

#endif

int main()
{
    std::cout << utf16_to_utf8(u"lémentaire, mon cher Watson!") << std::endl;

    return 0;
}

希望这个问题能在以后的版本中得到修复，否则#if条件需要细化.更新:不，在 VS 2017 中未修复.因此，我已将预处理器条件更新为 >=1900(最初为 ==1900).

Hopefully, the problem will be fixed in future releases, otherwise the #if condition will need refining. UPDATE: nope, not fixed in VS 2017. Therefore, I've updated the preprocessor conditional to >= 1900 (initially was == 1900).

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