Question

我正在尝试使用WideCharToMultiByte将std :: wstring转换为utf8 std :: string。这是我的代码：

const std::wstring & utf16("lorem ipsum"); // input

if (utf16.empty()) {
    return "";
}
cout << "wstring -> string, input: , size: " << utf16.size() << endl;
for (size_t i = 0; i < utf16.size(); ++i) {
    cout << i << ": " << static_cast<int>(utf16[i]) << endl;
}
for (size_t i = 0; i < utf16.size(); ++i) {
    wcout << static_cast<wchar_t>(utf16[i]);
}
cout << endl;
std::string res;
int required_size = 0;
if ((required_size = WideCharToMultiByte(
    CP_UTF8,
    0,
    utf16.c_str(),
    utf16.size(),
    nullptr,
    0,
    nullptr,
    nullptr
)) == 0) {
    throw std::invalid_argument("Cannot convert.");
}
cout << "required size: " << required_size << endl;
res.resize(required_size);
if (WideCharToMultiByte(
    CP_UTF8,
    0,
    utf16.c_str(),
    utf16.size(),
    &res[0],
    res.size(),
    nullptr,
    nullptr
) == 0) {
    throw std::invalid_argument("Cannot convert.");
}
cout << "Result: " << res << ", size: " << res.size() << endl;
for (size_t i = 0; i < res.size(); ++i) {
    cout << i << ": " << (int)static_cast<uint8_t>(res[i]) << endl;
}
exit(1);
return res;

它运行正常，没有例外，没有错误。只有结果是错误的。以下是运行代码的输出：

wstring -> string, input: , size: 11
0: 108
1: 111
2: 114
3: 101
4: 109
5: 32
6: 105
7: 112
8: 115
9: 117
10: 109
lorem ipsum
required size: 11
Result: lorem , size: 11
0: 108
1: 0
2: 111
3: 0
4: 114
5: 0
6: 101
7: 0
8: 109
9: 0
10: 32

我不明白为什么有空字节。我做错了什么？

Answer 1

_{总结评论：}

就WideCharToMultiByte逻辑和参数而言，您的代码是正确的;唯一的实际问题是utf16的初始化，需要用宽文字进行初始化。该代码提供了VC ++ 2015 RTM和Update 1的预期结果，因此这是您正在使用的WideCharToMultiByte仿真层中的错误。

也就是说，对于C ++ 11以后，有一个可行的便携式解决方案：std::wstring_convert与std::codecvt_utf8_utf16

结合使用

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

std::string test(std::wstring const& utf16)
{
    std::wcout << L"wstring -> string, input: " << utf16 << L", size: " << utf16.size() << L'\n';
    for (std::size_t i{}; i != utf16.size(); ++i)
        std::wcout << i << L": " << static_cast<int>(utf16[i]) << L'\n';
    for (std::size_t i{}; i != utf16.size(); ++i)
        std::wcout << utf16[i];
    std::wcout << L'\n';

    std::wstring_convert<std::codecvt_utf8_utf16<wchar_t>> cvt;
    std::string res = cvt.to_bytes(utf16);
    std::wcout << L"Result: " << res.c_str() << L", size: " << res.size() << L'\n';
    for (std::size_t i{}; i != res.size(); ++i)
        std::wcout << i << L": " << static_cast<int>(res[i]) << L'\n';
    return res;
}

int main()
{
    test(L"lorem ipsum");
}

Online Demo

WideCharToMultiByte在Wine中不起作用

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