c++ - 在std::wstring上使用std::remove_if的警告(MSVC C++ 20)

Question

当使用winapi HWND并返回wstring时，我有一个函数。

std::wstring utility::winapi::window_class(HWND hwnd) {
    int title_length = 20;
    std::wstring class_name;
    class_name.resize(title_length, '\0');
    GetClassName(hwnd, const_cast<LPWSTR>(class_name.c_str()), title_length);
    class_name.erase(std::remove_if(class_name.begin(), class_name.end(), [](const char &x){return x == '\0';}), class_name.end());
    return class_name;
}

我收到以下错误:

C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Tools\MSVC\14.27.29110\include\xmemory(2040): error C2220: the following warning is treated as an error
..\..\libs\utility\src\winapi.cpp(44): note: see reference to function template instantiation '_FwdIt std::remove_if<std::_String_iterator<std::_String_val<std::_Simple_types<_Elem>>>,utility::winapi::window_class::<lambda_1>>(_FwdIt,const _FwdIt,_Pr)' being compiled
        with
        [
            _FwdIt=std::_String_iterator<std::_String_val<std::_Simple_types<wchar_t>>>,
            _Elem=wchar_t,
            _Pr=utility::winapi::window_class::<lambda_1>
        ]
C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Tools\MSVC\14.27.29110\include\xmemory(2040): warning C4244: 'argument': conversion from 'wchar_t' to 'const char', possible loss of data
C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Tools\MSVC\14.27.29110\include\xutility(5667): warning C4244: 'argument': conversion from '_Elem' to 'const char', possible loss of data
        with
        [
            _Elem=wchar_t
        ]
C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Tools\MSVC\14.27.29110\include\xmemory(2036): note: see reference to function template instantiation '_InIt std::find_if<wchar_t*,_Fn>(_InIt,const _InIt,_Pr)' being compiled
        with
        [
            _InIt=wchar_t *,
            _Fn=utility::winapi::window_class::<lambda_1>,
            _Pr=utility::winapi::window_class::<lambda_1>
        ]
..\..\libs\utility\src\winapi.cpp(44): note: see reference to function template instantiation '_FwdIt std::remove_if<std::_String_iterator<std::_String_val<std::_Simple_types<_Elem>>>,utility::winapi::window_class::<lambda_1>>(_FwdIt,const _FwdIt,_Pr)' being compiled
        with
        [
            _FwdIt=std::_String_iterator<std::_String_val<std::_Simple_types<wchar_t>>>,
            _Elem=wchar_t,
            _Pr=utility::winapi::window_class::<lambda_1>
        ]
ninja: build stopped: subcommand failed.

此错误是什么意思？
UPD:已更新，并显示完整错误。

Answer 1

class_name声明为std::wstring。这是一串wchar_t值，但是您的lambda参数为const char &类型，它的范围更窄。警告告诉您该值可能会被截断。在应注意的情况下，这可能导致测试x == '\0'评估为true。这是编译器试图告诉您的:

warning C4244: 'argument': conversion from 'wchar_t' to 'const char', possible loss of data

附带说明，没有理由通过引用接受数字参数。在这种情况下，很可能会导致组装相同，因为几乎可以肯定会内联lambda，但不使用引用会更干净。
要解决警告，请更改lambda以接受wchar_t:

[](wchar_t x){return x == '\0';})

从C++ 14开始，您还可以使用auto作为参数的类型:

[](auto x){return x == '\0';})