c++ - 在 C++ 中使用 lambda 时出现奇怪的编译器消息

Question

所以我在 C++ 项目中添加了 lambda 的使用，现在编译器给出如下输出:

9>C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\include\xxcallobj(13): warning C4800: 'BOOL' : forcing value to bool 'true' or 'false' (performance warning)
3>          C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\include\xxfunction(386) : see reference to class template instantiation 'std::tr1::_Impl_no_alloc0<_Callable,_Rx>' being compiled
3>          with
3>          [
3>              _Callable=_MyWrapper,
3>              _Rx=bool
3>          ]
3>          C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\include\xxfunction(369) : see reference to function template instantiation 'void std::tr1::_Function_impl0<_Ret>::_Reset0o<_Myimpl,_Fty,std::allocator<_Ty>>(_Fty,_Alloc)' being compiled
3>          with
3>          [
3>              _Ret=bool,
3>              _Fty=`anonymous-namespace'::<lambda3>,
3>              _Ty=std::tr1::_Function_impl0<bool>,
3>              _Alloc=std::allocator<std::tr1::_Function_impl0<bool>>
3>          ]
3>          C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\include\functional(113) : see reference to function template instantiation 'void std::tr1::_Function_impl0<_Ret>::_Reset<_Fx>(_Fty)' being compiled
3>          with
3>          [
3>              _Ret=bool,
3>              _Fx=`anonymous-namespace'::<lambda3>,
3>              _Fty=`anonymous-namespace'::<lambda3>
3>          ]
3>          ..\..\Common\IOFile.cpp(1162) : see reference to function template instantiation 'std::tr1::function<_Fty>::function<`anonymous-namespace'::<lambda3>>(_Fx)' being compiled
3>          with
3>          [
3>              _Fty=bool (void),
3>              _Fx=`anonymous-namespace'::<lambda3>
3>          ]

我的 lambda 代码是:

在.h中:

typedef std::function<bool ()> RepeatingFunction;
static bool RepeatFileOperation(RepeatingFunction callback);

static bool Copy(CString file, CString copyFileName, bool failIfExists = true);

在 .cpp 中:

bool IOFile::RepeatFileOperation(RepeatingFunction callback)
{
    const int times_to_retry = 10;
    bool succeed = false;

    // Retry a few times if it doesn't work
    int retries = 0;
    do
    {
        // Perform the caller's action on the file
        succeed = callback();
    }
    while (!succeed && retries++ < times_to_retry);

    return succeed;
}

bool IOFile::Copy(CString file, CString copyFileName, bool failIfExists)
{
    return RepeatFileOperation([&] {
        return CopyFile(file, copyFileName, static_cast<BOOL>(failIfExists));
    });
}

程序仍然编译得很好。我用谷歌搜索了这个错误，发现人们收到了类似的消息，但在他们的情况下，程序没有构建。在所有情况下，他们的构建错误似乎都与前向声明有关，但正如我所说，我的构建很好，我包括了 <functional>在头文件中，因此它应该能够很好地找到它。

这些消息是我应该担心的事情还是它们只是预期的行为？

Answer 1

CopyFile不返回 bool ，但您将其结果隐式转换为 bool .只需添加一个显式转换 return static_cast<bool>(CopyFile(...