使用VC 15.4.0，std :: function赋值生成C2679（用15.3.5编译） [英] With VC 15.4.0, std::function assignment produces C2679 (compiled with 15.3.5)

问题描述

以下代码使用VC 15.3.5编译得很好：

The following code compiles just fine with VC 15.3.5:

#include <Windows.h>
#include <string>
#include <memory>
#include <optional>
#include <functional>

class TypeDictionary;
class IStructureField {};

using  FieldFactory = std::function<std::unique_ptr<IStructureField>(TypeDictionary& dict, LPCSTR szName, std::optional<ULONG> ulOffset, std::optional<std::string> strFieldType, std::optional<ULONG> ulFieldSize)>;


int main()
{
FieldFactory ff;

ff = [](TypeDictionary& dict, LPCSTR szName, std::optional<ULONG> ulOffset, std::optional<std::string> szFieldType, std::optional<ULONG> ulFieldSize) -> std::unique_ptr<IStructureField> {
    return  std::unique_ptr<IStructureField>();
};

    return 0;
}

它使用15.4.0生成以下错误消息：

It produces the following error message with 15.4.0:

1>f:\projects\fullpath\fullpath\fullpath.cpp(22): error C2679: binary '=': no operator found which takes a right-hand operand of type 'main::<lambda_36489dce1cc59e5a8de4329a29806b5a>' (or there is no acceptable conversion)
1>c:\program files (x86)\microsoft visual studio\2017\enterprise\vc\tools\msvc\14.11.25503\include\functional(656): note: could be 'std::function<std::unique_ptr<IStructureField,std::default_delete<_Ty>> (TypeDictionary &,LPCSTR,std::optional<ULONG>,std::optional<std::string>,std::optional<ULONG>)> &std::function<std::unique_ptr<_Ty,std::default_delete<_Ty>> (TypeDictionary &,LPCSTR,std::optional<ULONG>,std::optional<std::string>,std::optional<ULONG>)>::operator =(std::nullptr_t) noexcept'
1>        with
1>        [
1>            _Ty=IStructureField
1>        ]
1>c:\program files (x86)\microsoft visual studio\2017\enterprise\vc\tools\msvc\14.11.25503\include\functional(629): note: or       'std::function<std::unique_ptr<IStructureField,std::default_delete<_Ty>> (TypeDictionary &,LPCSTR,std::optional<ULONG>,std::optional<std::string>,std::optional<ULONG>)> &std::function<std::unique_ptr<_Ty,std::default_delete<_Ty>> (TypeDictionary &,LPCSTR,std::optional<ULONG>,std::optional<std::string>,std::optional<ULONG>)>::operator =(std::function<std::unique_ptr<_Ty,std::default_delete<_Ty>> (TypeDictionary &,LPCSTR,std::optional<ULONG>,std::optional<std::string>,std::optional<ULONG>)> &&)'
1>        with
1>        [
1>            _Ty=IStructureField
1>        ]
1>c:\program files (x86)\microsoft visual studio\2017\enterprise\vc\tools\msvc\14.11.25503\include\functional(612): note: or       'std::function<std::unique_ptr<IStructureField,std::default_delete<_Ty>> (TypeDictionary &,LPCSTR,std::optional<ULONG>,std::optional<std::string>,std::optional<ULONG>)> &std::function<std::unique_ptr<_Ty,std::default_delete<_Ty>> (TypeDictionary &,LPCSTR,std::optional<ULONG>,std::optional<std::string>,std::optional<ULONG>)>::operator =(const std::function<std::unique_ptr<_Ty,std::default_delete<_Ty>> (TypeDictionary &,LPCSTR,std::optional<ULONG>,std::optional<std::string>,std::optional<ULONG>)> &)'
1>        with
1>        [
1>            _Ty=IStructureField
1>        ]
1>f:\projects\fullpath\fullpath\fullpath.cpp(22): note: while trying to match the argument list '(FieldFactory, main::<lambda_36489dce1cc59e5a8de4329a29806b5a>)'
1>Done building project "FullPath.vcxproj" -- FAILED.

正如我在cppcon上看到的那样（Stephan的演讲），在< functional> ......这可能是相关的吗？我犯了什么错误？

As I saw on cppcon (Stephan's talk), a lot of work has been done on <functional>... Could this be related? Any mistake I made?

解决方法也没关系。

推荐答案

我想知道是否如果你真的定义了TypeDictionary，而不是只是向前声明它，它会工作。

I wonder whether it would work if you actually define TypeDictionary, rather than just forward-declaring it.

我的猜测是，较新的编译器实现了规范的这一部分（
http://en.cppreference.com/w/cpp/utility/functional/function/ operator％3D ）：

My guess is, the newer compiler implements this part of the spec ( http://en.cppreference.com/w/cpp/utility/functional/function/operator%3D ):

"此运算符不参与重载决策，除非f对于参数类型Args可调用并返回类型R.（因为C + +14）"

"This operator does not participate in overload resolution unless f is Callable for argument types Args... and return type R. (since C++14)"

当尝试确定lambda是否可调用时失败，因为其中一个参数属于不完整类型。

And it fails when trying to determine whether the lambda is callable, because one of the parameters is of an incomplete type.

错误spew列出了operator =编译器尝试过的所有重载，并且未提及您要使用的模板化的重载。这表明它已经从之前设置的过载中消除了。

The error spew lists all the overloads of operator= the compiler has tried, and the templated one you want to use is not mentioned. Which suggests that it got eliminated from the overload set earlier.

现在，我不确定它是否真的应该起作用。这需要通过"可调用"仔细解析标准的含义，这比我现在可以召唤的时间和耐心要多。

Now, I'm not sure whether or not it's actually supposed to work. That would require a careful parsing of what the standard means by "Callable", which would take more time and patience than I can summon at the moment.

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