将 unique_ptr 设为前向声明类型的解决方法 [英] Workaround for Making a unique_ptr to a Forward Declared Type

问题描述

鉴于我有一个转发声明类型:

class Foo;

我想制作一个 unique_ptr到这种类型:

I want to make a unique_ptr to this type:

unique_ptr<Foo> pFoo;

这在 visual-studio-2017 但我不能让它在 visual-studio-2012.

This works fine in visual-studio-2017 but I can't make it work in visual-studio-2012.

C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\memory(1150): error C2027: use of undefined type Foo (....\src\STETestbed\STETestbed.cpp)
O:\Engine\stetestbed\include\STETestbed\ComponentDirector.h(26) :参见 Foo
的声明C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\memory(1149) : while 编译类模板成员函数 void std::default_delete<_Ty>::operator ()(_Ty *) throw() 常量
与
[
_Ty=Foo
]
C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\memory(1444) :参见对函数模板实例化的引用 void std::default_delete<_Ty>::operator ()(_Ty *) throw() const 正在编译
与
[
_Ty=Foo
]
C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\type_traits(743) :参见正在编译的类模板实例std::default_delete<_Ty>的参考
与
[
_Ty=Foo
]
C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\memory(1281) :请参阅对正在编译的类模板实例化std::is_empty<_Ty> 的引用
与
[
_Ty=std::default_delete
]
O:\Engine\stetestbed\include\STETestbed\ComponentDirector.h(63) :参见正在编译的类模板实例std::unique_ptr<_Ty>的参考
与
[
_Ty=Foo
]
C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\memory(1151):错误 C2338:无法删除不完整的类型 (....\src\STETestbed\STETestbed.cpp)
C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\memory(1152):警告 C4150:删除指向不完整类型 Foo 的指针；没有调用析构函数 (....\src\STETestbed\STETestbed.cpp)
O:\Engine\stetestbed\include\STETestbed\ComponentDirector.h(26) :参见 Foo

C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\memory(1150): error C2027: use of undefined type Foo (....\src\STETestbed\STETestbed.cpp)
O:\Engine\stetestbed\include\STETestbed\ComponentDirector.h(26) : see declaration of Foo
C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\memory(1149) : while compiling class template member function void std::default_delete<_Ty>::operator ()(_Ty *) throw() const
with
[
_Ty=Foo
]
C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\memory(1444) : see reference to function template instantiation void std::default_delete<_Ty>::operator ()(_Ty *) throw() const being compiled
with
[
_Ty=Foo
]
C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\type_traits(743) : see reference to class template instantiation std::default_delete<_Ty> being compiled
with
[
_Ty=Foo
]
C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\memory(1281) : see reference to class template instantiation std::is_empty<_Ty> being compiled
with
[
_Ty=std::default_delete
]
O:\Engine\stetestbed\include\STETestbed\ComponentDirector.h(63) : see reference to class template instantiation std::unique_ptr<_Ty> being compiled
with
[
_Ty=Foo
]
C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\memory(1151): error C2338: can't delete an incomplete type (....\src\STETestbed\STETestbed.cpp)
C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\memory(1152): warning C4150: deletion of pointer to incomplete type Foo; no destructor called (....\src\STETestbed\STETestbed.cpp)
O:\Engine\stetestbed\include\STETestbed\ComponentDirector.h(26) : see declaration of Foo

在黑暗时代是否有解决方法，或者我可以不提前声明?

Was there a workaround for that back in the dark ages, or can I just not forward declare?

推荐答案

正如 Jarod42 提到的 问题似乎是 visual-studio-2012 需要完整的类型声明. visual-studio<的后期版本/a> 只需要调用括号运算符时的完整类型.

As mentioned by Jarod42 The problem seems to be that the default_deleter implemented by visual-studio-2012 required a complete type on declaration. Latter versions of visual-studio only required the complete type at the point the parenthesis operator is called.

我们可以通过提供一个函子来解决这个问题，它提供了一个不需要声明完整类型的删除器:

We can work around this by providing a functor which provides a deleter which does not require the complete type on declaration:

template <typename T>
void custom_deleter(T* param) {
    delete param;
}

要使用 custom_deleter 作为模板参数，我们需要将它变成一个函子，否则编译器会出错:

To use custom_deleter as a template parameter we'll need to make it into a functor otherwise the compiler will error:

error C2207: std::_Unique_ptr_base<_Ty,_Dx,_Empty_deleter>::_Mydel : 类模板的成员无法获取函数类型

error C2207: std::_Unique_ptr_base<_Ty,_Dx,_Empty_deleter>::_Mydel : a member of a class template cannot acquire a function type

因此，在 Foo 前向声明的标头中，我们需要将 unique_ptr 定义为:

So in the header where Foo is forward declared we'll need to define the unique_ptr as:

unique_ptr<Foo, function<void(Foo*)>> pFoo

在定义了 Foo 的实现文件中，我们需要将其赋值为:

And in the implementation file where Foo is defined we'll need to assign it as:

pFoo = decltype(pFoo)(new Foo, std::function<void(Foo*)>(custom_deleter<Foo>))

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