我的这段代码行不通,但是我认为目的很明确:
testmakeshared.cpp
#include <memory>
class A {
public:
static ::std::shared_ptr<A> create() {
return ::std::make_shared<A>();
}
protected:
A() {}
A(const A &) = delete;
const A &operator =(const A &) = delete;
};
::std::shared_ptr<A> foo()
{
return A::create();
}
但是在编译时会出现以下错误:
g++ -std=c++0x -march=native -mtune=native -O3 -Wall testmakeshared.cpp
In file included from /usr/lib/gcc/x86_64-redhat-linux/4.6.1/../../../../include/c++/4.6.1/bits/shared_ptr.h:52:0,
from /usr/lib/gcc/x86_64-redhat-linux/4.6.1/../../../../include/c++/4.6.1/memory:86,
from testmakeshared.cpp:1:
testmakeshared.cpp: In constructor ‘std::_Sp_counted_ptr_inplace<_Tp, _Alloc, _Lp>::_Sp_counted_ptr_inplace(_Alloc) [with _Tp = A, _Alloc = std::allocator<A>, __gnu_cxx::_Lock_policy _Lp = (__gnu_cxx::_Lock_policy)2u]’:
/usr/lib/gcc/x86_64-redhat-linux/4.6.1/../../../../include/c++/4.6.1/bits/shared_ptr_base.h:518:8: instantiated from ‘std::__shared_count<_Lp>::__shared_count(std::_Sp_make_shared_tag, _Tp*, const _Alloc&, _Args&& ...) [with _Tp = A, _Alloc = std::allocator<A>, _Args = {}, __gnu_cxx::_Lock_policy _Lp = (__gnu_cxx::_Lock_policy)2u]’
/usr/lib/gcc/x86_64-redhat-linux/4.6.1/../../../../include/c++/4.6.1/bits/shared_ptr_base.h:986:35: instantiated from ‘std::__shared_ptr<_Tp, _Lp>::__shared_ptr(std::_Sp_make_shared_tag, const _Alloc&, _Args&& ...) [with _Alloc = std::allocator<A>, _Args = {}, _Tp = A, __gnu_cxx::_Lock_policy _Lp = (__gnu_cxx::_Lock_policy)2u]’
/usr/lib/gcc/x86_64-redhat-linux/4.6.1/../../../../include/c++/4.6.1/bits/shared_ptr.h:313:64: instantiated from ‘std::shared_ptr<_Tp>::shared_ptr(std::_Sp_make_shared_tag, const _Alloc&, _Args&& ...) [with _Alloc = std::allocator<A>, _Args = {}, _Tp = A]’
/usr/lib/gcc/x86_64-redhat-linux/4.6.1/../../../../include/c++/4.6.1/bits/shared_ptr.h:531:39: instantiated from ‘std::shared_ptr<_Tp> std::allocate_shared(const _Alloc&, _Args&& ...) [with _Tp = A, _Alloc = std::allocator<A>, _Args = {}]’
/usr/lib/gcc/x86_64-redhat-linux/4.6.1/../../../../include/c++/4.6.1/bits/shared_ptr.h:547:42: instantiated from ‘std::shared_ptr<_Tp1> std::make_shared(_Args&& ...) [with _Tp = A, _Args = {}]’
testmakeshared.cpp:6:40: instantiated from here
testmakeshared.cpp:10:8: error: ‘A::A()’ is protected
/usr/lib/gcc/x86_64-redhat-linux/4.6.1/../../../../include/c++/4.6.1/bits/shared_ptr_base.h:400:2: error: within this context
Compilation exited abnormally with code 1 at Tue Nov 15 07:32:58
该消息基本上是说,从模板实例化堆栈中向下移动的某种随机方法::std::make_shared
无法访问构造函数,因为它是受保护的。
但是我真的很想同时使用这两种方法,::std::make_shared
并防止任何人制作此类未由a指向的对象::std::shared_ptr
。有什么办法可以做到这一点?
这个答案可能更好,我可能会接受。但是我也想出了一个比较丑陋的方法,但是它仍然让所有内容都内联并且不需要派生类:
#include <memory>
#include <string>
class A {
protected:
struct this_is_private;
public:
explicit A(const this_is_private &) {}
A(const this_is_private &, ::std::string, int) {}
template <typename... T>
static ::std::shared_ptr<A> create(T &&...args) {
return ::std::make_shared<A>(this_is_private{0},
::std::forward<T>(args)...);
}
protected:
struct this_is_private {
explicit this_is_private(int) {}
};
A(const A &) = delete;
const A &operator =(const A &) = delete;
};
::std::shared_ptr<A> foo()
{
return A::create();
}
::std::shared_ptr<A> bar()
{
return A::create("George", 5);
}
::std::shared_ptr<A> errors()
{
::std::shared_ptr<A> retval;
// Each of these assignments to retval properly generates errors.
retval = A::create("George");
retval = new A(A::this_is_private{0});
return ::std::move(retval);
}
编辑2017年1月6日:我对此进行了更改,以明确地表明此思想对于带有参数的构造函数很容易扩展,因为其他人正在按照这些思路提供答案,并且对此感到困惑。
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我来说两句