我有一个班级：
#include <memory>
class Object {
std::shared_ptr<void> object_ptr;
public:
Object() {}
template<typename T>
Object(T&& object)
: object_ptr {new T {std::move(object)} } {}
virtual ~Object() {};
};
我的主要cpp文件是：
#include <iostream>
#include "Object.hpp"
class Foo {};
int main() {
Object o {Foo{}};
}
它给了我错误：
test/test.cpp:13:20: required from here
include/Object.hpp:24:49: error: could not convert ‘{std::move<Foo&>((* & object))}’ from ‘<brace-enclosed initializer list>’ to ‘Foo’
: object_ptr {new T {std::move(object)} } {}
^
include/Object.hpp:24:49: error: no matching function for call to ‘std::shared_ptr<void>::shared_ptr(<brace-enclosed initializer list>)’
include/Object.hpp:24:49: note: candidates are:
In file included from /usr/include/c++/4.8/memory:82:0,
from include/Object.hpp:7,
from test/test.cpp:2:
/usr/include/c++/4.8/bits/shared_ptr.h:314:2: note: template<class _Alloc, class ... _Args> std::shared_ptr<_Tp>::shared_ptr(std::_Sp_make_shared_tag, const _Alloc&, _Args&& ...)
shared_ptr(_Sp_make_shared_tag __tag, const _Alloc& __a,
^
/usr/include/c++/4.8/bits/shared_ptr.h:314:2: note: template argument deduction/substitution failed:
/usr/include/c++/4.8/bits/shared_ptr.h:265:17: note: constexpr std::shared_ptr<_Tp>::shared_ptr(std::nullptr_t) [with _Tp = void; std::nullptr_t = std::nullptr_t]
constexpr shared_ptr(nullptr_t __p) noexcept
^
/usr/include/c++/4.8/bits/shared_ptr.h:265:17: note: no known conversion for argument 1 from ‘<type error>’ to ‘std::nullptr_t’
/usr/include/c++/4.8/bits/shared_ptr.h:257:2: note: template<class _Tp1, class _Del> std::shared_ptr<_Tp>::shared_ptr(std::unique_ptr<_Up, _Ep>&&)
shared_ptr(std::unique_ptr<_Tp1, _Del>&& __r)
^
/usr/include/c++/4.8/bits/shared_ptr.h:257:2: note: template argument deduction/substitution failed:
/usr/include/c++/4.8/bits/shared_ptr.h:253:2: note: template<class _Tp1> std::shared_ptr<_Tp>::shared_ptr(std::auto_ptr<_Up>&&)
shared_ptr(std::auto_ptr<_Tp1>&& __r);
^
/usr/include/c++/4.8/bits/shared_ptr.h:253:2: note: template argument deduction/substitution failed:
/usr/include/c++/4.8/bits/shared_ptr.h:248:11: note: template<class _Tp1> std::shared_ptr<_Tp>::shared_ptr(const std::weak_ptr<_Tp1>&)
explicit shared_ptr(const weak_ptr<_Tp1>& __r)
^
/usr/include/c++/4.8/bits/shared_ptr.h:248:11: note: template argument deduction/substitution failed:
/usr/include/c++/4.8/bits/shared_ptr.h:236:2: note: template<class _Tp1, class> std::shared_ptr<_Tp>::shared_ptr(std::shared_ptr<_Tp1>&&)
shared_ptr(shared_ptr<_Tp1>&& __r) noexcept
^
/usr/include/c++/4.8/bits/shared_ptr.h:236:2: note: template argument deduction/substitution failed:
/usr/include/c++/4.8/bits/shared_ptr.h:226:7: note: std::shared_ptr<_Tp>::shared_ptr(std::shared_ptr<_Tp>&&) [with _Tp = void]
shared_ptr(shared_ptr&& __r) noexcept
^
/usr/include/c++/4.8/bits/shared_ptr.h:226:7: note: no known conversion for argument 1 from ‘<type error>’ to ‘std::shared_ptr<void>&&’
/usr/include/c++/4.8/bits/shared_ptr.h:218:2: note: template<class _Tp1, class> std::shared_ptr<_Tp>::shared_ptr(const std::shared_ptr<_Tp1>&)
shared_ptr(const shared_ptr<_Tp1>& __r) noexcept
^
/usr/include/c++/4.8/bits/shared_ptr.h:218:2: note: template argument deduction/substitution failed:
/usr/include/c++/4.8/bits/shared_ptr.h:206:2: note: template<class _Tp1> std::shared_ptr<_Tp>::shared_ptr(const std::shared_ptr<_Tp1>&, _Tp*)
shared_ptr(const shared_ptr<_Tp1>& __r, _Tp* __p) noexcept
^
/usr/include/c++/4.8/bits/shared_ptr.h:206:2: note: template argument deduction/substitution failed:
/usr/include/c++/4.8/bits/shared_ptr.h:184:2: note: template<class _Deleter, class _Alloc> std::shared_ptr<_Tp>::shared_ptr(std::nullptr_t, _Deleter, _Alloc)
shared_ptr(nullptr_t __p, _Deleter __d, _Alloc __a)
^
/usr/include/c++/4.8/bits/shared_ptr.h:184:2: note: template argument deduction/substitution failed:
/usr/include/c++/4.8/bits/shared_ptr.h:165:2: note: template<class _Tp1, class _Deleter, class _Alloc> std::shared_ptr<_Tp>::shared_ptr(_Tp1*, _Deleter, _Alloc)
shared_ptr(_Tp1* __p, _Deleter __d, _Alloc __a)
^
/usr/include/c++/4.8/bits/shared_ptr.h:165:2: note: template argument deduction/substitution failed:
/usr/include/c++/4.8/bits/shared_ptr.h:146:2: note: template<class _Deleter> std::shared_ptr<_Tp>::shared_ptr(std::nullptr_t, _Deleter)
shared_ptr(nullptr_t __p, _Deleter __d)
^
/usr/include/c++/4.8/bits/shared_ptr.h:146:2: note: template argument deduction/substitution failed:
/usr/include/c++/4.8/bits/shared_ptr.h:129:2: note: template<class _Tp1, class _Deleter> std::shared_ptr<_Tp>::shared_ptr(_Tp1*, _Deleter)
shared_ptr(_Tp1* __p, _Deleter __d)
^
/usr/include/c++/4.8/bits/shared_ptr.h:129:2: note: template argument deduction/substitution failed:
/usr/include/c++/4.8/bits/shared_ptr.h:112:11: note: template<class _Tp1> std::shared_ptr<_Tp>::shared_ptr(_Tp1*)
explicit shared_ptr(_Tp1* __p)
^
/usr/include/c++/4.8/bits/shared_ptr.h:112:11: note: template argument deduction/substitution failed:
/usr/include/c++/4.8/bits/shared_ptr.h:103:7: note: std::shared_ptr<_Tp>::shared_ptr(const std::shared_ptr<_Tp>&) [with _Tp = void]
shared_ptr(const shared_ptr&) noexcept = default;
^
/usr/include/c++/4.8/bits/shared_ptr.h:103:7: note: no known conversion for argument 1 from ‘<type error>’ to ‘const std::shared_ptr<void>&’
/usr/include/c++/4.8/bits/shared_ptr.h:100:17: note: constexpr std::shared_ptr<_Tp>::shared_ptr() [with _Tp = void]
constexpr shared_ptr() noexcept
^
/usr/include/c++/4.8/bits/shared_ptr.h:100:17: note: candidate expects 0 arguments, 1 provided
make: *** [test.o] Error 1
但是，如果我将new T {std::move(object)}更改为new T (std::move(object))。它有效：
class Object {
std::shared_ptr<void> object_ptr;
public:
Object() {}
template<typename T>
Object(T&& object)
: object_ptr {new T (std::move(object)) } {}
virtual ~Object() {};
};
为什么大括号统一初始化在这里不起作用？在这种情况下，为什么被认为是初始化列表？ Foo甚至没有构造函数采用初始化列表？