为什么这个包装分配器的构造函数在模板替换期间采用了错误的类型（完美转发构造函数）？
Why does this wrapping allocator's constructor take in the wrong type during template substitution (perfect forwarding ctor)?
对于某些上下文，我正在编写一个分配器，它将基本分配器作为模板类型，除了将 allocate() 和 deallocate() 调用转发给底层分配器成员外什么都不做。使用此自定义分配器创建 std::vectors 效果很好。 我尝试编写一个默认使用 dummy_allocator<T, std::allocator<T>> 的 make_shared 包装器，但没有成功。 这是一个可重现的示例：
#include <memory>
namespace test {
template<typename T, typename base_allocator=std::allocator<T>>
class dummy_allocator {
public:
typedef typename std::allocator_traits<base_allocator>::size_type size_type;
typedef typename std::allocator_traits<base_allocator>::difference_type difference_type;
typedef typename std::allocator_traits<base_allocator>::pointer pointer;
typedef typename std::allocator_traits<base_allocator>::const_pointer const_pointer;
typedef typename std::allocator_traits<base_allocator>::value_type value_type;
template<class U>
struct rebind {
typedef dummy_allocator<U,
typename std::allocator_traits<base_allocator>::template rebind_alloc<U>> other;
};
template<typename... Args>
dummy_allocator(Args &&... args) noexcept : alloc(std::forward<Args>(args)...) {}
dummy_allocator(const dummy_allocator& a) = default;
[[nodiscard]] T *allocate(std::size_t n) {
T *p = alloc.allocate(n);
return p;
}
void deallocate(T *p, std::size_t size) noexcept {
alloc.deallocate(p, size);
}
private:
base_allocator alloc;
};
/// Allocate using a wrapped version of passed in allocator
template <typename T, typename Alloc, typename... Args>
std::shared_ptr<T> allocate_shared(const Alloc& alloc, Args&&... args) {
auto dummy_alloc = dummy_allocator<T, Alloc>(alloc);
return std::allocate_shared<T>(dummy_alloc, std::forward<Args>(args)...);
}
/// Create a shared pointer from a default stl allocator wrapped in profile allocator.
template <typename T, typename... Args>
std::shared_ptr<T> make_shared(Args&&... args) {
return test::allocate_shared<T>(std::allocator<T>(), std::forward<Args>(args)...);
}
} // namespace test
int main() {
auto ptr = test::make_shared<double>();
return 0;
}
当我运行上面的代码时，编译器产生了一些奇怪的模板替换失败错误：
/usr/include/c++/10.1.0/bits/shared_ptr_base.h:679:43: required from ‘std::__shared_count<_Lp>::__shared_count(_Tp*&, std::_Sp_alloc_shared_tag<_Alloc>, _Args&& ...) [with _Tp = double; _Alloc = test::dummy_allocator<double, std::allocator<double> >; _Args = {}; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]’
/usr/include/c++/10.1.0/bits/shared_ptr_base.h:1371:71: required from ‘std::__shared_ptr<_Tp, _Lp>::__shared_ptr(std::_Sp_alloc_shared_tag<_Tp>, _Args&& ...) [with _Alloc = test::dummy_allocator<double, std::allocator<double> >; _Args = {}; _Tp = double; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]’
/usr/include/c++/10.1.0/bits/shared_ptr.h:408:59: required from ‘std::shared_ptr<_Tp>::shared_ptr(std::_Sp_alloc_shared_tag<_Tp>, _Args&& ...) [with _Alloc = test::dummy_allocator<double, std::allocator<double> >; _Args = {}; _Tp = double]’
/usr/include/c++/10.1.0/bits/shared_ptr.h:859:14: required from ‘std::shared_ptr<_Tp> std::allocate_shared(const _Alloc&, _Args&& ...) [with _Tp = double; _Alloc = test::dummy_allocator<double, std::allocator<double> >; _Args = {}]’
/home/ray/home/testing/src/alloc.cpp:44:35: required from ‘std::shared_ptr<_Tp> test::allocate_shared(const Alloc&, Args&& ...) [with T = double; Alloc = std::allocator<double>; Args = {}]’
/home/ray/home/testing/src/alloc.cpp:50:36: required from ‘std::shared_ptr<_Tp> test::make_shared(Args&& ...) [with T = double; Args = {}]’
/home/ray/home/testing/src/alloc.cpp:86:46: required from here
/home/ray/home/testing/src/alloc.cpp:25:82: error: no matching function for call to ‘std::allocator<std::_Sp_counted_ptr_inplace<double, test::dummy_allocator<double, std::allocator<double> >, __gnu_cxx::_S_atomic> >::allocator(const test::dummy_allocator<double, std::allocator<double> >&)’
25 | dummy_allocator(Args &&... args) noexcept : alloc(std::forward<Args>(args)...) {}
| ^
In file included from /usr/include/c++/10.1.0/list:61,
from /home/ray/home/testing/src/alloc.cpp:3:
/usr/include/c++/10.1.0/bits/allocator.h:157:2: note: candidate: ‘template<class _Tp1> constexpr std::allocator< <template-parameter-1-1> >::allocator(const std::allocator<_Tp1>&) [with _Tp1 = _Tp1; _Tp = std::_Sp_counted_ptr_inplace<double, test::dummy_allocator<double, std::allocator<double> >, __gnu_cxx::_S_atomic>]’
157 | allocator(const allocator<_Tp1>&) _GLIBCXX_NOTHROW { }
| ^~~~~~~~~
/usr/include/c++/10.1.0/bits/allocator.h:157:2: note: template argument deduction/substitution failed:
/home/ray/home/testing/src/alloc.cpp:25:82: note: ‘const test::dummy_allocator<double, std::allocator<double> >’ is not derived from ‘const std::allocator<_Up>’
25 | dummy_allocator(Args &&... args) noexcept : alloc(std::forward<Args>(args)...) {}
| ^
In file included from /usr/include/c++/10.1.0/list:61,
from /home/ray/home/testing/src/alloc.cpp:3:
/usr/include/c++/10.1.0/bits/allocator.h:147:7: note: candidate: ‘constexpr std::allocator< <template-parameter-1-1> >::allocator(const std::allocator< <template-parameter-1-1> >&) [with _Tp = std::_Sp_counted_ptr_inplace<double, test::dummy_allocator<double, std::allocator<double> >, __gnu_cxx::_S_atomic>]’
147 | allocator(const allocator& __a) _GLIBCXX_NOTHROW
| ^~~~~~~~~
/usr/include/c++/10.1.0/bits/allocator.h:147:34: note: no known conversion for argument 1 from ‘const test::dummy_allocator<double, std::allocator<double> >’ to ‘const std::allocator<std::_Sp_counted_ptr_inplace<double, test::dummy_allocator<double, std::allocator<double> >, __gnu_cxx::_S_atomic> >&’
147 | allocator(const allocator& __a) _GLIBCXX_NOTHROW
| ~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/10.1.0/bits/allocator.h:144:7: note: candidate: ‘constexpr std::allocator< <template-parameter-1-1> >::allocator() [with _Tp = std::_Sp_counted_ptr_inplace<double, test::dummy_allocator<double, std::allocator<double> >, __gnu_cxx::_S_atomic>]’
144 | allocator() _GLIBCXX_NOTHROW { }
| ^~~~~~~~~
/usr/include/c++/10.1.0/bits/allocator.h:144:7: note: candidate expects 0 arguments, 1 provided
... (The above error basically is repeated 2 more times)
特别是，这个错误似乎代表了正在发生的问题：
error: no matching function for call to ‘std::allocator<std::_Sp_counted_ptr_inplace<double, test::dummy_allocator<double, std::allocator<double> >, __gnu_cxx::_S_atomic> >::allocator(const test::dummy_allocator<double, std::allocator<double> >&)’
基本上说的是"in the constructor of dummy_allocator, you can't pass in a dummy_allocator to the std::allocator's constructor"。但我没有那样做。在 allocate_shared 中，我将 std::allocator 传入 dummy_allocator。
阅读编译器错误时我真的抓狂了，但没有得出我做错了什么的结论。任何帮助将不胜感激！
编辑：我想我有一种预感，无论shared_ptr在下面做什么，它都在尝试做我的dummy_allocator的复制构造函数，而完美转发是捕获复制构造而不是实际的复制构造函数。 但是，我没有知道如何解决这个问题，因为它是一个可变参数模板，我不能在完美的转发构造函数中使用 std::is_same<Args, dummy_allocator> 作为 requires 子句.
因此，正如 Daniel Langr 所指出的，仅复制构建 dummy_allocator 将失败。我用以下 requires 子句解决了这个问题：
...
template <typename T1, typename ...TV>
struct is_dummy : std::is_same<typename std::decay<T1>::type, dummy_allocator<T, base_allocator>>{
};
template<typename... Args>
requires (!is_dummy<Args...>::value)
dummy_allocator(Args &&... args) noexcept : alloc(std::forward<Args>(args)...) {}
dummy_allocator() = default;
dummy_allocator(const dummy_allocator& a) = default;
...
但是，这还不足以解决 shared_ptr 问题，其中的错误有点长...*
所以这是你得到的错误：
#include <memory>
namespace test {
template<typename T, typename base_allocator=std::allocator<T>>
class dummy_allocator {
public:
/// Necessary for allocators, propagate exactly what the base_allocator
/// wants.
typedef typename std::allocator_traits<base_allocator>::size_type size_type;
typedef typename std::allocator_traits<base_allocator>::difference_type difference_type;
typedef typename std::allocator_traits<base_allocator>::pointer pointer;
typedef typename std::allocator_traits<base_allocator>::const_pointer const_pointer;
typedef typename std::allocator_traits<base_allocator>::value_type value_type;
template<class U>
struct rebind {
typedef dummy_allocator<U,
typename std::allocator_traits<base_allocator>::template rebind_alloc<U>> other;
};
template <typename T1, typename ...TV>
struct is_dummy : std::is_same<typename std::decay<T1>::type, dummy_allocator<T, base_allocator>>{
};
template<typename... Args>
requires (!is_dummy<Args...>::value)
dummy_allocator(Args &&... args) noexcept : alloc(std::forward<Args>(args)...) {}
dummy_allocator() = default;
dummy_allocator(const dummy_allocator& a) = default;
[[nodiscard]] T *allocate(std::size_t n) {
T *p = alloc.allocate(n);
return p;
}
void deallocate(T *p, std::size_t size) noexcept {
alloc.deallocate(p, size);
}
private:
base_allocator alloc;
};
/// Allocate using a wrapped version of passed in allocator
template <typename T, typename Alloc, typename... Args>
auto allocate_shared(const Alloc& alloc, Args&&... args) {
auto dummy_alloc = dummy_allocator<T, Alloc>(alloc);
return std::allocate_shared<T>(dummy_alloc, std::forward<Args>(args)...);
}
/// Create a shared pointer from a default stl allocator wrapped in profile allocator.
template <typename T, typename... Args>
auto make_shared(Args&&... args) {
return test::allocate_shared<T>(std::allocator<T>(), std::forward<Args>(args)...);
}
} // namespace test
int main() {
// This will fail
auto ptr = test::make_shared<double>();
// This will now work
auto dummy_alloc = test::dummy_allocator<int, std::allocator<int>>();
auto dummy_alloc2 = test::dummy_allocator<int, std::allocator<int>>(dummy_alloc);
return 0;
}
相关错误：
/usr/include/c++/10.1.0/bits/shared_ptr_base.h:679:43: required from ‘std::__shared_count<_Lp>::__shared_count(_Tp*&, std::_Sp_alloc_shared_tag<_Alloc>, _Args&& ...) [with _Tp = double; _Alloc = test::dummy_allocator<double, std::allocator<double> >; _Args = {}; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]’
/usr/include/c++/10.1.0/bits/shared_ptr_base.h:1371:71: required from ‘std::__shared_ptr<_Tp, _Lp>::__shared_ptr(std::_Sp_alloc_shared_tag<_Tp>, _Args&& ...) [with _Alloc = test::dummy_allocator<double, std::allocator<double> >; _Args = {}; _Tp = double; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]’
/usr/include/c++/10.1.0/bits/shared_ptr.h:408:59: required from ‘std::shared_ptr<_Tp>::shared_ptr(std::_Sp_alloc_shared_tag<_Tp>, _Args&& ...) [with _Alloc = test::dummy_allocator<double, std::allocator<double> >; _Args = {}; _Tp = double]’
/usr/include/c++/10.1.0/bits/shared_ptr.h:859:14: required from ‘std::shared_ptr<_Tp> std::allocate_shared(const _Alloc&, _Args&& ...) [with _Tp = double; _Alloc = test::dummy_allocator<double, std::allocator<double> >; _Args = {}]’
/home/ray/home/test/src/alloc.cpp:53:35: required from ‘auto test::allocate_shared(const Alloc&, Args&& ...) [with T = double; Alloc = std::allocator<double>; Args = {}]’
/home/ray/home/test/src/alloc.cpp:59:36: required from ‘auto test::make_shared(Args&& ...) [with T = double; Args = {}]’
/home/ray/home/test/src/alloc.cpp:95:46: required from here
/home/ray/home/test/src/alloc.cpp:31:82: error: no matching function for call to ‘std::allocator<std::_Sp_counted_ptr_inplace<double, test::dummy_allocator<double, std::allocator<double> >, __gnu_cxx::_S_atomic> >::allocator(const test::dummy_allocator<double, std::allocator<double> >&)’
31 | dummy_allocator(Args &&... args) noexcept : alloc(std::forward<Args>(args)...) {}
| ^
In file included from /usr/include/c++/10.1.0/list:61,
from /home/ray/home/test/src/alloc.cpp:3:
/usr/include/c++/10.1.0/bits/allocator.h:157:2: note: candidate: ‘template<class _Tp1> constexpr std::allocator< <template-parameter-1-1> >::allocator(const std::allocator<_Tp1>&) [with _Tp1 = _Tp1; _Tp = std::_Sp_counted_ptr_inplace<double, test::dummy_allocator<double, std::allocator<double> >, __gnu_cxx::_S_atomic>]’
157 | allocator(const allocator<_Tp1>&) _GLIBCXX_NOTHROW { }
| ^~~~~~~~~
/usr/include/c++/10.1.0/bits/allocator.h:157:2: note: template argument deduction/substitution failed:
/home/ray/home/test/src/alloc.cpp:31:82: note: ‘const test::dummy_allocator<double, std::allocator<double> >’ is not derived from ‘const std::allocator<_Up>’
31 | dummy_allocator(Args &&... args) noexcept : alloc(std::forward<Args>(args)...) {}
| ^
In file included from /usr/include/c++/10.1.0/list:61,
from /home/ray/home/test/src/alloc.cpp:3:
/usr/include/c++/10.1.0/bits/allocator.h:147:7: note: candidate: ‘constexpr std::allocator< <template-parameter-1-1> >::allocator(const std::allocator< <template-parameter-1-1> >&) [with _Tp = std::_Sp_counted_ptr_inplace<double, test::dummy_allocator<double, std::allocator<double> >, __gnu_cxx::_S_atomic>]’
147 | allocator(const allocator& __a) _GLIBCXX_NOTHROW
| ^~~~~~~~~
/usr/include/c++/10.1.0/bits/allocator.h:147:34: note: no known conversion for argument 1 from ‘const test::dummy_allocator<double, std::allocator<double> >’ to ‘const std::allocator<std::_Sp_counted_ptr_inplace<double, test::dummy_allocator<double, std::allocator<double> >, __gnu_cxx::_S_atomic> >&’
147 | allocator(const allocator& __a) _GLIBCXX_NOTHROW
| ~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/10.1.0/bits/allocator.h:144:7: note: candidate: ‘constexpr std::allocator< <template-parameter-1-1> >::allocator() [with _Tp = std::_Sp_counted_ptr_inplace<double, test::dummy_allocator<double, std::allocator<double> >, __gnu_cxx::_S_atomic>]’
144 | allocator() _GLIBCXX_NOTHROW { }
| ^~~~~~~~~
/usr/include/c++/10.1.0/bits/allocator.h:144:7: note: candidate expects 0 arguments, 1 provided
只需删除可变构造函数并添加这两个：
dummy_allocator(const base_allocator& a) : alloc(a)
{}
template<class U, class Alloc>
dummy_allocator(const dummy_allocator<U, Alloc>& a) : alloc(a.alloc)
{}
This doesn't solve the issue if I wanted to forward arguments to an allocator that requires arguments :(
那么你可以使用std::is_constructible:
template<class... Args>
dummy_allocator(Args&&... args)
requires(std::is_constructible_v<base_allocator, Args...>)
: alloc(std::forward<Args>(args)...)
{}
template<class U, class Alloc>
dummy_allocator(const dummy_allocator<U, Alloc>& a) : alloc(a.alloc)
{}
请注意，在这两种情况下都需要 template<class U, class Alloc> dummy_allocator 构造函数，因为 dummy_allocator a(b) should be well-formed 对于任何 b 重新绑定的分配器类型。
我认为你的预感是正确的。人们总是必须小心完美的转发构造函数和普通的复制构造函数。我认为有多种解决方案，但我还没有测试过任何一种。
首先，您可以通过向普通复制构造函数添加 dummy_allocator(dummy_allocator&) 转发来提供比完美转发构造函数更好的匹配。当传递的 dummy_allocator 不是 const 并且因此复制构造函数必须进行 const 转换时，完美转发构造函数比复制构造函数匹配得更好。但这还需要重载移动构造函数（我认为），否则当某个地方有人试图移动 dummy_allocator.
时，你会得到同样的错误。
执行以下操作有点棘手但可能更安全：
template<class... Args>
dummy_allocator(Args&&... args) requires (sizeof...(Args) != 1) : alloc(std::forward<Args>(args)...) {}
template<class Arg>
dummy_allocator(Arg&& arg) requires (!std::is_same_v<std::decay_t<Arg>, dummy_allocator>) : alloc(std::forward<Arg>(arg)) {}
dummy_allocator(const dummy_allocator&) = default;
我希望我的 requires 子句是正确的，我还没有做太多的工作。但是在构造函数中做 SFINAE 是一件非常痛苦的事情。
@Evg 的答案在使参数可构造的实现中是正确的，但它还没有解决为什么我得到 shared_ptr 问题的问题。我解决了使用他的实现，因为我觉得使用 is_constructible::value 是最好的方法，但我们需要添加一个允许不同类型重新绑定的构造函数：
template<typename T, typename base_allocator=std::allocator<T>>
class dummy_allocator {
public:
/// Necessary for allocators, propagate exactly what the base_allocator
/// wants.
typedef typename std::allocator_traits<base_allocator>::size_type size_type;
typedef typename std::allocator_traits<base_allocator>::difference_type difference_type;
typedef typename std::allocator_traits<base_allocator>::pointer pointer;
typedef typename std::allocator_traits<base_allocator>::const_pointer const_pointer;
typedef typename std::allocator_traits<base_allocator>::value_type value_type;
template<class U>
struct rebind {
typedef dummy_allocator<U,
typename std::allocator_traits<base_allocator>::template rebind_alloc<U>> other;
};
template <typename T1, typename ...TV>
struct is_dummy : std::is_same<typename std::decay<T1>::type, dummy_allocator<T, base_allocator>>{
};
template<typename... Args>
dummy_allocator(Args &&... args) requires (std::is_constructible_v<base_allocator, Args...>) : alloc(std::forward<Args>(args)...) {}
template <typename U, typename A> friend class dummy_allocator;
// Construct a dummy allocator from another dummy allocator with the same base_allocator but with different type.
template <typename U>
dummy_allocator(
const dummy_allocator<U,
typename std::allocator_traits<base_allocator>::template rebind_alloc<U>>& other) noexcept :
alloc(other.alloc) {}
dummy_allocator() = default;
dummy_allocator(const dummy_allocator& a) : alloc(a.alloc) {}
[[nodiscard]] T *allocate(std::size_t n) {
T *p = alloc.allocate(n);
return p;
}
void deallocate(T *p, std::size_t size) noexcept {
alloc.deallocate(p, size);
}
private:
base_allocator alloc;
};
特别是这里的这些行：
template <typename U, typename A> friend class dummy_allocator;
// Construct a dummy allocator from another dummy allocator with the same base_allocator but with different type.
template <typename U>
dummy_allocator(
const dummy_allocator<U,
typename std::allocator_traits<base_allocator>::template rebind_alloc<U>>& other) noexcept :
alloc(other.alloc) {}
因为我们需要在分配器的不同底层类型之间有构造函数。
对于某些上下文，我正在编写一个分配器，它将基本分配器作为模板类型，除了将 allocate() 和 deallocate() 调用转发给底层分配器成员外什么都不做。使用此自定义分配器创建 std::vectors 效果很好。 我尝试编写一个默认使用 dummy_allocator<T, std::allocator<T>> 的 make_shared 包装器，但没有成功。 这是一个可重现的示例：
#include <memory>
namespace test {
template<typename T, typename base_allocator=std::allocator<T>>
class dummy_allocator {
public:
typedef typename std::allocator_traits<base_allocator>::size_type size_type;
typedef typename std::allocator_traits<base_allocator>::difference_type difference_type;
typedef typename std::allocator_traits<base_allocator>::pointer pointer;
typedef typename std::allocator_traits<base_allocator>::const_pointer const_pointer;
typedef typename std::allocator_traits<base_allocator>::value_type value_type;
template<class U>
struct rebind {
typedef dummy_allocator<U,
typename std::allocator_traits<base_allocator>::template rebind_alloc<U>> other;
};
template<typename... Args>
dummy_allocator(Args &&... args) noexcept : alloc(std::forward<Args>(args)...) {}
dummy_allocator(const dummy_allocator& a) = default;
[[nodiscard]] T *allocate(std::size_t n) {
T *p = alloc.allocate(n);
return p;
}
void deallocate(T *p, std::size_t size) noexcept {
alloc.deallocate(p, size);
}
private:
base_allocator alloc;
};
/// Allocate using a wrapped version of passed in allocator
template <typename T, typename Alloc, typename... Args>
std::shared_ptr<T> allocate_shared(const Alloc& alloc, Args&&... args) {
auto dummy_alloc = dummy_allocator<T, Alloc>(alloc);
return std::allocate_shared<T>(dummy_alloc, std::forward<Args>(args)...);
}
/// Create a shared pointer from a default stl allocator wrapped in profile allocator.
template <typename T, typename... Args>
std::shared_ptr<T> make_shared(Args&&... args) {
return test::allocate_shared<T>(std::allocator<T>(), std::forward<Args>(args)...);
}
} // namespace test
int main() {
auto ptr = test::make_shared<double>();
return 0;
}
当我运行上面的代码时，编译器产生了一些奇怪的模板替换失败错误：
/usr/include/c++/10.1.0/bits/shared_ptr_base.h:679:43: required from ‘std::__shared_count<_Lp>::__shared_count(_Tp*&, std::_Sp_alloc_shared_tag<_Alloc>, _Args&& ...) [with _Tp = double; _Alloc = test::dummy_allocator<double, std::allocator<double> >; _Args = {}; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]’
/usr/include/c++/10.1.0/bits/shared_ptr_base.h:1371:71: required from ‘std::__shared_ptr<_Tp, _Lp>::__shared_ptr(std::_Sp_alloc_shared_tag<_Tp>, _Args&& ...) [with _Alloc = test::dummy_allocator<double, std::allocator<double> >; _Args = {}; _Tp = double; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]’
/usr/include/c++/10.1.0/bits/shared_ptr.h:408:59: required from ‘std::shared_ptr<_Tp>::shared_ptr(std::_Sp_alloc_shared_tag<_Tp>, _Args&& ...) [with _Alloc = test::dummy_allocator<double, std::allocator<double> >; _Args = {}; _Tp = double]’
/usr/include/c++/10.1.0/bits/shared_ptr.h:859:14: required from ‘std::shared_ptr<_Tp> std::allocate_shared(const _Alloc&, _Args&& ...) [with _Tp = double; _Alloc = test::dummy_allocator<double, std::allocator<double> >; _Args = {}]’
/home/ray/home/testing/src/alloc.cpp:44:35: required from ‘std::shared_ptr<_Tp> test::allocate_shared(const Alloc&, Args&& ...) [with T = double; Alloc = std::allocator<double>; Args = {}]’
/home/ray/home/testing/src/alloc.cpp:50:36: required from ‘std::shared_ptr<_Tp> test::make_shared(Args&& ...) [with T = double; Args = {}]’
/home/ray/home/testing/src/alloc.cpp:86:46: required from here
/home/ray/home/testing/src/alloc.cpp:25:82: error: no matching function for call to ‘std::allocator<std::_Sp_counted_ptr_inplace<double, test::dummy_allocator<double, std::allocator<double> >, __gnu_cxx::_S_atomic> >::allocator(const test::dummy_allocator<double, std::allocator<double> >&)’
25 | dummy_allocator(Args &&... args) noexcept : alloc(std::forward<Args>(args)...) {}
| ^
In file included from /usr/include/c++/10.1.0/list:61,
from /home/ray/home/testing/src/alloc.cpp:3:
/usr/include/c++/10.1.0/bits/allocator.h:157:2: note: candidate: ‘template<class _Tp1> constexpr std::allocator< <template-parameter-1-1> >::allocator(const std::allocator<_Tp1>&) [with _Tp1 = _Tp1; _Tp = std::_Sp_counted_ptr_inplace<double, test::dummy_allocator<double, std::allocator<double> >, __gnu_cxx::_S_atomic>]’
157 | allocator(const allocator<_Tp1>&) _GLIBCXX_NOTHROW { }
| ^~~~~~~~~
/usr/include/c++/10.1.0/bits/allocator.h:157:2: note: template argument deduction/substitution failed:
/home/ray/home/testing/src/alloc.cpp:25:82: note: ‘const test::dummy_allocator<double, std::allocator<double> >’ is not derived from ‘const std::allocator<_Up>’
25 | dummy_allocator(Args &&... args) noexcept : alloc(std::forward<Args>(args)...) {}
| ^
In file included from /usr/include/c++/10.1.0/list:61,
from /home/ray/home/testing/src/alloc.cpp:3:
/usr/include/c++/10.1.0/bits/allocator.h:147:7: note: candidate: ‘constexpr std::allocator< <template-parameter-1-1> >::allocator(const std::allocator< <template-parameter-1-1> >&) [with _Tp = std::_Sp_counted_ptr_inplace<double, test::dummy_allocator<double, std::allocator<double> >, __gnu_cxx::_S_atomic>]’
147 | allocator(const allocator& __a) _GLIBCXX_NOTHROW
| ^~~~~~~~~
/usr/include/c++/10.1.0/bits/allocator.h:147:34: note: no known conversion for argument 1 from ‘const test::dummy_allocator<double, std::allocator<double> >’ to ‘const std::allocator<std::_Sp_counted_ptr_inplace<double, test::dummy_allocator<double, std::allocator<double> >, __gnu_cxx::_S_atomic> >&’
147 | allocator(const allocator& __a) _GLIBCXX_NOTHROW
| ~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/10.1.0/bits/allocator.h:144:7: note: candidate: ‘constexpr std::allocator< <template-parameter-1-1> >::allocator() [with _Tp = std::_Sp_counted_ptr_inplace<double, test::dummy_allocator<double, std::allocator<double> >, __gnu_cxx::_S_atomic>]’
144 | allocator() _GLIBCXX_NOTHROW { }
| ^~~~~~~~~
/usr/include/c++/10.1.0/bits/allocator.h:144:7: note: candidate expects 0 arguments, 1 provided
... (The above error basically is repeated 2 more times)
特别是，这个错误似乎代表了正在发生的问题：
error: no matching function for call to ‘std::allocator<std::_Sp_counted_ptr_inplace<double, test::dummy_allocator<double, std::allocator<double> >, __gnu_cxx::_S_atomic> >::allocator(const test::dummy_allocator<double, std::allocator<double> >&)’
基本上说的是"in the constructor of dummy_allocator, you can't pass in a dummy_allocator to the std::allocator's constructor"。但我没有那样做。在 allocate_shared 中，我将 std::allocator 传入 dummy_allocator。
阅读编译器错误时我真的抓狂了，但没有得出我做错了什么的结论。任何帮助将不胜感激！
编辑：我想我有一种预感，无论shared_ptr在下面做什么，它都在尝试做我的dummy_allocator的复制构造函数，而完美转发是捕获复制构造而不是实际的复制构造函数。 但是，我没有知道如何解决这个问题，因为它是一个可变参数模板，我不能在完美的转发构造函数中使用 std::is_same<Args, dummy_allocator> 作为 requires 子句.
因此，正如 Daniel Langr 所指出的，仅复制构建 dummy_allocator 将失败。我用以下 requires 子句解决了这个问题：
...
template <typename T1, typename ...TV>
struct is_dummy : std::is_same<typename std::decay<T1>::type, dummy_allocator<T, base_allocator>>{
};
template<typename... Args>
requires (!is_dummy<Args...>::value)
dummy_allocator(Args &&... args) noexcept : alloc(std::forward<Args>(args)...) {}
dummy_allocator() = default;
dummy_allocator(const dummy_allocator& a) = default;
...
但是，这还不足以解决 shared_ptr 问题，其中的错误有点长...*
所以这是你得到的错误：
#include <memory>
namespace test {
template<typename T, typename base_allocator=std::allocator<T>>
class dummy_allocator {
public:
/// Necessary for allocators, propagate exactly what the base_allocator
/// wants.
typedef typename std::allocator_traits<base_allocator>::size_type size_type;
typedef typename std::allocator_traits<base_allocator>::difference_type difference_type;
typedef typename std::allocator_traits<base_allocator>::pointer pointer;
typedef typename std::allocator_traits<base_allocator>::const_pointer const_pointer;
typedef typename std::allocator_traits<base_allocator>::value_type value_type;
template<class U>
struct rebind {
typedef dummy_allocator<U,
typename std::allocator_traits<base_allocator>::template rebind_alloc<U>> other;
};
template <typename T1, typename ...TV>
struct is_dummy : std::is_same<typename std::decay<T1>::type, dummy_allocator<T, base_allocator>>{
};
template<typename... Args>
requires (!is_dummy<Args...>::value)
dummy_allocator(Args &&... args) noexcept : alloc(std::forward<Args>(args)...) {}
dummy_allocator() = default;
dummy_allocator(const dummy_allocator& a) = default;
[[nodiscard]] T *allocate(std::size_t n) {
T *p = alloc.allocate(n);
return p;
}
void deallocate(T *p, std::size_t size) noexcept {
alloc.deallocate(p, size);
}
private:
base_allocator alloc;
};
/// Allocate using a wrapped version of passed in allocator
template <typename T, typename Alloc, typename... Args>
auto allocate_shared(const Alloc& alloc, Args&&... args) {
auto dummy_alloc = dummy_allocator<T, Alloc>(alloc);
return std::allocate_shared<T>(dummy_alloc, std::forward<Args>(args)...);
}
/// Create a shared pointer from a default stl allocator wrapped in profile allocator.
template <typename T, typename... Args>
auto make_shared(Args&&... args) {
return test::allocate_shared<T>(std::allocator<T>(), std::forward<Args>(args)...);
}
} // namespace test
int main() {
// This will fail
auto ptr = test::make_shared<double>();
// This will now work
auto dummy_alloc = test::dummy_allocator<int, std::allocator<int>>();
auto dummy_alloc2 = test::dummy_allocator<int, std::allocator<int>>(dummy_alloc);
return 0;
}
相关错误：
/usr/include/c++/10.1.0/bits/shared_ptr_base.h:679:43: required from ‘std::__shared_count<_Lp>::__shared_count(_Tp*&, std::_Sp_alloc_shared_tag<_Alloc>, _Args&& ...) [with _Tp = double; _Alloc = test::dummy_allocator<double, std::allocator<double> >; _Args = {}; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]’
/usr/include/c++/10.1.0/bits/shared_ptr_base.h:1371:71: required from ‘std::__shared_ptr<_Tp, _Lp>::__shared_ptr(std::_Sp_alloc_shared_tag<_Tp>, _Args&& ...) [with _Alloc = test::dummy_allocator<double, std::allocator<double> >; _Args = {}; _Tp = double; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]’
/usr/include/c++/10.1.0/bits/shared_ptr.h:408:59: required from ‘std::shared_ptr<_Tp>::shared_ptr(std::_Sp_alloc_shared_tag<_Tp>, _Args&& ...) [with _Alloc = test::dummy_allocator<double, std::allocator<double> >; _Args = {}; _Tp = double]’
/usr/include/c++/10.1.0/bits/shared_ptr.h:859:14: required from ‘std::shared_ptr<_Tp> std::allocate_shared(const _Alloc&, _Args&& ...) [with _Tp = double; _Alloc = test::dummy_allocator<double, std::allocator<double> >; _Args = {}]’
/home/ray/home/test/src/alloc.cpp:53:35: required from ‘auto test::allocate_shared(const Alloc&, Args&& ...) [with T = double; Alloc = std::allocator<double>; Args = {}]’
/home/ray/home/test/src/alloc.cpp:59:36: required from ‘auto test::make_shared(Args&& ...) [with T = double; Args = {}]’
/home/ray/home/test/src/alloc.cpp:95:46: required from here
/home/ray/home/test/src/alloc.cpp:31:82: error: no matching function for call to ‘std::allocator<std::_Sp_counted_ptr_inplace<double, test::dummy_allocator<double, std::allocator<double> >, __gnu_cxx::_S_atomic> >::allocator(const test::dummy_allocator<double, std::allocator<double> >&)’
31 | dummy_allocator(Args &&... args) noexcept : alloc(std::forward<Args>(args)...) {}
| ^
In file included from /usr/include/c++/10.1.0/list:61,
from /home/ray/home/test/src/alloc.cpp:3:
/usr/include/c++/10.1.0/bits/allocator.h:157:2: note: candidate: ‘template<class _Tp1> constexpr std::allocator< <template-parameter-1-1> >::allocator(const std::allocator<_Tp1>&) [with _Tp1 = _Tp1; _Tp = std::_Sp_counted_ptr_inplace<double, test::dummy_allocator<double, std::allocator<double> >, __gnu_cxx::_S_atomic>]’
157 | allocator(const allocator<_Tp1>&) _GLIBCXX_NOTHROW { }
| ^~~~~~~~~
/usr/include/c++/10.1.0/bits/allocator.h:157:2: note: template argument deduction/substitution failed:
/home/ray/home/test/src/alloc.cpp:31:82: note: ‘const test::dummy_allocator<double, std::allocator<double> >’ is not derived from ‘const std::allocator<_Up>’
31 | dummy_allocator(Args &&... args) noexcept : alloc(std::forward<Args>(args)...) {}
| ^
In file included from /usr/include/c++/10.1.0/list:61,
from /home/ray/home/test/src/alloc.cpp:3:
/usr/include/c++/10.1.0/bits/allocator.h:147:7: note: candidate: ‘constexpr std::allocator< <template-parameter-1-1> >::allocator(const std::allocator< <template-parameter-1-1> >&) [with _Tp = std::_Sp_counted_ptr_inplace<double, test::dummy_allocator<double, std::allocator<double> >, __gnu_cxx::_S_atomic>]’
147 | allocator(const allocator& __a) _GLIBCXX_NOTHROW
| ^~~~~~~~~
/usr/include/c++/10.1.0/bits/allocator.h:147:34: note: no known conversion for argument 1 from ‘const test::dummy_allocator<double, std::allocator<double> >’ to ‘const std::allocator<std::_Sp_counted_ptr_inplace<double, test::dummy_allocator<double, std::allocator<double> >, __gnu_cxx::_S_atomic> >&’
147 | allocator(const allocator& __a) _GLIBCXX_NOTHROW
| ~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/10.1.0/bits/allocator.h:144:7: note: candidate: ‘constexpr std::allocator< <template-parameter-1-1> >::allocator() [with _Tp = std::_Sp_counted_ptr_inplace<double, test::dummy_allocator<double, std::allocator<double> >, __gnu_cxx::_S_atomic>]’
144 | allocator() _GLIBCXX_NOTHROW { }
| ^~~~~~~~~
/usr/include/c++/10.1.0/bits/allocator.h:144:7: note: candidate expects 0 arguments, 1 provided
只需删除可变构造函数并添加这两个：
dummy_allocator(const base_allocator& a) : alloc(a)
{}
template<class U, class Alloc>
dummy_allocator(const dummy_allocator<U, Alloc>& a) : alloc(a.alloc)
{}
This doesn't solve the issue if I wanted to forward arguments to an allocator that requires arguments :(
那么你可以使用std::is_constructible:
template<class... Args>
dummy_allocator(Args&&... args)
requires(std::is_constructible_v<base_allocator, Args...>)
: alloc(std::forward<Args>(args)...)
{}
template<class U, class Alloc>
dummy_allocator(const dummy_allocator<U, Alloc>& a) : alloc(a.alloc)
{}
请注意，在这两种情况下都需要 template<class U, class Alloc> dummy_allocator 构造函数，因为 dummy_allocator a(b) should be well-formed 对于任何 b 重新绑定的分配器类型。
我认为你的预感是正确的。人们总是必须小心完美的转发构造函数和普通的复制构造函数。我认为有多种解决方案，但我还没有测试过任何一种。
首先，您可以通过向普通复制构造函数添加 dummy_allocator(dummy_allocator&) 转发来提供比完美转发构造函数更好的匹配。当传递的 dummy_allocator 不是 const 并且因此复制构造函数必须进行 const 转换时，完美转发构造函数比复制构造函数匹配得更好。但这还需要重载移动构造函数（我认为），否则当某个地方有人试图移动 dummy_allocator.
执行以下操作有点棘手但可能更安全：
template<class... Args>
dummy_allocator(Args&&... args) requires (sizeof...(Args) != 1) : alloc(std::forward<Args>(args)...) {}
template<class Arg>
dummy_allocator(Arg&& arg) requires (!std::is_same_v<std::decay_t<Arg>, dummy_allocator>) : alloc(std::forward<Arg>(arg)) {}
dummy_allocator(const dummy_allocator&) = default;
我希望我的 requires 子句是正确的，我还没有做太多的工作。但是在构造函数中做 SFINAE 是一件非常痛苦的事情。
@Evg 的答案在使参数可构造的实现中是正确的，但它还没有解决为什么我得到 shared_ptr 问题的问题。我解决了使用他的实现，因为我觉得使用 is_constructible::value 是最好的方法，但我们需要添加一个允许不同类型重新绑定的构造函数：
template<typename T, typename base_allocator=std::allocator<T>>
class dummy_allocator {
public:
/// Necessary for allocators, propagate exactly what the base_allocator
/// wants.
typedef typename std::allocator_traits<base_allocator>::size_type size_type;
typedef typename std::allocator_traits<base_allocator>::difference_type difference_type;
typedef typename std::allocator_traits<base_allocator>::pointer pointer;
typedef typename std::allocator_traits<base_allocator>::const_pointer const_pointer;
typedef typename std::allocator_traits<base_allocator>::value_type value_type;
template<class U>
struct rebind {
typedef dummy_allocator<U,
typename std::allocator_traits<base_allocator>::template rebind_alloc<U>> other;
};
template <typename T1, typename ...TV>
struct is_dummy : std::is_same<typename std::decay<T1>::type, dummy_allocator<T, base_allocator>>{
};
template<typename... Args>
dummy_allocator(Args &&... args) requires (std::is_constructible_v<base_allocator, Args...>) : alloc(std::forward<Args>(args)...) {}
template <typename U, typename A> friend class dummy_allocator;
// Construct a dummy allocator from another dummy allocator with the same base_allocator but with different type.
template <typename U>
dummy_allocator(
const dummy_allocator<U,
typename std::allocator_traits<base_allocator>::template rebind_alloc<U>>& other) noexcept :
alloc(other.alloc) {}
dummy_allocator() = default;
dummy_allocator(const dummy_allocator& a) : alloc(a.alloc) {}
[[nodiscard]] T *allocate(std::size_t n) {
T *p = alloc.allocate(n);
return p;
}
void deallocate(T *p, std::size_t size) noexcept {
alloc.deallocate(p, size);
}
private:
base_allocator alloc;
};
特别是这里的这些行：
template <typename U, typename A> friend class dummy_allocator;
// Construct a dummy allocator from another dummy allocator with the same base_allocator but with different type.
template <typename U>
dummy_allocator(
const dummy_allocator<U,
typename std::allocator_traits<base_allocator>::template rebind_alloc<U>>& other) noexcept :
alloc(other.alloc) {}
因为我们需要在分配器的不同底层类型之间有构造函数。