c++ - 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)？

Question

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，除了转发 allocate() 之外什么都不做。和 deallocate()调用底层分配器成员。创建std::vector使用此自定义分配器可以正常工作。 我试着写一个make_shared将使用 dummy_allocator<T, std::allocator<T>> 的包装器默认情况下，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，编译器生成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> >&)’

它基本上说“在 dummy_allocator 的构造函数中，您不能将 dummy_allocator 传递给 std::allocator 的构造函数”。但我没有那样做。在 allocate_shared ，我正在传递 std::allocator进入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，而完美的转发是捕获复制构造而不是实际的复制构造函数。 但是，我不知道如何解决这个问题，因为它是一个可变参数模板，我不能使用 std::is_same<Args, dummy_allocator>作为完美转发构造函数中的 requires 子句。

正如 Daniel Langr 所指出的，仅复制构造 dummy_allocator 将会失败。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，这还不足以修复 shared_ptr问题，错误有点长...*

所以这里是你得到的错误:

#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

Answer 1

只需删除可变参数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)
{}

Demo

---

This doesn't solve the issue if I wanted to forward arguments to an allocator that requires arguments :(

然后你可以使用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)
{}

Demo

---

请注意 template<class U, class Alloc> dummy_allocator两种情况都需要构造函数，因为 dummy_allocator a(b) should be well-formed对于任何 b重新绑定(bind)的分配器类型。