[英]Template parametric type Allocator in C++
我有以下有效的玩具代碼:
#include <cstdlib>
#include <vector>
template <typename T>
struct Dummy {
using value_type = T;
using size_type = size_t;
using difference_type = ptrdiff_t;
using pointer = value_type*;
using const_pointer = const value_type*;
using reference = value_type&;
using const_reference = const value_type&;
Dummy() = default;
Dummy(const Dummy&) = default;
pointer allocate(size_type n, const void* hint = 0) {
(void)hint;
return static_cast<pointer>(aligned_alloc(128, n * sizeof(T)));
}
void deallocate(void* p, size_type) {
if (p)
free(p);
}
};
int main()
{
std::vector<int, Dummy<int, 16>> v;
for (size_t i = 1; i < 5000; ++i)
v.push_back(i);
return 0;
}
現在我想將 alignment 模板化(而不是放入構造函數中)。 我嘗試在模板中添加一個 integer 參數
template <typename T, size_t align>
struct Dummy {
using value_type = T;
using size_type = size_t;
using difference_type = ptrdiff_t;
using pointer = value_type*;
using const_pointer = const value_type*;
using reference = value_type&;
using const_reference = const value_type&;
Dummy() = default;
Dummy(const Dummy&) = default;
pointer allocate(size_type n, const void* hint = 0) {
(void)hint;
return static_cast<pointer>(aligned_alloc(align, n * sizeof(T)));
}
void deallocate(void* p, size_type) {
if (p)
free(p);
}
};
int main()
{
std::vector<int, Dummy<int, 128>> v;
for (size_t i = 1; i < 5000; ++i)
v.push_back(i);
return 0;
}
我在使用g++
和clang++
時遇到很多編譯錯誤:例如
/usr/include/c++/10.2.0/bits/alloc_traits.h:78:11: error: no type named 'type' in 'struct std::__allocator_traits_base::__rebind<Dummy<int, 128>, int, void>'
78 | using __alloc_rebind
| ^~~~~~~~~~~~~~
In file included from /usr/include/c++/10.2.0/vector:67,
from aligned.cpp:4:
/usr/include/c++/10.2.0/bits/stl_vector.h: In instantiation of 'class std::vector<int, Dummy<int, 128> >':
aligned.cpp:62:43: required from here
/usr/include/c++/10.2.0/bits/stl_vector.h:474:20: error: '_M_allocate' has not been declared in 'std::_Base<int, Dummy<int, 128> >'
474 | using _Base::_M_allocate;
| ^~~~~~~~~~~
/usr/include/c++/10.2.0/bits/stl_vector.h:475:20: error: '_M_deallocate' has not been declared in 'std::_Base<int, Dummy<int, 128> >'
475 | using _Base::_M_deallocate;
我不明白發生了什么事。 我注意到,即使我不在 class 中使用align
,編譯器也會以同樣的方式抱怨。 相反,如果我在模板中放置了一個未使用的typename
,那么一切都可以正常編譯。 你能幫助我嗎?
我想我找到了解決問題的方法,即使我不確定我是否完全理解所有細節。
template <typename T, size_t align>
struct Dummy {
using value_type = T;
using size_type = size_t;
using difference_type = ptrdiff_t;
using pointer = value_type*;
using const_pointer = const value_type*;
using reference = value_type&;
using const_reference = const value_type&;
template<class Other>
struct rebind { using other = Dummy<Other, align>; };
Dummy() {};
Dummy(const Dummy&) {};
template <typename Other>
Dummy(const Dummy<Other, align>&) {}
pointer allocate(size_type n, const void* hint = 0) {
(void)hint;
return static_cast<pointer>(aligned_alloc(align, n * sizeof(T)));
}
void deallocate(void* p, size_type) {
if (p)
free(p);
}
};
它編譯並且似乎具有正確的行為。
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