问题描述
#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;
}
现在我想对对齐进行模板化(而不是放入构造函数中)。我尝试向模板添加一个整数参数
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,128>> v;
for (size_t i = 1; i < 5000; ++i)
v.push_back(i);
return 0;
}
我在使用 g++
和 clang++
时遇到很多编译错误: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,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,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,128> >'
475 | using _Base::_M_deallocate;
我不明白发生了什么。我注意到即使我不在类中使用 align
编译器也会以同样的方式抱怨。相反,如果我在模板中放置了一个未使用的 typename
,那么一切都可以正常编译。
你能帮我吗?
解决方法
我想我找到了解决问题的方法,尽管我不确定我是否完全理解了所有细节。
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);
}
};
它编译并似乎具有正确的行为。