I have the following toy code that works:
#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;
}
Now I would like to templetize (not to put in the constructor) the alignment. I try to add to the template a integer parameter
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;
}
I'm getting a lot of compilation error both with g++
and clang++
: for instance
/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;
I do not understand what is going on. I notice that even if I do not use align
in the class the compiler complaints the same way. If instead I put an unused typename
inside the template everything is compiling fine. Can you help me?
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