[英]C++ STL: Why allocators don't increase memory footprint of containers?
The following code snippet( see on godbolt ) shows that big allocators won't increase the memory footprint of STL containers, but big comparators will.下面的代码片段(参见 Godbolt )表明大分配器不会增加 STL 容器的内存占用,但大比较器会。 Why is it the case?为什么会这样?
// compiled with x86-64 gcc 10.3, -std=c++17
#include <functional>
#include <iostream>
#include <memory>
#include <set>
struct MyLess : public std::less<int>
{
char dummy[1024];
};
struct MyAllocator : public std::allocator<int>
{
char dummy[1024];
};
int main()
{
std::cout << sizeof(std::set<int, MyLess>) << std::endl; // prints 1064
std::cout << sizeof(std::set<int, std::less<int>, MyAllocator>) << std::endl; // prints 48
return 0;
}
Your allocator is not being used.您的分配器未被使用。
By default, std::set
receives std::allocator<int>
, but it needs to allocate some kind of nodes, not int
s.默认情况下, std::set
接收std::allocator<int>
,但它需要分配某种节点,而不是int
s。 It usesstd::allocator_traits::rebind
to get a different allocator for its internal node type.它使用std::allocator_traits::rebind
为其内部节点类型获取不同的分配器。
Pre-C++20 std::allocator
has a rebind
member type, which you inherit, and which std::allocator_traits::rebind
finds. Pre-C++20 std::allocator
有一个rebind
成员类型,你继承它,并且std::allocator_traits::rebind
找到。 That rebind
points to std::allocator
, so that's what you get. rebind
指向std::allocator
,这就是你得到的。
Starting from C++20, there's no rebind
in std::allocator
, so std::allocator_traits::rebind
falls back to directly modifying the first template parameter of your allocator, and since it's not a template, you get a compilation error.从 C++20 开始, std::allocator
没有rebind
,因此std::allocator_traits::rebind
退回到直接修改分配器的第一个模板参数,并且由于它不是模板,因此会出现编译错误。
A possible solution is to make your allocator a template, and to provide your own rebind
(which can be malformed, then the template parameter will be replaced automatically):一个可能的解决方案是让你的分配器成为一个模板,并提供你自己的rebind
(这可能是格式错误的,然后模板参数将被自动替换):
template <typename T>
struct MyAllocator : public std::allocator<T>
{
char dummy[1024];
struct rebind {}; // Malformed `rebind` to hide the inherited one, if any.
};
Then 1072
is printed for me.然后为我打印1072
。
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