如何将std :: scoped_allocator_adaptor用于C中的嵌套容器？

使用std::scoped_allocator_adaptor可以确保嵌套容器共享同一自定义分配器，1. 需将外层容器的分配器用std::scoped_allocator_adaptor包装；2. 构造时使用std::allocator_arg显式传递分配器；3. 内层容器必须支持std::uses_allocator特性；4. 分配器需可复制且类型兼容；5. 所有嵌套层级将自动使用同一分配器完成内存分配，从而实现一致的内存管理行为。

Using std::scoped_allocator_adaptor in C allows you to propagate a custom allocator through nested containers — for example, when you have a std::vector<:vector>></:vector> and want both the outer and inner vectors to use the same allocator. This is especially useful when working with custom allocators (like arena-based or pool allocators) and you want consistent memory allocation behavior across all levels.

Here’s how to use std::scoped_allocator_adaptor effectively:

1. Understanding the Purpose

By default, nested containers like std::vector<:vector>></:vector> don’t automatically share allocators. The inner vectors use their own default-constructed allocator, even if the outer vector uses a custom one.

std::scoped_allocator_adaptor wraps your allocator and makes it propagate to any nested containers created within the outer container. It does this by adjusting the value_type's allocator when constructing inner objects.

2. Basic Usage with a Custom Allocator

Let’s say you have a simple custom allocator (e.g., a wrapper around std::allocator for illustration):

#include <memory>
#include <vector>
#include <scoped_allocator>

template <typename T>
using MyAlloc = std::allocator<T>;

// Define a nested vector using scoped allocator
using InnerVec = std::vector<int, MyAlloc<int>>;
using OuterVec = std::vector<InnerVec, std::scoped_allocator_adaptor<MyAlloc<InnerVec>>>;

Now, when you construct an OuterVec, the allocator will be passed to each inner vector:

int main() {
    MyAlloc<InnerVec> alloc;
    OuterVec vec(std::allocator_arg, alloc);  // Use allocator with std::allocator_arg

    vec.resize(3);  // Creates 3 inner vectors

    // All inner vectors use the same allocator 'alloc'
    for (auto& inner : vec) {
        inner.push_back(42);
    }

    return 0;
}

Note:

• You must use std::allocator_arg to pass the allocator explicitly.
• The scoped_allocator_adaptor automatically forwards the allocator to inner container constructions.

3. How Allocator Propagation Works

When you do vec.resize(3), the outer vector needs to construct three InnerVec objects. Normally, they’d use InnerVec’s default allocator. But because the outer allocator is a scoped_allocator_adaptor, it intercepts the construction and provides the outer allocator to the inner vectors.

Internally:

• OuterVec::allocator_type is std::scoped_allocator_adaptor<MyAlloc<InnerVec>>
• When constructing an InnerVec, the runtime uses std::uses_allocator and passes the outer allocator via std::allocator_arg.

This ensures inner vectors are constructed with:

InnerVec(std::allocator_arg, outer_allocator, args...)

4. Key Requirements and Notes

• Allocator must be copyable: The outer allocator is copied into each inner container.
• Use std::allocator_arg syntax: Required when constructing objects that accept scoped allocators.
• Nested types must support std::uses_allocator: Most standard containers do.
• Inner container’s allocator type must be compatible: The outer allocator should be convertible to the inner one.

Example with deeper nesting (vector<vector<vector<int>>>):

using Vec1D = std::vector<int, MyAlloc<int>>;
using Vec2D = std::vector<Vec1D, std::scoped_allocator_adaptor<MyAlloc<Vec1D>>>;
using Vec3D = std::vector<Vec2D, std::scoped_allocator_adaptor<MyAlloc<Vec2D>>>;

MyAlloc<Vec2D> alloc;
Vec3D vec3d(std::allocator_arg, alloc);
vec3d.resize(2);
vec3d[0].resize(3);
vec3d[0][0].push_back(1);
// All levels use the same allocator

5. Practical Example: Pool Allocator Use Case

Suppose you’re using a memory pool:

struct PoolAllocator {
    // Simplified — in practice, implement allocate/deallocate
    template <typename T>
    struct rebind {
        using other = PoolAllocator;
    };

    T* allocate(size_t n) { /* use pool */ }
    void deallocate(T* p, size_t n) { /* return to pool */ }

    // Copyable
    PoolAllocator() = default;
    template <typename U>
    PoolAllocator(const PoolAllocator<U>&) {}
};

Now use it with scoped_allocator_adaptor to ensure all vectors (inner and outer) pull memory from the same pool.

Summary

To use std::scoped_allocator_adaptor:

• Wrap your allocator in std::scoped_allocator_adaptor for the outer container.
• Use std::allocator_arg when constructing the container.
• Ensure your allocator is compatible and supports the necessary traits.
• Inner containers will automatically receive the same allocator.

It’s a powerful tool for consistent memory management in deeply nested structures — though in practice, such setups are rare unless you're doing high-performance or embedded work.

Basically, it’s not something you need every day, but when you do, it solves a very specific and tricky problem cleanly.

以上是如何将std :: scoped_allocator_adaptor用于C中的嵌套容器？的详细内容。更多信息请关注PHP中文网其他相关文章！