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Here we discuss the various allocator models used in the C++ standard, followed by an explanation of the model used in this library and its benefits. Finally we discuss how the library interoperates with existing code that uses polymorphic allocators.
Here we discuss the various allocator models used in the C++ standard, followed by an explanation of the model used in this library and its benefits. Finally we discuss how the library interoperates with existing code that uses polymorphic allocators.本文将首先讨论C++标准中使用的各种分配器模型，随后介绍本库所采用的模型及其优势，最后说明该库如何与使用多态分配器的现有代码进行互操作。
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= Allocators
= Allocators分配器
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Boost.JSON solves this problem by introducing a new smart pointer called <<ref_storage_ptr>> which builds upon {cpp}17's memory allocation interfaces, accomplishing the goals above. As a result, libraries which use this type compose more easily and enjoy faster compilation, as member functions for containers which use the type can be defined out-of-line.
Boost.JSON solves this problem by introducing a new smart pointer called <<ref_storage_ptr>> which builds upon {cpp}17's memory allocation interfaces, accomplishing the goals above. As a result, libraries which use this type compose more easily and enjoy faster compilation, as member functions for containers which use the type can be defined out-of-line.通过引入一个名为&lt;<ref_storage_ptr>&gt; 的新型智能指针解决此问题，该指针基于{cpp}17的内存分配接口构建，实现了上述目标。因此，使用该类型的库更易于组合，并能获得更快的编译速度，因为使用该类型的容器的成员函数可以定义在类外（out-of-line）。</ref_storage_ptr>
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`T` interoperates with code already using `std::pmr`
`T` interoperates with code already using 能与已使用`std::pmr`的代码互操作
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`T` supports both reference semantics or shared ownership
`T` supports both reference semantics or shared ownership同时支持引用语义和共享所有权
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`T` references a {ref_memory_resource}
`T` references a引用一个 {ref_memory_resource}
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`T` is not a class template
`T` is not a class template不是类模板
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We would like an allocator model using a single type `T` with the following properties:
We would like an allocator model using a single type `T` with the following properties:我们希望采用一种使用单一类型`T`的分配器模型，该模型具有以下特性：
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Problem Statement
Problem Statement问题陈述
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Workarounds for this problem are worse than the problem itself. The library could return a pair with the vector and `std::unique_ptr<std::pmr::memory_resource>` which the caller must manage. Or the library could change its function signatures to accept a {ref_memory_resource}``*`` provided by the caller, where the library also makes public the desired memory resources (`my_resource` above).
Workarounds for this problem are worse than the problem itself. The library could return a pair with the针对此问题的变通方案往往比问题本身更糟糕。该库可以返回一个包含 vector and 和`std::unique_ptr<std::pmr::memory_resource>` which the caller must manage. Or the library could change its function signatures to accept a {ref_memory_resource}``*`` provided by the caller, where the library also makes public the desired memory resources (`my_resource` above).的pair，必须由调用方自行管理。或者，该库也可以修改其函数签名，接受调用方提供的{ref_memory_resource}``*``，同时将所需的内存资源（如上文的`my_resource`）公开。</std::pmr::memory_resource>
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This can be worked around by declaring the container to use a custom allocator (perhaps using a `std::shared_ptr< std::pmr::memory_resource >` as a data member). This hinders library composition; every library now exports unique, incompatible container types. A raw memory resource pointer is also easy to misuse:
This can be worked around by declaring the container to use a custom allocator (perhaps using a 可通过声明容器使用自定义分配器（可能使用`std::shared_ptr<&lt; std::pmr::memory_resource >` as a data member). This hinders library composition; every library now exports unique, incompatible container types. A raw memory resource pointer is also easy to misuse:&gt;`作为数据成员）来解决此问题。这会阻碍库的组合；每个库都会导出各自独特且互不兼容的容器类型。原始内存资源指针也容易被误用：
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However, sometimes shared ownership is needed. Specifically, that the lifetime extension of the memory resource should be automatic. For example, if a library wants to return a container which owns an instance of the library's custom memory resource as shown below:
However, sometimes shared ownership is needed. Specifically, that the lifetime extension of the memory resource should be automatic. For example, if a library wants to return a container which owns an instance of the library's custom memory resource as shown below:然而，有时需要共享所有权，即内存资源的生命周期应自动延长。例如，若某个库希望返回一个容器，该容器拥有该库自定义内存资源的实例，如下所示：
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The memory resource is passed by pointer; ownership is not transferred. The caller is responsible for extending the lifetime of the memory resource until the last container which is using it goes out of scope, otherwise the behavior is undefined. Sometimes this is the correct model, such as in this example which uses a monotonic resource constructed from a local stack buffer:
The memory resource is passed by pointer; ownership is not transferred. The caller is responsible for extending the lifetime of the memory resource until the last container which is using it goes out of scope, otherwise the behavior is undefined. Sometimes this is the correct model, such as in this example which uses a monotonic resource constructed from a local stack buffer:内存资源通过指针传递；所有权不发生转移。调用方需负责确保该内存资源的生命周期持续到所有使用它的容器都离开作用域为止，否则行为未定义。在某些场景下，这种模型是合适的，例如以下示例中使用了一个基于局部栈缓冲区构造的单调内存资源：
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A polymorphic allocator constructs with a pointer to a memory resource:
A polymorphic allocator constructs with a pointer to a memory resource:多态分配器通过一个指向内存资源的指针进行构造：
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The class template {ref_polymorphic_allocator} wraps a {ref_memory_resource} pointer and meets the requirements of {req_Allocator}, allowing it to be used where an allocator is expected. The standard provides type aliases using the polymorphic allocator for standard containers:
The class template 类模板{ref_polymorphic_allocator} wraps a 包装了{ref_memory_resource} pointer and meets the requirements of {req_Allocator}, allowing it to be used where an allocator is expected. The standard provides type aliases using the polymorphic allocator for standard containers:指针，并满足{req_Allocator}要求，使其可在需要分配器的地方使用。标准库为使用多态分配器的标准容器提供了类型别名：
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{cpp}17 improves the allocator model by representing the low-level allocation operation with an abstract interface called {ref_memory_resource}, which is not parameterized on the element type, and has no traits:
{cpp}17 improves the allocator model by representing the low-level allocation operation with an abstract interface called {ref_memory_resource}, which is not parameterized on the element type, and has no traits:通过引入一个名为 {ref_memory_resource} 的抽象接口来表示底层分配操作，从而改进分配器模型。该接口未在元素类型上参数化，且无特征：
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Polymorphic Allocators
Polymorphic Allocators多态分配器
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Allocator-based programs which use multiple allocator types incur a greater number of function template instantiations and are generally slower to compile because class template function definitions must be visible at all call sites.
Allocator-based programs which use multiple allocator types incur a greater number of function template instantiations and are generally slower to compile because class template function definitions must be visible at all call sites.使用多种分配器类型的基于分配器的程序会引发更多的函数模板实例化，且通常编译速度更慢，因为类模板的函数定义必须在所有调用点可见。
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is complicated and error-prone.
is complicated and error-prone.是复杂且容易出错的。
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