boost :: any/std :: any是否将小对象存储在原位? [英] Does boost::any / std::any store small objects in-place?

问题描述

要容纳任意大的对象，boost::any/std::any当然需要为对象分配堆空间.但是，对于大小小于或等于指针(int,char,bool,...)的小型类型，any可以将值直接存储在指针插槽或某些其他内部内存中，而不分配堆空间.但是实现可以做到这一点吗?

To hold arbitrarily large objects, boost::any / std::any surely needs to allocate heap space for objects. However, for small types whose size is less or equal to a pointer (int,char,bool,...), any could instead store the value in-place in the pointer slot or in some other in-place memory and not allocate heap space. But does the implementation do this?

我遇到的情况是，我经常将小类型存储在any中，而有时仅将大类型存储在string中.代码很热，因此我要问这个问题.如果不执行优化，我可能会采用自己的实施方式来更好地存储小型类型.

I have a scenario where I often store small types in an any and only sometimes larger types like strings. The code is quite hot and therefore I am asking the question. If the optimization is not performed, I might be better off with an own implementation that stores small types in-place.

推荐答案

没有保证，但是 [any.class] 那个

实现应避免为包含的小对象使用动态分配的内存. [示例:构造的对象仅包含int的情况. [示例]这种小对象优化只能应用于is_­nothrow_­move_­constructible_­v<T>为true的T类型.

不幸的是，它没有建议应将 small 视为什么，只是说int应该能够就位存储.

Unfortunately it does not give a recommendation for what should be considered small except to say a int should be able to be stored in place.

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