c++ - 为什么 C++ 分配器避免就地重新分配

Question

我对 realloc 的理解是，如果内存在分配点之外连续可用，它可以尝试扩展当前分配而不进行复制。

阅读本文时 https://github.com/facebook/folly/blob/master/folly/docs/FBVector.md我开始知道大多数分配器都避免就地重新分配

. Many memory allocators do not support in- place reallocation, although most of them could. This comes from the now notorious design of realloc() to opaquely perform either in-place reallocation or an allocate-memcpy-deallocate cycle. Such lack of control subsequently forced all clib-based allocator designs to avoid in-place reallocation, and that includes C++'s new and std:allocator.

正如在另一个关于 C++ 中缺少重新分配器的问题 ( Why is there no reallocation functionality in C++ allocators? ) 中所回答的那样，接受的答案提到这样的分配器会禁止 realloc 使用 form C 库，但没有回答为什么不尝试使 realloc 扩展当前内存(如果可能)？

Answer 1

为什么不做一个？实际上，有一些建议可以做到这一点:

• http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1953.html
• http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2045.html
• http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3495.htm