c++ - 为什么不是 std::string::max_size() == std::string::allocator::max_size()

Question

最近我注意到给定 std::string s 的情况下以下陈述不正确.

s.max_size() == s.get_allocator().max_size();

我发现这很有趣，默认情况下 std::string将使用 std::allocator<char>其理论极限为 size_type(-1) (是的，我知道我假设 2 的补码，但这与实际问题无关)。我知道实际限制会比这少得多。在典型的 32 位 x86 系统上，内核将占用 2GB(可能是 1GB)的地址空间，实际上限要小得多。

无论如何，GNU libstdc++ 的 std::basic_string<>::max_size()似乎返回相同的值，不管它使用的分配器说什么(类似于 1073741820 )。

所以问题仍然存在，为什么不 std::basic_string<>::max_size()只需返回 get_allocator().max_size() ？在我看来，这是假设的上限。如果分配不足，它只会抛出 std::bad_alloc。 ，为什么不试试呢？

这比其他任何事情都更让人好奇，我只是想知道为什么至少在这个实现中将两者分开定义。

Answer 1

在Microsoft Connect已发布与您的问题相关的错误。微软对此有一个有趣的答案:

We've resolved it as By Design according to our interpretation of the Standard, which doesn't clearly explain what the intended purpose for max_size() is. Allocator max_size() is described as "the largest value that can meaningfully be passed to X::allocate()" (C++03 20.1.5 [lib.allocator.requirements]/Table 32), but container max_size() is described as "size() of the largest possible container" (23.1 [lib.container.requirements]/Table 65). Nothing describes whether or how container max_size() should be derived from allocator max_size(). Our implementation for many years has derived container max_size() directly from allocator max_size() and then used this value for overflow checks and so forth. Other interpretations of the Standard, such as yours, are possible, but aren't unambiguously correct to us. The Standard's wording could certainly benefit from clarification here. Unless and until that happens, we've decided to leave our current implementation unchanged for two reasons: (1) other customers may be depending on our current behavior, and (2) max_size() fundamentally doesn't buy anything. At most, things that consume allocators (like containers) could use allocator max_size() to predict when allocate() will fail - but simply calling allocate() is a better test, since the allocator will then decide to give out memory or not. Things that consume containers could use container max_size() as a guarantee of how large size() could be, but a simpler guarantee is size_type's range.

另外here你可以找到 Core Issue #197。委员会考虑了改进标准措辞的请求，但被拒绝了。

所以您的问题“为什么..”的答案是标准没有清楚地解释 max_size() 的预期目的是什么。