是libstdc ++支持std :: unordered_map不完整吗？ [英] Is libstdc++ support for std::unordered_map incomplete?

问题描述

相关于在CodeReview上的这个问题，我试图使用 std :: unordered_map 使用自定义分配器，但显然这不适用于gcc / clang和libstdc ++。该错误可以通过使用 std :: allocator

Related to this question on CodeReview, I tried to use std::unordered_map with a custom allocator but apparently this does not work with gcc/clang and libstdc++. The error can be generated from initializing an empty hash map with a std::allocator

#include <unordered_map>

int main()
{
    typedef std::allocator<std::pair<const int, int>> A;
    typedef std::unordered_map<int, int, std::hash<int>, std::equal_to<int>, A> H;    

    auto h = H{A()}; // ERROR, cannot find constructor H::H(const A&)
}

活生生的例子 。

问：是的libstdc ++构建支持的std :: unordered_map 用一个分配器作为参数不全

Question: is libstdc++ support for constructing std::unordered_map with a single allocator as argument incomplete?

UPDATE ：进一步检查表明，对于除 std :: vector 之外的几乎所有容器， libstdc ++中的分配器直接访问typedef和成员函数，而不是通过 std :: allocator_traits 。这适用于 std :: allocator ，但对所有自定义分配器失败，除非他们直接添加这些成员和typedef。

UPDATE: further inspection shows that, for almost all containers other than std::vector, uses of allocators in libstdc++ access typedefs and member functions of allocators directly, rather than through std::allocator_traits. This works for std::allocator but fails for all custom allocators, unless they verbosely add those members and typedefs directly.

推荐答案

在最新的的doxygen docs 在2013-08-01生成，它在第178行：

In latest doxygen docs generated on 2013-08-01, it's there on line 178:

explicit
unordered_map(const allocator_type& __a)
: _M_h(__a)
{ }

不过在文档的 4.8.1 它没有，这是我的本地一样。至于g ++ 4.8涉及它的未实现。

However, in the docs for 4.8.1 it's not there, which is the same as my local one. As as far as g++4.8 is concerned its not implemented.

发现链接补丁。它是日期为2013-04-22，这是一个稍后的4.8版本。

Found the link to the patch. It was dated 2013-04-22, which was a little after the release of 4.8.

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