c++ - 使用 clang++ v4 和 gcc 6.3 库的自定义分配器

Question

在库随 gcc 6.3.1 一起提供的系统 (arch linux) 上，我在使用带有 clang++ 4.0.0 的自定义分配器时遇到了很多麻烦。这是一个最小的非工作示例:

#include <string>

struct myalloc : std::allocator<char> {
  using std::allocator<char>::allocator;
};

struct mystring 
  : std::basic_string<char, std::char_traits<char>, myalloc> {
  using std::basic_string<char, std::char_traits<char>, myalloc>::basic_string;
};

int
main()
{
  mystring r = "hello";
  mystring s (std::move(r));
}

我的意图显然是让 myalloc 成为一个自定义分配器，其行为与系统 std::allocator 和 mystring 与 std::string 相同，只是它使用 myalloc。这应该是最不可能引起问题的情况。 (很明显，一旦这开始工作，我想进一步自定义分配器。)

代码使用 g++ -std=c++14 -Wall -Werror 编译干净，但是 clang++ -std=c++14 失败:

In file included from strerror.cc:1:
In file included from /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1/../../../../include/c++/6.3.1/string:52:
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1/../../../../include/c++/6.3.1/bits/basic_string.h:477:9: error: 
      no matching constructor for initialization of
      'std::__cxx11::basic_string<char, std::char_traits<char>,
      myalloc>::_Alloc_hider'
      : _M_dataplus(_M_local_data(), std::move(__str._M_get_allocator()))
        ^           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
strerror.cc:7:8: note: in instantiation of member function
      'std::__cxx11::basic_string<char, std::char_traits<char>,
      myalloc>::basic_string' requested here
struct mystring 
       ^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1/../../../../include/c++/6.3.1/bits/basic_string.h:109:2: note: 
      candidate constructor not viable: no known conversion from 'typename
      std::remove_reference<allocator<char> &>::type' (aka
      'std::allocator<char>') to 'const myalloc' for 2nd argument
        _Alloc_hider(pointer __dat, const _Alloc& __a = _Alloc())
        ^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1/../../../../include/c++/6.3.1/bits/basic_string.h:107:14: note: 
      candidate constructor (the implicit move constructor) not viable: requires
      1 argument, but 2 were provided
      struct _Alloc_hider : allocator_type // TODO check __is_final
             ^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1/../../../../include/c++/6.3.1/bits/basic_string.h:107:14: note: 
      candidate constructor (the implicit copy constructor) not viable: requires
      1 argument, but 2 were provided
1 error generated.

这只是 clang 或 gcc 库中的错误，还是我的代码在概念上有问题？

Answer 1

最小示例的最小修复是将此成员添加到 struct myalloc

template<class> struct rebind {
    using other = myalloc;
};

当然最好不要从 std::allocator 继承(在那种情况下你不需要重新绑定(bind))，也不要从 string 继承，这些类不打算作为公共(public)基础。