const类型容器的const_iterator类型

const_iterator type of a container of const types

本文关键字：const 类型 iterator      更新时间：2023-10-16

如果我编译以下代码：

#include <list>
using iter_t = std::list<const unsigned>::const_iterator;
int main(int, char**) {
  return 0;
}

我得到以下编译错误：

In file included from /usr/local/Cellar/gcc/4.9.2_1/include/c++/4.9.2/x86_64-apple-darwin13.4.0/bits/c++allocator.h:33:0,
             from /usr/local/Cellar/gcc/4.9.2_1/include/c++/4.9.2/bits/allocator.h:46,
             from /usr/local/Cellar/gcc/4.9.2_1/include/c++/4.9.2/list:61,
             from main.cpp:1:
/usr/local/Cellar/gcc/4.9.2_1/include/c++/4.9.2/ext/new_allocator.h: In instantiation of 'class __gnu_cxx::new_allocator<const unsigned int>':
/usr/local/Cellar/gcc/4.9.2_1/include/c++/4.9.2/bits/allocator.h:92:11:   required from 'class std::allocator<const unsigned int>'
/usr/local/Cellar/gcc/4.9.2_1/include/c++/4.9.2/bits/stl_list.h:315:9:   required from 'class std::_List_base<const unsigned int, std::allocator<const unsigned int> >'
/usr/local/Cellar/gcc/4.9.2_1/include/c++/4.9.2/bits/stl_list.h:447:11:   required from 'class std::list<const unsigned int>'
main.cpp:3:41:   required from here
/usr/local/Cellar/gcc/4.9.2_1/include/c++/4.9.2/ext/new_allocator.h:93:7: error: 'const _Tp* __gnu_cxx::new_allocator<_Tp>::address(__gnu_cxx::new_allocator<_Tp>::const_reference) const [with _Tp = const unsigned int; __gnu_cxx::new_allocator<_Tp>::const_pointer = const unsigned int*; __gnu_cxx::new_allocator<_Tp>::const_reference = const unsigned int&]' cannot be overloaded
   address(const_reference __x) const _GLIBCXX_NOEXCEPT
   ^
/usr/local/Cellar/gcc/4.9.2_1/include/c++/4.9.2/ext/new_allocator.h:89:7: error: with '_Tp* __gnu_cxx::new_allocator<_Tp>::address(__gnu_cxx::new_allocator<_Tp>::reference) const [with _Tp = const unsigned int; __gnu_cxx::new_allocator<_Tp>::pointer = const unsigned int*; __gnu_cxx::new_allocator<_Tp>::reference = const unsigned int&]'
   address(reference __x) const _GLIBCXX_NOEXCEPT
   ^

如果将容器类型从std::list更改为std::vector、std::deque或std::forward_list，则会得到大致相同的错误消息。如果我将模板参数从const unsigned更改为unsigned，它编译得很好。

这是怎么回事？我不明白为什么它不能编译。

Visual C++2015提供了明确的诊断

"C++标准禁止const元素的容器，因为allocator<const T>格式不正确。

据我所知，这是因为分配器的allocate方法返回一个指向未初始化的T项数组的指针，如果T是const，则这些项永远无法初始化

我找不到任何关于allocator<const T>格式错误的明确声明，所以这可能是语义的结果

正如Dieter Lücking在评论中指出的那样，在提供reference和const_reference地址的两个成员函数之间，const类型T存在直接冲突，因为当类型为const时，这两个函数是相等的。

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