c++ - C++11 是否要求分配器是默认可构造的，libstdc++ 和 libc++ 不同意？

Question

使用稍作修改的 Howard Hinnants's C++11 stack allocator 版本这是documented here和 here ，使用 std::basic_string 并使用使用 libstdc++ 的 gcc 进行编译，以下示例 ( see it live ):

const unsigned int N = 200;

arena<N> a;
short_alloc<char, N> ac(a) ;

std::basic_string<char,std::char_traits<char>,short_alloc<char, N>> empty(ac);

给出以下错误(以及其他):

error: no matching function for call to 'short_alloc<char, 200ul>::short_alloc()'
   if (__n == 0 && __a == _Alloc())
                       ^

然而，当使用 clang 和使用 libc++ 编译时，它可以正常工作( see it live )。

std::basic_string 的 stdlibc++ 实现期望分配器具有默认构造函数。

C++11 是否要求分配器默认可构造？哪个实现是正确的？

Answer 1

不，C++11 不要求分配器具有默认构造函数，如果我们查看草案 C++11 标准部分 17.6.3.5 [allocator.requirements] 它包含表 28 分配器要求，其中不包含对默认构造函数的要求，稍后在本节中提供了一个最小的符合接口(interface):

[ Example: the following is an allocator class template supporting the minimal interface that satisfies the requirements of Table 28:

template <class Tp>
struct SimpleAllocator {
    typedef Tp value_type;
    SimpleAllocator(ctor args );

    template <class T> SimpleAllocator(const SimpleAllocator<T>& other);

    Tp *allocate(std::size_t n);
    void deallocate(Tp *p, std::size_t n);
};

—end example ]

不包含默认构造函数。

有一个 libstdc++ 错误报告:basic_string assumes that allocators are default-constructible其中说:

The empty-string optimization of basic_string assumes that allocators are default constructible. While this used to be the case in C++98, it is no longer true in C++11, as now allocators are allowed to have state.

Consider the attached example program. Compiling with

g++ -std=c++11 -c t.cpp

produces an error message, even though it should compile fine. The problem is the the "_S_construct" calls "_Alloc()", which does not exist.

Note that the C++11 standard does not require default constructors. (Section 17.6.3.5, Table 28). In particular, the SimpleAllocator example from Section 17.6.3.5 would trigger the same bug, too.

响应是:

This is hardly the only C++11 allocator requirement missing from std::string, ALL of the new requirements are missing, and unlikely to be implemented until we switch to a non-COW string implementation.

自 gcc 5.0 起已修复:

Fixed for GCC 5 (when using the new string ABI)

我们可以使用 gcc 5 on wandbox 确认这一点