在其他容器中使用 boost::container::static_vector 时，gcc 编译错误“将‘const s’绑定到‘s&’类型的引用丢弃限定符”

Question

在 std::vector 等另一个容器中使用 boost::container::static_vector 时，gcc 返回编译错误。下面的测试用例重现了错误：

#include <boost/container/static_vector.hpp>
#include <vector>

struct s
{
    boost::container::static_vector<int,6> a;
};

int main()
{
    std::vector<s> b;
    b.resize(6);
}

在 Ubuntu Xenial 上遵循 g++ template_test2.cpp 会导致以下错误：

In file included from /usr/include/c++/5/bits/char_traits.h:39:0,
                 from /usr/include/c++/5/string:40,
                 from /usr/include/c++/5/stdexcept:39,
                 from /usr/include/boost/container/throw_exception.hpp:26,
                 from /usr/include/boost/container/new_allocator.hpp:24,
                 from /usr/include/boost/container/vector.hpp:28,
                 from /usr/include/boost/container/static_vector.hpp:25,
                 from template_test2.cpp:1:
/usr/include/c++/5/bits/stl_algobase.h: In instantiation of ‘typename __gnu_cxx::__enable_if<(! std::__is_scalar<_Tp>::__value), void>::__type std::__fill_a(_ForwardIterator, _ForwardIterator, const _Tp&) [with _ForwardIterator = s*; _Tp = s; typename __gnu_cxx::__enable_if<(! std::__is_scalar<_Tp>::__value), void>::__type = void]’:
/usr/include/c++/5/bits/stl_algobase.h:747:20:   required from ‘void std::fill(_ForwardIterator, _ForwardIterator, const _Tp&) [with _ForwardIterator = s*; _Tp = s]’
/usr/include/c++/5/bits/vector.tcc:469:14:   required from ‘void std::vector<_Tp, _Alloc>::_M_fill_insert(std::vector<_Tp, _Alloc>::iterator, std::vector<_Tp, _Alloc>::size_type, const value_type&) [with _Tp = s; _Alloc = std::allocator<s>; std::vector<_Tp, _Alloc>::iterator = __gnu_cxx::__normal_iterator<s*, std::vector<s> >; typename std::_Vector_base<_Tp, _Alloc>::pointer = s*; std::vector<_Tp, _Alloc>::size_type = long unsigned int; std::vector<_Tp, _Alloc>::value_type = s]’
/usr/include/c++/5/bits/stl_vector.h:1073:23:   required from ‘void std::vector<_Tp, _Alloc>::insert(std::vector<_Tp, _Alloc>::iterator, std::vector<_Tp, _Alloc>::size_type, const value_type&) [with _Tp = s; _Alloc = std::allocator<s>; std::vector<_Tp, _Alloc>::iterator = __gnu_cxx::__normal_iterator<s*, std::vector<s> >; typename std::_Vector_base<_Tp, _Alloc>::pointer = s*; std::vector<_Tp, _Alloc>::size_type = long unsigned int; std::vector<_Tp, _Alloc>::value_type = s]’
/usr/include/c++/5/bits/stl_vector.h:716:10:   required from ‘void std::vector<_Tp, _Alloc>::resize(std::vector<_Tp, _Alloc>::size_type, std::vector<_Tp, _Alloc>::value_type) [with _Tp = s; _Alloc = std::allocator<s>; std::vector<_Tp, _Alloc>::size_type = long unsigned int; std::vector<_Tp, _Alloc>::value_type = s]’
template_test2.cpp:12:11:   required from here
/usr/include/c++/5/bits/stl_algobase.h:701:11: error: binding ‘const s’ to reference of type ‘s&’ discards qualifiers
  *__first = __value;

仅当struct s 包含静态向量时才会出现此错误。此错误的原因是什么，是否有可用的解决方法？

Answer 1

编译器生成的默认复制赋值运算符好像有签名

s& operator= (s&);

并且您的代码在fill 算法的这一行中失败（在resize 方法中以某种方式调用）：

  for (; __first != __last; ++__first)
      *__first = __value;  // <----- call operator=(s&) passing const s& object

因为__value 是对s 对象的常量引用，它不能作为参数传递给 operator=(s&)（常量对象不能修改）。

关于为类生成默认复制赋值运算符，您可以阅读here。 static_vector 的实现有些奇怪，默认复制赋值运算符将 s& 作为参数而不是 const s&。

要解决此问题，您可以实现您自己的此运算符版本：

 s& operator = (const s& other) {
     a = other.a;
     return *this;
 }