在其他容器中使用 boost：：container：：static_vector 时，GCC 编译错误"将'const s'绑定到类型's&'的引用丢弃限定符"

Question

当在另一个容器(如std：：vector(中使用boost：：container：：static_vector时，gcc会返回编译错误。以下测试用例重现该错误：

#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);
}

符合 Ubuntu Xenial 上的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，才会发生此错误。此错误的原因是什么，是否有可用的解决方法？

Answer 1

看起来编译器生成的默认复制辅助运算符具有签名

s& operator= (s&);

并且您的代码在以下fill算法中失败(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，并且不能作为参数传递给operator=(s&)(常量对象无法修改(。

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

若要解决此问题，可以实现此运算符的自己的版本：

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