使用 g++ 4.6 和 boost::unordered_map 的 C++11 相关编译错误

Question

我有一段 C++ 代码会因编译错误而失败，除非它在 ​​C++11 模式下运行，但我无法弄清楚为什么会这样，因为代码没有(明确地)使用 C++11 特点:

#include <vector>
#include <map>
#include <boost/unordered_map.hpp>

struct SomeStruct {
  boost::unordered_map<int, int> intMap;
};

int main(int argc, const char* argv[]) {
  std::vector<SomeStruct> vals;

  vals.resize(100);
}

在使用 gcc 4.6.3-1ubuntu5 和 boost 1.48 的 Ubuntu 12.04 64 位机器上编译时:

g++ test.cpp

然后我得到这个编译错误:

In file included from /usr/include/c++/4.6/vector:61:0,
                 from test.cpp:1:
/usr/include/c++/4.6/bits/stl_algobase.h: In function 'typename __gnu_cxx::__enable_if<(! std::__is_scalar<_Tp>::__value), void>::__type std::__fill_a(_ForwardIterator, _ForwardIterator, const _Tp&) [with _ForwardIterator = SomeStruct*, _Tp = SomeStruct, typename __gnu_cxx::__enable_if<(! std::__is_scalar<_Tp>::__value), void>::__type = void]':
/usr/include/c++/4.6/bits/stl_algobase.h:722:7:   instantiated from 'void std::fill(_ForwardIterator, _ForwardIterator, const _Tp&) [with _ForwardIterator = SomeStruct*, _Tp = SomeStruct]'
/usr/include/c++/4.6/bits/vector.tcc:397:5:   instantiated 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 = SomeStruct, _Alloc = std::allocator<SomeStruct>, std::vector<_Tp, _Alloc>::iterator = __gnu_cxx::__normal_iterator<SomeStruct*, std::vector<SomeStruct> >, typename std::_Vector_base<_Tp, _Alloc>::_Tp_alloc_type::pointer = SomeStruct*, std::vector<_Tp, _Alloc>::size_type = long unsigned int, std::vector<_Tp, _Alloc>::value_type = SomeStruct]'
/usr/include/c++/4.6/bits/stl_vector.h:944:9:   instantiated from 'void std::vector<_Tp, _Alloc>::insert(std::vector<_Tp, _Alloc>::iterator, std::vector<_Tp, _Alloc>::size_type, const value_type&) [with _Tp = SomeStruct, _Alloc = std::allocator<SomeStruct>, std::vector<_Tp, _Alloc>::iterator = __gnu_cxx::__normal_iterator<SomeStruct*, std::vector<SomeStruct> >, typename std::_Vector_base<_Tp, _Alloc>::_Tp_alloc_type::pointer = SomeStruct*, std::vector<_Tp, _Alloc>::size_type = long unsigned int, std::vector<_Tp, _Alloc>::value_type = SomeStruct]'
/usr/include/c++/4.6/bits/stl_vector.h:632:4:   instantiated from 'void std::vector<_Tp, _Alloc>::resize(std::vector<_Tp, _Alloc>::size_type, std::vector<_Tp, _Alloc>::value_type) [with _Tp = SomeStruct, _Alloc = std::allocator<SomeStruct>, std::vector<_Tp, _Alloc>::size_type = long unsigned int, std::vector<_Tp, _Alloc>::value_type = SomeStruct]'
test.cpp:12:18:   instantiated from here
/usr/include/c++/4.6/bits/stl_algobase.h:676:2: error: no match for 'operator=' in '* __first = __value'
/usr/include/c++/4.6/bits/stl_algobase.h:676:2: note: candidate is:
test.cpp:5:8: note: SomeStruct& SomeStruct::operator=(SomeStruct&)
test.cpp:5:8: note:   no known conversion for argument 1 from 'const SomeStruct' to 'SomeStruct&'

在 gcc 中启用 C++11 支持时编译正常:

g++ -std=c++0x test.cpp

有人可以解释为什么这只适用于 C++11 模式吗？

编辑:

使用 Vagrant 重现的步骤:

vagrant init precise64
vagrant up
vagrant ssh
sudo apt-get install -y build-essential libboost1.48-all-dev
echo "#include <vector>
#include <map>
#include <boost/unordered_map.hpp>

struct SomeStruct {
  boost::unordered_map<int, int> intMap;
};

int main(int argc, const char* argv[]) {
  std::vector<SomeStruct> vals;

  vals.resize(100);
}" > test.cpp
g++ test.cpp

Answer 1

boost::unordered_map 的复制赋值运算符声明为:

unordered_map& operator=(unordered_map &t)

这反过来导致 (C++98 12.8 [#10]) SomeStruct 的隐式复制赋值运算符被声明为:

SomeStruct &operator= (SomeStruct &);

添加显式复制赋值运算符

SomeStruct &operator= (const SomeStruct &x) { intMap = x.intMap; return *this; }

解决了这个问题，它通过调用 unordered_map::operator= 的不同重载来工作。

此问题已在 Boost 1.52 中修复。将运算符声明为:

unordered_map& operator=(unordered_map const& x);