Question

我有一些C ++代码因编译错误而失败，除非它在C ++ 11模式下运行，但我无法弄清楚为什么会出现这种情况，因为代码没有（明确地）使用C ++ 11功能：

#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

然后我得到了这个编译错误：

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支持时编译很好：

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？

修改

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)

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 &);

添加显式的复制赋值运算符

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=的不同重载来实现。

此问题已在Boost 1.52中修复。它将运算符声明为：

unordered_map& operator=(unordered_map const& x);

与g ++ 4.6和boost :: unordered_map相关的C ++ 11相关编译错误

1 个答案: