我在使用const成员的类的实例上使用'emplace_back'时遇到问题。
请参阅下面的示例代码清单。
#include <vector>
using std::vector;
class Data {
public:
Data(int a, int b, int c) : a(a), b(b), c(c) {}
// Removing any/all of these constructors changes nothing!
Data& operator=(const Data& s) = default;
Data(const Data& s) = default;
Data(Data&& s) = default;
~Data() = default;
const int a;
const int b;
const int c;
};
int main() {
vector<Data> v;
Data e(1,2,3);
// Problem line
v.emplace_back(4,5,6);
}
编译错误：
% clang++ a.cc -std=c++11
In file included from a.cc:1:
In file included from /usr/include/c++/4.6/vector:69:
/usr/include/c++/4.6/bits/vector.tcc:319:16: error: overload resolution selected
deleted operator '='
*__position = _Tp(std::forward<_Args>(__args)...);
~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/4.6/bits/vector.tcc:102:4: note: in instantiation of function
template specialization 'std::vector<Data, std::allocator<Data>
>::_M_insert_aux<int, int, int>' requested here
_M_insert_aux(end(), std::forward<_Args>(__args)...);
^
a.cc:25:5: note: in instantiation of function template specialization
'std::vector<Data, std::allocator<Data> >::emplace_back<int, int, int>'
requested here
v.emplace_back(4,5,6);
^
a.cc:9:11: note: candidate function has been explicitly deleted
Data& operator=(const Data& s) = default;
^
In file included from a.cc:1:
In file included from /usr/include/c++/4.6/vector:60:
/usr/include/c++/4.6/bits/stl_algobase.h:546:18: error: overload resolution
selected deleted operator '='
*--__result = std::move(*--__last);
~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/4.6/bits/stl_algobase.h:578:14: note: in instantiation of
function template specialization 'std::__copy_move_backward<true, false,
std::random_access_iterator_tag>::__copy_move_b<Data *, Data *>' requested
here
return std::__copy_move_backward<_IsMove, __simple,
^
/usr/include/c++/4.6/bits/stl_algobase.h:588:19: note: in instantiation of
function template specialization 'std::__copy_move_backward_a<true, Data
*, Data *>' requested here
return _BI2(std::__copy_move_backward_a<_IsMove>
^
/usr/include/c++/4.6/bits/stl_algobase.h:659:14: note: in instantiation of
function template specialization 'std::__copy_move_backward_a2<true, Data
*, Data *>' requested here
return std::__copy_move_backward_a2<true>(std::__miter_base(__first),
^
/usr/include/c++/4.6/bits/vector.tcc:313:4: note: in instantiation of function
template specialization 'std::move_backward<Data *, Data *>' requested
here
_GLIBCXX_MOVE_BACKWARD3(__position.base(),
^
/usr/include/c++/4.6/bits/stl_algobase.h:664:48: note: expanded from:
#define _GLIBCXX_MOVE_BACKWARD3(_Tp, _Up, _Vp) std::move_backward(_Tp, _Up, _Vp)
^
/usr/include/c++/4.6/bits/vector.tcc:102:4: note: in instantiation of function
template specialization 'std::vector<Data, std::allocator<Data>
>::_M_insert_aux<int, int, int>' requested here
_M_insert_aux(end(), std::forward<_Args>(__args)...);
^
a.cc:25:5: note: in instantiation of function template specialization
'std::vector<Data, std::allocator<Data> >::emplace_back<int, int, int>'
requested here
v.emplace_back(4,5,6);
^
a.cc:9:11: note: candidate function has been explicitly deleted
Data& operator=(const Data& s) = default;
^
2 errors generated.
看到此错误消息：
a.cc:9:11: note: candidate function has been explicitly deleted
Data& operator=(const Data& s) = default;
我尝试定义赋值运算符（以及移动和复制构造函数以及析构函数）以确保隐式生成的运算符不会以某种方式变为私有。这并没有改变任何事情。在这种情况下，我不明白为什么赋值运算符被“显式删除”。
请注意，将类成员更改为非const不会导致编译错误。