生成的运算符的签名=(）?

Signature of Generated operator=()?

本文关键字：运算符      更新时间：2023-10-16

我有以下代码：

#include <string>
#include <vector>
struct S {
const std::string str;
};
int main() {
std::vector<S> v;
const std::string test("test");
S s;
v.push_back(s);
}

它使用 g++ 4.8.5 编译得很好：

$ g++ --version
g++ (GCC) 4.8.5 20150623 (Red Hat 4.8.5-4)
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

但是当我尝试使用 g++ 4.6.2 编译它时，我遇到了以下错误：

In file included from /usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/vector:70:0,
from compilerTest.cpp:2:
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/vector.tcc: In member function ‘void std::vector<_Tp, _Alloc>::_M_insert_aux(std::vector<_Tp, _Alloc>::iterator, _Args&& ...) [with _Args = {const S&}, _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>::_Tp_alloc_type::pointer = S*]’:
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/stl_vector.h:834:4:   instantiated from ‘void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = S, _Alloc = std::allocator<S>, std::vector<_Tp, _Alloc>::value_type = S]’
compilerTest.cpp:12:14:   instantiated from here
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/vector.tcc:319:4: error: use of deleted function ‘S& S::operator=(const S&)’
compilerTest.cpp:4:8: error: ‘S& S::operator=(const S&)’ is implicitly deleted because the default definition would be ill-formed:
compilerTest.cpp:4:8: error: passing ‘const string {aka const std::basic_string<char>}’ as ‘this’ argument of ‘std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(const std::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>, std::basic_string<_CharT, _Traits, _Alloc> = std::basic_string<char>]’ discards qualifiers [-fpermissive]
In file included from /usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/char_traits.h:41:0,
from /usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/string:42,
from compilerTest.cpp:1:
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/stl_algobase.h: In static member function ‘static _BI2 std::__copy_move_backward<true, false, std::random_access_iterator_tag>::__copy_move_b(_BI1, _BI1, _BI2) [with _BI1 = S*, _BI2 = S*]’:
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/stl_algobase.h:581:18:   instantiated from ‘_BI2 std::__copy_move_backward_a(_BI1, _BI1, _BI2) [with bool _IsMove = true, _BI1 = S*, _BI2 = S*]’
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/stl_algobase.h:590:34:   instantiated from ‘_BI2 std::__copy_move_backward_a2(_BI1, _BI1, _BI2) [with bool _IsMove = true, _BI1 = S*, _BI2 = S*]’
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/stl_algobase.h:661:15:   instantiated from ‘_BI2 std::move_backward(_BI1, _BI1, _BI2) [with _BI1 = S*, _BI2 = S*]’
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/vector.tcc:313:4:   instantiated from ‘void std::vector<_Tp, _Alloc>::_M_insert_aux(std::vector<_Tp, _Alloc>::iterator, _Args&& ...) [with _Args = {const S&}, _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>::_Tp_alloc_type::pointer = S*]’
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/stl_vector.h:834:4:   instantiated from ‘void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = S, _Alloc = std::allocator<S>, std::vector<_Tp, _Alloc>::value_type = S]’
compilerTest.cpp:12:14:   instantiated from here
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/stl_algobase.h:546:6: error: use of deleted function ‘S& S::operator=(const S&)’

为什么S& S::operator=(const S&)被"删除"？

我正在使用命令进行编译：

g++ -Wall compilerTest.cpp -o compilerTest -std=c++0x

即使我使用 emplace_back()，它也给了我类似的错误：

In file included from /usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/x86_64-redhat-linux/bits/c++allocator.h:34:0,
from /usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/allocator.h:48,
from /usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/string:43,
from compilerTest.cpp:1:
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/ext/new_allocator.h: In member function ‘void __gnu_cxx::new_allocator<_Tp>::construct(__gnu_cxx::new_allocator<_Tp>::pointer, _Args&& ...) [with _Args = {std::basic_string<char>&}, _Tp = S, __gnu_cxx::new_allocator<_Tp>::pointer = S*]’:
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/vector.tcc:97:6:   instantiated from ‘void std::vector<_Tp, _Alloc>::emplace_back(_Args&& ...) [with _Args = {std::basic_string<char>&}, _Tp = S, _Alloc = std::allocator<S>]’
compilerTest.cpp:11:18:   instantiated from here
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/ext/new_allocator.h:114:4: error: no matching function for call to ‘S::S(std::basic_string<char>&)’
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/ext/new_allocator.h:114:4: note: candidates are:
compilerTest.cpp:4:8: note: S::S()
compilerTest.cpp:4:8: note:   candidate expects 0 arguments, 1 provided
compilerTest.cpp:4:8: note: S::S(const S&)
compilerTest.cpp:4:8: note:   no known conversion for argument 1 from ‘std::basic_string<char>’ to ‘const S&’
In file included from /usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/vector:70:0,
from compilerTest.cpp:2:
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/vector.tcc: In member function ‘void std::vector<_Tp, _Alloc>::_M_insert_aux(std::vector<_Tp, _Alloc>::iterator, _Args&& ...) [with _Args = {std::basic_string<char>&}, _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>::_Tp_alloc_type::pointer = S*]’:
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/vector.tcc:102:4:   instantiated from ‘void std::vector<_Tp, _Alloc>::emplace_back(_Args&& ...) [with _Args = {std::basic_string<char>&}, _Tp = S, _Alloc = std::allocator<S>]’
compilerTest.cpp:11:18:   instantiated from here
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/vector.tcc:319:4: error: no matching function for call to ‘S::S(std::basic_string<char>&)’
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/vector.tcc:319:4: note: candidates are:
compilerTest.cpp:4:8: note: S::S()
compilerTest.cpp:4:8: note:   candidate expects 0 arguments, 1 provided
compilerTest.cpp:4:8: note: S::S(const S&)
compilerTest.cpp:4:8: note:   no known conversion for argument 1 from ‘std::basic_string<char>’ to ‘const S&’
In file included from /usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/char_traits.h:41:0,
from /usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/string:42,
from compilerTest.cpp:1:
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/stl_algobase.h: In static member function ‘static _BI2 std::__copy_move_backward<true, false, std::random_access_iterator_tag>::__copy_move_b(_BI1, _BI1, _BI2) [with _BI1 = S*, _BI2 = S*]’:
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/stl_algobase.h:581:18:   instantiated from ‘_BI2 std::__copy_move_backward_a(_BI1, _BI1, _BI2) [with bool _IsMove = true, _BI1 = S*, _BI2 = S*]’
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/stl_algobase.h:590:34:   instantiated from ‘_BI2 std::__copy_move_backward_a2(_BI1, _BI1, _BI2) [with bool _IsMove = true, _BI1 = S*, _BI2 = S*]’
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/stl_algobase.h:661:15:   instantiated from ‘_BI2 std::move_backward(_BI1, _BI1, _BI2) [with _BI1 = S*, _BI2 = S*]’
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/vector.tcc:313:4:   instantiated from ‘void std::vector<_Tp, _Alloc>::_M_insert_aux(std::vector<_Tp, _Alloc>::iterator, _Args&& ...) [with _Args = {std::basic_string<char>&}, _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>::_Tp_alloc_type::pointer = S*]’
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/vector.tcc:102:4:   instantiated from ‘void std::vector<_Tp, _Alloc>::emplace_back(_Args&& ...) [with _Args = {std::basic_string<char>&}, _Tp = S, _Alloc = std::allocator<S>]’
compilerTest.cpp:11:18:   instantiated from here
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/stl_algobase.h:546:6: error: use of deleted function ‘S& S::operator=(const S&)’
compilerTest.cpp:4:8: error: ‘S& S::operator=(const S&)’ is implicitly deleted because the default definition would be ill-formed:
compilerTest.cpp:4:8: error: passing ‘const string {aka const std::basic_string<char>}’ as ‘this’ argument of ‘std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(const std::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>, std::basic_string<_CharT, _Traits, _Alloc> = std::basic_string<char>]’ discards qualifiers [-fpermissive]
为什么S& S：

：operator=(const S&)被"删除"？

因为S有一个const成员str这使得默认的复制赋值运算符被定义为已删除。

如果满足以下任一条件，则类T的缺省复制赋值运算符定义为已删除：

T具有无法复制分配的非静态数据成员或直接或虚拟基类(复制分配的重载解析失败，或者选择已删除或无法访问的函数);

不可能在const string上调用operator=，它是一个非常量成员函数。

而std::vector的类型要求从C++11开始变化：

T必须满足 CopyAssignable 和 CopyConstructable 的要求。 (至C++11)

对元素施加的要求取决于实际 对容器执行的操作。通常，要求 元素类型是完整的类型，符合 可擦除，但许多成员函数提出了更严格的要求。 (自C++11起)

因此，从 C++11 开始，要求取决于您执行的操作。事实上，std::vector::push_back不需要类型T是可复制的，复制可插入就可以了。

类型要求
- T 必须满足 CopyInsertable 的要求才能使用重载 (1)。

这就是为什么它使用 gcc4.8.5 编译，但 gcc4.6.2 抱怨复制赋值运算符被删除。(支持AFAIK gcc C++4.8.1的11)

当您具有非静态 const 成员时，将隐式删除复制赋值

这是因为您复制到的对象没有重新初始化，因此您有一个无法重新分配到的现有 const。

在您的情况下，不完全支持 C++11，这要求向量成员具有复制赋值

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