生成运算符的签名=（）？答案

【问题标题】：Signature of Generated operator=()?生成运算符的签名=（）？
【发布时间】：2016-06-22 14:26:44
【问题描述】：

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);
}

使用 g++ 4.8.5 可以正常编译：

$ 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.

但是当我尝试用 g++ 4.6.2 编译它时，我得到了以下错误：

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”了？

我是用命令编译的：

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

即使我使用 emplace_back()，它也会给我类似的错误：

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]

【问题讨论】：

非常有趣的是push_back使用复制赋值，试试emplace_back()
@gbehar 同样的错误。我认为众所周知 push_back() 将调用赋值运算符，因为 std 容器在插入元素时会进行复制。
实际上，您有一个隐式复制构造函数，而在 c++11 中，这就是 push_back 使用的。我想即使使用 -std=c++0x 也没有对此的完全支持。在 c++03 中，向量项需要复制构造函数和复制赋值。在您的情况下，复制分配被删除
对，我在这篇文章中的问题是试图找出赋值运算符被删除的原因。
嗯，那是因为你有一个 const 成员

标签： c++ c++11 g++

【解决方案1】：

为什么 S& S::operator=(const S&) 被“删除”了？

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deleted。

如果以下任何一项为真，则类 T 的默认复制赋值运算符被定义为已删除：

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（复制分配的重载解决方案失败，或选择了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。

并且std::vector的类型要求从C++11改变了：

T 必须满足 CopyAssignable 和 CopyConstructible 的要求。（直到 C++11）

对元素的要求取决于实际情况对容器执行的操作。一般来说，要求元素类型是一个完整的类型，并且满足要求可擦除，但许多成员函数提出了更严格的要求。 (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 T 类型为 CopyAssignable，CopyInsertable 就可以了。

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

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

【讨论】：

非常详细的答案。我只是想知道 CopyInsertable 上是否有更详细的描述？您包含的链接提供了有关其含义的正式描述。但是有点难以看出什么是“std::allocator_traits::construct(m, p, v);”是单词中格式正确的意思（除了表达式可以编译没有错误）。谢谢。
@Hei 重点是（也写在开头）“可以复制构建”。对于您的情况，它将是（也写在最后）“由 ::new((void*)p) T(v)”。 S 有一个 const 成员，则不能由 operator=() 复制分配，但它可以由其复制构造函数复制构造。这就是区别。
我明白你的观点，即 const 成员不能由 operator=() 复制分配。你介意给我一个不能复制的例子吗？（我知道这与原始帖子略有不同；希望您不要介意）谢谢。
@Hei 当然。您可以通过编写S(const S&) = delete; 显式创建复制构造函数delete，或者使S 具有无法复制构造的成员，例如std::unique_ptr<int> p;。
啊，所以花哨的术语“可复制构造”只是意味着一个类有一个复制构造函数？

【解决方案2】：

当你有一个非静态的 const 成员时，复制赋值被隐式删除

那是因为你复制到的对象没有重新初始化，所以你有一个不能重新分配到的现有 const。

在您的情况下，不完全支持 C++11，这需要向量成员进行复制分配

【讨论】：