为什么 std::sort 抱怨已删除的复制者？

假设我们有一个简单的类，其中包含std::string:

class StringWrapper {
public:
    const std::string s;

    StringWrapper(const std::string s)
        : s(s) {}

    // We want this to be moveable but not copyable
    ~StringWrapper() = default;
    StringWrapper(const StringWrapper&) = delete;
    StringWrapper& operator=(const StringWrapper&) = delete;
    StringWrapper(StringWrapper&&) noexcept = default;
    StringWrapper& operator=(StringWrapper&&) = default;
};

在这里，我们试图使类可移动，但不可复制。我明白，一个const成员变量通常会阻止生成默认的移动向量，但在这里我们尝试显式生成它 - 这至少可以编译。

现在我们尝试排序std::vector<StringWrapper>（比较函数在这里并不重要）：

std::vector<StringWrapper> strings;
std::sort(strings.begin(), strings.end(), [](auto const& a, auto const& b) { return true; });

这无法编译，但错误非常神秘：

1>...\include\algorithm(7419,25): error C2280: 'StringWrapper &StringWrapper::operator =(const StringWrapper &)': attempting to reference a deleted function
1>...src\example.cpp(476): message : see declaration of 'StringWrapper::operator ='
1>...src\example.cpp(476,24): message : 'StringWrapper &StringWrapper::operator =(const StringWrapper &)': function was explicitly deleted
1>...\include\algorithm(7541): message : see reference to function template instantiation '_BidIt std::_Insertion_sort_unchecked<_RanIt,_Pr>(const _BidIt,const _BidIt,_Pr)' being compiled
1>        with
1>        [
1>            _BidIt=StringWrapper *,
1>            _RanIt=StringWrapper *,
1>            _Pr=myFunc::<lambda_8668e50965d967f7b587b72f59fcb0cf>
1>        ]
1>...\include\algorithm(7571): message : see reference to function template instantiation 'void std::_Sort_unchecked<StringWrapper*,_Fn>(_RanIt,_RanIt,int,_Pr)' being compiled
1>        with
1>        [
1>            _Fn=myFunc::<lambda_8668e50965d967f7b587b72f59fcb0cf>,
1>            _RanIt=StringWrapper *,
1>            _Pr=myFunc::<lambda_8668e50965d967f7b587b72f59fcb0cf>
1>        ]
1>...src\example.cpp(488): message : see reference to function template instantiation 'void std::sort<std::_Vector_iterator<std::_Vector_val<std::_Simple_types<_Ty>>>,myFunc::<lambda_8668e50965d967f7b587b72f59fcb0cf>>(const _RanIt,const _RanIt,_Pr)' being compiled
1>        with
1>        [
1>            _Ty=StringWrapper,
1>            _RanIt=std::_Vector_iterator<std::_Vector_val<std::_Simple_types<StringWrapper>>>,
1>            _Pr=myFunc::<lambda_8668e50965d967f7b587b72f59fcb0cf>
1>        ]
1>...\include\algorithm(7422,28): error C2280: 'StringWrapper &StringWrapper::operator =(const StringWrapper &)': attempting to reference a deleted function
1>...src\example.cpp(476): message : see declaration of 'StringWrapper::operator ='
1>...src\example.cpp(476,24): message : 'StringWrapper &StringWrapper::operator =(const StringWrapper &)': function was explicitly deleted
1>...\include\algorithm(7425,24): error C2280: 'StringWrapper &StringWrapper::operator =(const StringWrapper &)': attempting to reference a deleted function
1>...src\example.cpp(476): message : see declaration of 'StringWrapper::operator ='
1>...src\example.cpp(476,24): message : 'StringWrapper &StringWrapper::operator =(const StringWrapper &)': function was explicitly deleted

具体来说，它建议std::sort正在尝试引用已删除的复制者，但我不知道为什么会出现这种情况；我希望std::sort仅使用移动操作。

谁能向我解释一下这是怎么回事？

这已经在评论中涵盖了，但对于后代来说值得注意的是，使用default关键字不保证编译器始终会尝试创建特殊成员的默认实现。这（像往常一样）在关于移动赋值运算符的部分中得到了很好的解释参考参数 https://en.cppreference.com/w/cpp/language/move_assignment#Deleted_implicitly-declared_move_assignment_operator.

如果满足以下任一条件，则类 T 的隐式声明或默认移动赋值运算符被定义为已删除：

• T 有一个非静态数据成员，即const.

编译器也会像这里一样抱怨这种情况。例如铿锵：

warning: explicitly defaulted move assignment operator is implicitly deleted [-Wdefaulted-function-deleted]
    StringWrapper& operator=(StringWrapper&&) = default;
                   ^
note: move assignment operator of 'StringWrapper' is implicitly deleted because field 's' has no move assignment operator
    const std::string s;

请注意，这是警告而不是错误，这就是您最终遇到这种情况的原因。一般来说，最好不要假设default关键字允许您绕过语言限制。