为什么 std：：sort 抱怨已删除的副本 ctor？-解网

问：

假设我们有一个简单的类，它包含一个：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。我知道成员变量通常会阻止生成默认的移动 ctor，但在这里我们尝试显式生成它 - 这至少可以编译。const

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

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

这无法编译，但错误非常隐晦：

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，它表明正在尝试引用已删除的副本 ctor，但我不知道为什么会这样;我希望只使用移动操作。std::sortstd::sort

谁能向我解释一下这是怎么回事？

C++

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，这是一个警告，而不是一个错误，这就是你最终遇到这种情况的方式。一般来说，最好不要假设关键字允许您绕过语言限制。default

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为什么 std：：sort 抱怨已删除的副本 ctor？

Why is std::sort complaining about a deleted copy ctor?

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