命名参数习语和不可复制的类

Question

我正在实现一个使用命名参数惯用语初始化其成员的类：

class Person
{
    public:
        Person & first_name(std::string n) { m_first_name = n; return *this; }
        Person & last_name(std::string l) { m_last_name = l; return *this; }
        Person & age(int a) { m_age = a; return *this; }

        std::string get_first_name() const { return m_first_name; }
        std::string get_last_name() const { return m_last_name; }
        int get_age() const { return m_age; }

    private:
        std::string m_first_name;
        std::string m_last_name;
        int m_age;
};

到目前为止，一切正常。

#include <string>
#include <iostream>

class Person
{
    public:
        Person & first_name(std::string n) { m_first_name = n; return *this; }
        Person & last_name(std::string l) { m_last_name = l; return *this; }
        Person & age(int a) { m_age = a; return *this; }

        std::string get_first_name() const { return m_first_name; }
        std::string get_last_name() const { return m_last_name; }
        int get_age() const { return m_age; }

    private:
        std::string m_first_name;
        std::string m_last_name;
        int m_age;
};

void print(Person const & p)
{
    std::cout << p.get_last_name() << ", " << p.get_first_name() << ", " << p.get_age() << '\n';
}

int main()
{
    auto p1 = Person().first_name("Joe").age(20);
    auto p2 = Person().last_name("Kane").age(50);

    print(p1);
    print(p2);
}

以上代码输出

, Joe, 20
Kane, , 50

按预期。

但是，一旦我使我的班级不可复制（在我的情况下，通过添加std::unique_ptr成员，上述代码将无法编译]]

#include <string>
#include <memory>
#include <iostream>

class Person
{
    public:
        Person & first_name(std::string n) { m_first_name = n; return *this; }
        Person & last_name(std::string l) { m_last_name = l; return *this; }
        Person & age(int a) { m_age = a; return *this; }

        std::string get_first_name() const { return m_first_name; }
        std::string get_last_name() const { return m_last_name; }
        int get_age() const { return m_age; }

    private:
        std::string m_first_name;
        std::string m_last_name;
        int m_age;
        std::unique_ptr<int> m_ptr; // added member
};

void print(Person const & p)
{
    std::cout << p.get_last_name() << ", " << p.get_first_name() << ", " << p.get_age() << '\n';
}

int main()
{
    auto p1 = Person().first_name("Joe").age(20);
    auto p2 = Person().last_name("Kane").age(50);

    print(p1);
    print(p2);
}

带有G ++的以下错误：

prog.cc: In function 'int main()':
prog.cc:30:48: error: use of deleted function 'Person::Person(const Person&)'
   30 |     auto p1 = Person().first_name("Joe").age(20);
      |                                                ^
prog.cc:5:7: note: 'Person::Person(const Person&)' is implicitly deleted because the default definition would be ill-formed:
    5 | class Person
      |       ^~~~~~
prog.cc:5:7: error: use of deleted function 'std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = int; _Dp = std::default_delete<int>]'
In file included from /opt/wandbox/gcc-head/include/c++/10.0.0/memory:82,
                 from prog.cc:2:
/opt/wandbox/gcc-head/include/c++/10.0.0/bits/unique_ptr.h:456:7: note: declared here
  456 |       unique_ptr(const unique_ptr&) = delete;
      |       ^~~~~~~~~~
prog.cc:31:48: error: use of deleted function 'Person::Person(const Person&)'
   31 |     auto p2 = Person().last_name("Kane").age(50);
      |                
我通过返回右值来解决此问题

：

class Person
{
    public:
        Person && first_name(std::string n) { m_first_name = n; return std::move(*this); }
        Person && last_name(std::string l) { m_last_name = l; return std::move(*this); }
        Person && age(int a) { m_age = a; return std::move(*this); }

        // ...
};
这是正确的解决方法吗？我应该考虑任何陷阱吗？

我正在实现一个使用命名参数惯用语初始化其成员的类：类Person {public：Person＆first_name（std :: string n）{m_first_name = n;返回* this; } ...

Answer 1

问题是在p1和p2的声明中>

//      V <-- first copy
auto p1 = Person().first_name("Joe").age(20);
auto p2 = Person().last_name("Kane").age(50);
//      ^ <-- second copy