有错误的引用变量，必须在构造函数基类/成员初始化器中初始化

Reference variable with error, must be initialized in constructor base/member initializer(有错误的引用变量，必须在构造函数基类/成员初始化器中初始化)

问题描述

当我尝试编译下面的源代码时出现以下错误.任何人都可以描述为什么存在此错误以及我如何解决它?

I got following error when I try to compile the source-code below. Can anybody describe why this error exists and how I can fix it?

错误 1 ​​错误 C2758:'A::s_':必须在构造函数基类/成员初始化器中初始化

#include <iostream>
#include <string>

using namespace std;

class A
{
public:
    A(string& s) : s_(s) { cout << "A::ctor" << endl; }
    A(const A& rhs)      { cout << "A::copy" << endl; }
    ~A()                 { cout << "A::dtor" << endl; }

    A& operator=(const A& rhs) { cout << "A::copyassign" << endl; }

private:
    string& s_;    
};

int main()
{

    return 0;
}

推荐答案

首先，你的 A::s_ 是对 std::string 的引用；这意味着它正在引用某处必须存在的东西.

First of all, your A::s_ is a reference to a std::string; that means that it's referencing something that must exists somewhere.

由于他的引用类型，以及引用必须在创建时初始化的事实，您必须在所有 A 中初始化 A::s_> 构造函数(由其他用户指出):

Due of his reference type, and the fact that the references must be initialized at the moment they're created, you must initialize A::s_ in ALL the A constructors (as pointed by other users):

class A
{
public:
    A(string& s) : s_(s)
    { cout << "A::ctor" << endl; }

    A(const A& rhs) : s_(rhs.s_) // <-- here too!!
    { cout << "A::copy" << endl; }

    ~A()
    { cout << "A::dtor" << endl; }

    A& operator=(const A& rhs)
    { cout << "A::copyassign" << endl; }

private:
    string& s_;    
};

现在，回到我提到的第一件事；A::s_ 必须引用存在的东西，所以一定要注意一些东西，看看下面的代码:

And now, back to the first thing I mentioned; the A::s_ must reference something that exists, so you must be aware of some things, take a look at the following code:

int main()
{
    // New A instance:
    A a("hello world");

    return 0;
}

构造这个 A 实例，我们提供了一个 const char[12] 值，这个值是一个临时的 std::string创建并提供给 A::A(string& s) 构造函数.构造函数结束后 A::s_ 在哪里引用?创建的临时 std::string 会发生什么?当 A 构造函数结束时，它的生命周期会延长还是死亡?您确定需要参考吗?

Constructing this A instance we're providing a const char[12] value, with this value a temporary std::string is created and is given to the A::A(string& s) constructor. Where A::s_ is referencing after the constructor ends? What happens with the temporary std::string created? It's lifetime is extended or it just die when the Aconstructor ends? Are you sure that a reference is what you need?

std::string s("hello world");

int main()
{
    // New A instance:
    A a(s);

    return 0;
}

使用上面的代码，创建了一个新的 A 实例，调用相同的 A::A(string& s) 构造函数，但提供的字符串位于全局作用域，因此它不会被销毁，并且 a 实例中的 A::s_ 将在其整个生命周期中引用一个有效的字符串，但真正的威胁在于复制构造函数:

With the code above, a new A instance is created calling the same A::A(string& s) constructor, but with a provided string lying in the global scope, so it doesn't be destroyed and the A::s_ from the a instance would reference a valid string all its lifetime, but the real threat is in the copy constructor:

std::string s("hello world");

int main()
{
    A a(s);    // a.s_ references the global s.
    A b(a);    // b.s_ references the a.s_ that references the global s.

    return 0;
}

复制的对象值将引用给定对象的std::string！这就是你想要的吗?

The copied object value will reference the std::string of the given object! Is that what you want?

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