关于string的移动构造 - 编程论坛

在观察string移动构造时发现一个问题，为什么移动构造前后原来应该被置空的对象的c_str()没有变化，而且新构造的对象的c_str()和原对象不等呢？
我期望的行为应该和我自己实现的String对象的行为类似啊

程序代码：

#include "bits/stdc++.h"
using namespace std;

class String {
public:
    String() {
        cout << "String()" << endl;
    }
    String(const char* p) {
        cout << "String(const char* p)" << endl;
        this->len = strlen(p);
        this->p = new char[this->len + 1];
        strcpy(this->p, p);
    }

    String(const String& s) {
        cout << "String(const String& s)" << endl;
        this->len = s.len;
        this->p = new char[this->len + 1];
        strcpy(this->p, s.p);
    }

    String(String&& s) {
        cout << "String(String&& s)" << endl;
        this->len = s.len;
        this->p = s.p;
        s.p = nullptr;
        s.len = 0;
    }

    ~String() {
        cout << "~String()" << endl;
        if(this->p) {
            delete this->p;
        }
    }

    String& operator = (const String& s) {
        cout << "String& operator = (const String& s)" << endl;
        if(this->p) {
            delete this->p;
        }
        this->len = s.len;
        this->p = new char[this->len + 1];
        strcpy(this->p, s.p);
        return *this;
    }

    String& operator = (String&& s) {
        cout << "String& operator = (String&& s)" << endl;
        if(this->p) {
            delete this->p;
        }
        this->len = s.len;
        this->p = s.p;
        s.p = nullptr;
        s.len = 0;
        return *this;
    }

    const char* c_str() const {
        return p;
    }

private:
    char* p = nullptr;
    int len = 0;

    friend ostream& operator << (ostream& out, const String& s);
};

ostream& operator << (ostream& out, const String& s) {
    if(s.c_str()) {
        out << s.c_str();
    } else {
        out << "";
    }
    return out;
}

void test1() {
    String a = "abc";
    printf("%p\n", a.c_str());
    String b = move(a);
    printf("%p\n", a.c_str());
    printf("%p\n", b.c_str());
    cout << a << endl;
    cout << b << endl;
    move(b);
    cout << b << endl;
}

void test2() {
    using String = std::string;

    String a = "abc";
    printf("%p\n", a.c_str());
    String b = move(a);
    printf("%p\n", a.c_str());
    printf("%p\n", b.c_str());
    cout << a << endl;
    cout << b << endl;
    move(b);
    cout << b << endl;
}

int _main_() {
    test1();
    test2();

    return 0;
}