一个C++的有意思的例子

最近为了学习QT，重新学习C++，拿起了厚厚的《C++ Primer Plus（第6版）》

第12章：类与动态内存分配

Stringbad.h

#pragma once
#include <iostream>
class Stringbad
{
private:
    char* str;
    int len;
    static int num_strings;
public:
    Stringbad(const char* s);    
    Stringbad();
    ~Stringbad();    
    friend std::ostream& operator<<(std::ostream& os, const Stringbad& st);
    //Stringbad& operator=(const Stringbad& st);
    //Stringbad(const Stringbad& st);
};

strngbad.cpp

#include "strngbad.h"
#include <cstring>

using std::cout;
int Stringbad::num_strings = 0;

Stringbad::Stringbad(const char* s) {
    len = std::strlen(s);
    str = new char[len + 1];
    std::strcpy(str, s);
    num_strings++;
    cout << num_strings << ":\"" << str << "\" object created\n";
}

Stringbad::Stringbad() {
    len = 4;
    str = new char[4];
    std::strcpy(str, "C++");
    num_strings++;
    cout << num_strings << ":\"" << str << "\" default object created\n";
}

Stringbad::~Stringbad() {
    cout << "\"" << str << "\" object deleted,";
    --num_strings;
    cout << num_strings << " left\n";
    delete[] str;
}

std::ostream& operator<<(std::ostream& os, const Stringbad& st) {
    os << st.str;
    return os;
}
/*
Stringbad& Stringbad::operator=(const Stringbad& st) {
    cout << "object assign " << st.str << std::endl;
    if (this == &st)
        return *this;
    delete[] str;
    len = st.len;
    str = new char(len + 1);
    std::strcpy(str, st.str);
    return *this;
}

Stringbad::Stringbad(const Stringbad& st) {
    len = st.len;
    str = new char[len + 1];
    std::strcpy(str, st.str);
    num_strings++;
    cout << num_strings << ":\"" << str << "\" object copy.\n";
}
*/

看似没有问题的类定义，如下测试

#include <iostream>
#include <string>
#include "Stock.h"
#include "strngbad.h"

void callme1(Stringbad&);
void callme2(Stringbad);

int stonetolb(int st) {

    return 14 * st;
}
using std::cout;
int main()
{    
    using std::endl;     
    {
        cout << "String an inner block.\n";
        Stringbad headline1("Celera Stalks at Midnight");
        Stringbad headline2("Lettuce Prey");
        Stringbad sports("Spinach Leaves Bowl for Dollars");

        cout << "headline1:" << headline1 << endl;
        cout << "headline2:" << headline2 << endl;
        cout << "sports:" << sports << endl;

        callme1(headline1);
        cout << "headline1:" << headline1 << endl;
        callme2(headline2);
        cout << "headline2:" << headline2 << endl;
        cout << "Initialize one object to another:\n";
        Stringbad knot;
        knot = headline1;
        cout << "knot: " << knot << endl;
        cout << "Exiting the block.\n";
    }
    cout << "End of main()\n";
    return 0;
}

void callme1(Stringbad& rsb) {
    cout << "String passed by reference:\n";
    cout << "     \"" << rsb << "\"\n";
}

void callme2(Stringbad rsb) {
    cout << "String passed by reference:\n";
    cout << "     \"" << rsb << "\"\n";
}

结果，又是负数，又是乱码。以下是我的vs.net2019的运行结果，因为是异常退出，且输出的结果有时会少了一行，所以以下截图看不到-1的结果

 

 

上面的代码你会发现我注释了部分代码，恢复后，你会发现程序死得更早。因为在赋值函数如此创建的数组

str = new char(len + 1);

而如此delete

delete [] str;