C++ Problems

1. no default constructor exists for class "Apple"

#include <iostream>
using namespace std;
//默认情况下，类的所有成员都是私有的.只有类和友元函数可以访问私有成员
//protected（受保护）成员在派生类（即子类）中是可访问的
class Box
{public:double length; double Width;double GetStr(void)//无参数可以加void，{return (length + Width)*2;}double GetArea();//无参数也可以不加void，
};
//类继承
class BoxOne :protected Box
{double x;//不指定访问符，默认是Private.
};double Box::GetArea()
{return length * Width;
}class Apple
{public:Apple(double len);//构造函数声明；无返回值；但可以带参数。~Apple();//析构函数声明；无返回值；且无参数;用于释放资源double length;
};Apple::Apple(double len)
{//构造函数是类进入时进行cout << "这是构造函数！" << endl;length = len;
}Apple::~Apple()
{//析构函数是类退出时进行cout << "这是析构函数！" << endl;
}int main()
{Box box1;double area,str;box1.length = 2;box1.Width = 4;area = box1.GetArea();str = box1.GetStr();cout << "Area:"<< area<<endl;cout << "str:" << str << endl;Apple apple;apple.length = 10;cout << "Apple.Length:"<<apple.length << endl;cout << "Test 析构顺序！" << endl;return 0;
}

报错如下：
纠正措施如下：在C++中要给构造函数的参数赋默认值。

Apple::Apple(double len)

修改为下面形式：

Apple::Apple(double len=0)

2. Error LNK2019 unresolved external symbol __imp__pthread_exit referenced in function _main
Error LNK2019 unresolved external symbol __imp__pthread_create referenced in function _main

使用如下代码发生如上报错：

#include <iostream>
// 必须的头文件
#include <pthread.h>
using namespace std;
#define NUM_THREADS 5// 线程的运行函数
void* say_hello(void* args)
{cout << "Hello Runoob！" << endl;return 0;
}int main()
{// 定义线程的 id 变量，多个变量使用数组pthread_t tids[NUM_THREADS];for (int i = 0; i < NUM_THREADS; ++i){//参数依次是：创建的线程id，线程参数，调用的函数，传入的函数参数int ret = pthread_create(&tids[i], NULL, say_hello, NULL);if (ret != 0){cout << "pthread_create error: error_code=" << ret << endl;}}//等各个线程退出后，进程才结束，否则进程强制结束了，线程可能还没反应过来；pthread_exit(NULL);
}

报错的原因很简单，编译该cpp时没找到连接的库，需要手动加进来，如下：

打开：https://www.sourceware.org/pthreads-win32/
点击Mirror
点击US http
找到 pthreads-win32/
下载pthreads-w32-2-9-1-release.zip
解压该文件，将pthreads-w32-2-9-1-release\Pre-built.2\dll\x86文件夹下的pthreadVC2.dll拷到你存放.cpp的文件夹下；将C:\Users\LP\Desktop\pthreads-w32-2-9-1-release\Pre-built.2\lib\x86文件夹下的pthreadVC2.lib也拷贝到你存放.cpp的文件夹下。
在代码中加入预编译命令#pragma comment(lib, "pthreadVC2.lib")，目的是让应用程序预编译时能找到pthread_create等线程函数连接的库
完整代码如下：

#include <iostream>
// 必须的头文件
#include <pthread.h>
using namespace std;
#pragma comment(lib, "pthreadVC2.lib")
#define NUM_THREADS 5// 线程的运行函数
void* say_hello(void* args)
{cout << "Hello Runoob！" << endl;return 0;
}int main()
{// 定义线程的 id 变量，多个变量使用数组pthread_t tids[NUM_THREADS];for (int i = 0; i < NUM_THREADS; ++i){//参数依次是：创建的线程id，线程参数，调用的函数，传入的函数参数int ret = pthread_create(&tids[i], NULL, say_hello, NULL);if (ret != 0){cout << "pthread_create error: error_code=" << ret << endl;}}//等各个线程退出后，进程才结束，否则进程强制结束了，线程可能还没反应过来；pthread_exit(NULL);
}

再次运行即可成功。

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