为什么类和 main() 函数中也有动态内存分配

Question

#include<iostream>
#include<cstring>
using namespace std;

class Employee
{
private:
    char *firstName;
    char *lastName;
    static int count;
public:
    Employee( const char * const first, const char * const last){
        firstName = new char[ strlen( first ) + 1 ];
        strcpy( firstName, first );

        lastName = new char[ strlen( last ) + 1 ];
        strcpy( lastName, last );

        count++;
        cout << "Employee constructor for " << firstName
             << ' ' << lastName << " called." << endl;
    }
    ~Employee(){
        cout << "~Employee() called for " << firstName
             << ' ' << lastName << endl;

        delete [] firstName; // release memory                                                                     
        delete [] lastName; // release memory                                                                      
        count--;
    }
    const char *getFirstName() const{
        return firstName;
    }
    const char *getLastName() const{
        return lastName;
    }
    static int getCount(){
        return count;
    }
}; // end class Employee                                                                                           

int Employee::count = 0;

main(){
    cout<<"Count = "<<Employee::getCount()<<endl;
    Employee *e1Ptr = new Employee( "Ahmad", "Ali" );
    Employee *e2Ptr = new Employee( "Saeed", "Khalid" );
    cout<<"Count = "<<e1Ptr->getCount()<<endl;
    cout << "\n\nEmployee 1: "
         << e1Ptr->getFirstName()<<" "<<e1Ptr->getLastName()
         << "\nEmployee 2: "
         << e2Ptr->getFirstName()<<" "<<e2Ptr->getLastName()<<"\n";
    delete e1Ptr;
    e1Ptr = 0;
    delete e2Ptr;
    e2Ptr = 0;
    cout<<"Count = "<<Employee::getCount()<<endl;
    system("pause");
}

Q- 为什么我们需要在 main() 和 class 中进行动态内存分配？在类定义或 main() 函数中分配动态内存是否不够？

Q- 为什么 e1Ptr = 0;而 e2Ptr = 0;正在使用中。

我是新手。所以，请详细说明一下。谢谢

Answer 1

这很简单。 new Employee( "Ahmad", "Ali" ) 为 Employee 类的实例分配内存并初始化，new char[ strlen( first ) + 1 ]; 在类内部分配一个字符串。

然而，那是糟糕的 C++ 代码。您应该很少直接管理内存，而是使用类来帮助您。这些被称为智能指针，将根据需要自动分配、初始化、销毁和取消分配。手动管理原始指针容易出错且不必要。请记住：您编写的代码越少，可能出现错误的地方就越少。

因此，而不是

Employee *e1Ptr = new Employee( "Ahmad", "Ali" );

你会使用

std::unique_ptr<Employee> e1Ptr = std::make_unique<Employee>("Ahmad", "Ali");

或

auto e1Ptr = std::make_unique<Employee>("Ahmad", "Ali");

并删除delete。

此外，您可以使用std::string，而不是分配字符串。所以不是

char* firstName = new char[ strlen( first ) + 1 ];
strcpy( firstName, first );

你会使用

std::string firstName = first;

（也删除delete[]）