[C++知识库]C++中创建对象的方式

C++中创建对象的方式

无参数的情况

1. 隐式创建：A a1;
2. 显式创建：A a2 = A();
3. 通过new关键字创建，返回的是指针：A *a3 = new A();

#include <iostream>

using namespace std;

class A {
    public:
    	int id;
}; 

int main() {
    A a1;
    a1.id = 1;
    A a2 = A();
    a2.id = 2;
    A *a3 = new A();
    a3->id = 3;
    
    cout << "a1.id == " << a1.id << endl;
    cout << "a2.id == " << a2.id << endl;
    cout << "a3->id == " << a3->id << endl;
    return 0;
}

有参数的情况

1. 隐式创建：A a1;
2. 显式创建：A a2 = A();
3. 通过new关键字创建，返回的是指针：A *a3 = new A();

#include <iostream>

using namespace std;

class A {
    public:
    	int id;
    	A();
    	A(int);
}; 

A::A() {
    id = -1;
}

A::A(int id) {
    this->id = id;
}

int main() {
    A a1(1);
    A a2 = A(2);
    A *a3 = new A(3);
    
    cout << "a1.id == " << a1.id << endl;
    cout << "a2.id == " << a2.id << endl;
    cout << "a3->id == " << a3->id << endl;
    return 0;
}

什么时候使用new关键字

1. 申请对象占内存特别大的时候

2. 作为函数的返回值

   #include <iostream>
   
   using namespace std;
   
   class A {
       public:
       	int id;
       	A();
       	A(int);
   }; 
   
   A::A() {
       id = -1;
   }
   
   A::A(int id) {
       this->id = id;
   }
   
   A getObject1(int id) {
       A a1(id);
       return a1;
   }
   
   A* getObject2(int id) {
       A *a2 = new A(id);
       return a2;
   }
   
   int main() {
       A a1 = getObject1(1);
       A *a2 = getObject2(2);
       cout << "a1.id == " << a1.id << endl;
       cout << "a2->id == " << a2->id << endl;
       return 0;
   }
   

3. 两个类互相引用

   #include <iostream>
   
   using namespace std;
   
   class A;
   class B;
   
   class A {
       public:
       	int id;
       	B *b;
       	A(int);
       	A(B*);
   }; 
   
   A::A(int id) {
       this->id = id;
   }
   
   A::A(B *b) {
       this->b = b;
   }
   
   class B {
       public:
       	int id;
       	A *a;
       	B(int);
       	B(A*);
   }; 
   
   B::B(int id) {
       this->id = id;
   }
   
   B::B(A *a) {
       this->a = a;
   }
   
   int main() {
       A *a = new A(1);
       B *b = new B(2);
       a->b = b;
       b->a = a;
       cout << "a->id == " << a->id <<endl;
       cout << "a->b->id == " << a->b->id <<endl;
       cout << "b->id == " << b->id <<endl;
       cout << "b->a->id == " << b->a->id <<endl;
       return 0;
   }
   

   如果不使用指针，编译无法通过

   #include <iostream>
   
   using namespace std;
   
   class A;
   class B;
   
   class A {
       public:
       	int id;
       	B b;
   };
   
   class B {
       public:
       	int id;
       	A a;
   }; 
   
   int main() {
   	return 0;
   }