10. C ++-constructor initialization list, object construction sequence, destructor, 10. c Constructor

10. C ++-constructor initialization list, object construction sequence, destructor, 10. c Constructor

First, recall,Previously learned const

Separate useConst ModificationVariables are defined.ConstantFor example, const int I = 1;

UseVolatile const ModificationVariable is definedRead-Only variables

UseConst & ModifierVariable is definedRead-Only variables

Can a const be defined in a class?Member?

Directly write the code:

#include <stdio.h>class Test{private:  const int ci;public://       Test()//       {//           ci=10;//       }  int getCI()  {       return ci;  }};int main(){       Test t;       printf("%d\n",t.getCI());       return 0;}

Compilation error:

test.cpp: In function ‘int main()’:test.cpp:21: error: structure ‘t’ with uninitialized const members

According to the compilation information, because the const Member of the struct t is not initialized, an error occurred while executing printf.

Next, unmask the preceding example and use the const learned in the previous chapter to initialize the const.

Compilation or error:

test.cpp: In constructor ‘Test::Test()’:test.cpp:8: error: uninitialized member ‘Test::ci’ with ‘const’ type ‘const int’test.cpp:10: error: assignment of read-only data-member ‘Test::ci’

From the compilation information, the const variable cannot be directly initialized in the Test: Test () constructor.

 

So,In C ++Medium,The constructor initialization list (In addition to initializing member variables, you can also initialize a const member.)

Initialization listLocated in the constructor nameRight sideToStart with ColonAnd then the variable to be initialized,Separated by commasFor example:

Class Example {private: int I; float j; const int ci; int * p; public: Test (): j (1.5), I (2), ci (10) // initialize I = 2, j = 1.5, ci = 10 {p = new int; * p = 3 ;}};

Note:

-List member'sInitialization orderOnly for MembersDeclaration OrderSame, irrelevant to the location of the initialization list

For example, in the previous example, the initialization sequence of the initialization list is: I = 2, j = 1.5, ci = 10.

-When the constructor is called for initializationInitialize the list firstAnd then execute the content in the constructor.

 

Is the const member in the class A constant or a read-only variable?

See the following example:

#include <stdio.h>class Test{private:  const int ci;public:      Test():ci(10)      { }  int getCI()  {       return ci;  }  void setCI(int val)  {     int *p=const_cast<int *>(&ci);     *p=val;  }};int main(){    Test t;    t.setCI(5);    printf("%d\n",t.getCI());    return 0;}

Compile and run:

5   

So the const member in the class definesRead-Only variables

 

 

Object Construction Order

Classes in C ++ can define multiple objects. What is the order of Object Construction?

For local objects (stacks)

-When the program executes the Definition Statement of the object, it constructs

For objects created through new (HEAP)

-Like a local object, the new statement is constructed when the program runs.

For global objects (static storage area)

-The object construction sequence is uncertain, so we needAvoidMutual dependency between multiple global objects.

 

Object destruction-Destructor

We have learned how to initialize an object by using constructors.

Similarly, some cleanup work should be performed before the object is destroyed. Therefore, a special cleanup function is introduced in C ++-Destructor

• Functions and constructor of destructorOppositeAutomatically called when the object is destroyed
• DestructorNo Parameters, AlsoNo return value type declaration

Definition :~ Class_name (), for example:

Class Test {public: Test () {}// constructor ~ Test () {}// destructor };

Note:

• In the class, when the Destructor is defined, the compiler will not provide the default constructor, so you need to define a constructor yourself.
• The object variable created using new must be deleted when not in use before the Destructor can be called.

See the following example:

#include <stdio.h>class Test{  int val;public:  Test(int i)  {    val=i;    printf("Test() val=%d\n",val);   }  ~Test()  {      printf("~Test() val=%d\n",val);  }};int main(){    Test t1(1);    Test* t2 = new Test(2);//  delete t2;    return 0;}

Compile and run:

Test(1)Test(2)~Test(1)

From the print result, we can see that the destructor of t2 is not printed, so it only prints :~ Test (1)

Run again after unblocking:

Test(1)Test(2)~Test(2)~Test(1)

 

Summary:

When there are Members in the class that need memory application, file opening, and database connection, you needDefine destructorTo Recycle resources.

(Similar to the copy constructor)