Constructor, copy function and destructor

The default function

because c++ Mainly for object-oriented , So it has three basic characteristics ： encapsulation , Inherit , polymorphic , and c++ The six default functions in the class of are mainly used to encapsulate the process , namely ： Constructors , Copy function , Destructor , Overloading of assignment operands , Overload of address operator ,const The modified address fetching operator overloads
Here we mainly talk about constructors , Copy function , The difference between destructors

Constructors

The function name of the constructor must have the same name as the class name , And the constructor has no return value , The main function of constructor is to initialize function

class A {
public:
	A(int a,int b,char c) {
		_a = a;
		_b = b;
		_c = c;
	}

	int _a;
	int _b;
	char _c;
};

int main() {
	A i(1,2,'s');
	cout << i._a << i._b << i._c << endl;
	return 0;
}

Let's look at the code above , You can see that we have defined a class type A, The object is created in the main function i, And put 1,2,s The data is transmitted to _a,_b,_c Member variables , And in the class A Functions are constructors . We should note that , Class _a,_b,_c Variables are just Declare objects , There is no room to create objects , They are in Constructors A Initializing objects in
So if we don't create constructors , Can it be successful ？
The answer is yes , Because the compiler will have its own default constructor , The default constructor is the compiler's automatic initialization function

class A {
public:
	int _a=1;
	int _b=2;
	char _c='s';
	int main() {
	A i;
	cout << i._a << i._b << i._c << endl;
	return 0;
};

Here is to initialize the default constructor directly , The output of the above code is also 12s, But we can use a more convenient default constructor , This is also the default constructor

class A {
public:
	A(int a = 1, int b = 2, char c = 's') {// Default constructor 
		_a = a;
		_b = b;
		_c = c;
	}

	int _a;
	int _b;
	char _c;
};

int main() {
	A i1(3,4,'t');
	A i2;
	cout << i1._a << i1._b << i1._c << endl;
	cout << i2._a << i2._b << i2._c << endl;
	return 0;
}

The code output above is 34t and 12s, When transferring parameters, use the parameter value , If there is no parameter transfer, the default initialization

It is worth noting that , When we don't write a constructor , The compiler default constructor will have an eccentric treatment ：
1、 Built in types are not initialized 2、 The custom type will call its constructor to initialize , If the custom type has no constructor , It will not be initialized

Destructor

Destructor has no return value , No parameters , Destruct name adds a before the class name ~, If the class is undefined , The compiler will automatically generate the default destructor , At the end of the object life cycle , The destructor is automatically called
The main function of the destructor is to release such as malloc Objects that open up memory space on the heap , Clean up

class Stack {
    
public:
	Stack(int capacity = 4) {
    
		_a = (int*)malloc(sizeof(int) * capacity);
		if (_a == nullptr) {
    
			cout << "malloc fail" << endl;
			exit(-1);
		}
		_top = _capacity = 0;
	}
	~Stack() {
    
		free(_a);
		_a = nullptr;
		_top = _capacity = 0;
	}

	int* _a;
	int _top;
	int _capacity;
};
int main() {
    
	Stack i;
	return 0;
}

Copy function

no return value , Take the class name as the function name

class A {
    
public:
	A(int a = 1, int b = 2, char c = 's') {
    
		_a = a;
		_b = b;
		_c = c;
	}

	A(const A& d) {
    // Copy function 
		_a = d._a;
		_b = d._b;
		_c = d._c;
	}

	int _a;
	int _b;
	char _c;
};

int main() {
    
	A i;
	A i2(i);//A i2=i; Also call the copy function 
	cout << i2._a << i2._b << i2._c << endl;
	return 0;
}

After debugging , We can find out A i2(i); The code here calls the copy function
It is worth noting that , Here, the parameters of the copy function pass references , signify d It's a class A Of i References to , If there is no quotation , That will cause me to copy the definition in the function d Also copy functions , This will cause a problem of infinite recursion in passing values , So we need to add one &, To quote
If you pass the reference , Not as an output parameter , It is best to const & For protection （ That is, the internal member variables will not change ）
notes ： Pass values and parameters to classes in functions , And the return value is to call the copy construction
For the copy function automatically generated by the compiler , Do not distinguish between built-in types and custom types , Metropolitan processing
1、 Built in type , Shallow copy of byte order
2、 Custom type , Will call his copy construct
But because it is a shallow copy , For those who open up memory units by themselves （ As above Stack） It is difficult to realize the default copy structure , Will generate a crash

int main(){
    
	Stack st1;
	Stack st2(st1);
	return 0;
}

The above code will crash , because st1 and st2 Generate space in the heap , First create st1, To create a st2, When destructor , First pair st2 Of _a To release space , turn st1 When , because _a Space has been released , Then there is no harm in the second release , So it will cause the program to crash

summary

function	function
Constructors	Initialize object
Copy function	Copy objects
Destructor	Release the space opened up on the heap

The above is the constructor , The difference between copy function and destructor