Classes and objects - initialization and cleanup of objects - constructor call rules

By default ,c++ The compiler adds... To at least one class 3 A function

1. Default constructor ( No arguments , Function body is empty )
2. Default destructor ( No arguments , Function body is empty )
3. Default copy constructor , Copy the value of the property

The rules for calling constructors are as follows ：

1. If the user defines a parameter constructor ,c++ No default parameterless construction is provided , But the default copy structure will be provided
2. If you define a copy constructor ,c++ No more constructors

#include<iostream>
using namespace std;

// Call rules for constructors 
//1、 Create a class ,C++ The compiler adds at least 3 A function 
// Default structure （ Empty implementation ）
// Destructor （ Empty implementation ）
// Copy structure （ Value copy ）

//2、
// If we write a parametric constructor , The compiler no longer provides a default construct , Copy structure will still be provided 
// If we write a copy constructor , The compiler will no longer provide other ordinary constructors 

class Person { 
    
public:

	/*Person() { cout << "Person The default constructor for calls " << endl; }*/

	/*Person(int age) { cout << "Person The parameterized constructor of " << endl; m_Age = age; }*/

	Person(const Person& p) { 
    
		cout << "Person Copy constructor call for " << endl;
		m_Age = p.m_Age;
	}

	~Person() { 
    
		cout << "Person Destructor call for " << endl;
	}


	int m_Age;
};

//void test01() { 
    
// Person p1;
// p1.m_Age = 18;
//
// Person p2(p1);
//
// cout << "p2 The age is ：" << p2.m_Age << endl;
//}

void test02() { 
    
	//Person p3(28);
	
	//Person p4(p3);

	//cout << "p4 The age is ：" << p4.m_Age << endl;
}

int main() { 
    

	//test01();
	//test02();

	system("pause");

	return 0;
}

By default ,c++ The compiler adds... To at least one class 3 A function

1. Default constructor ( No arguments , Function body is empty )
2. Default destructor ( No arguments , Function body is empty )
3. Default copy constructor , Copy the value of the property

class Person { 
    
public:

	Person() { 
    
		cout << "Person The default constructor for calls " << endl;
	}

	Person(int age) { 
    
		cout << "Person The parameterized constructor of " << endl;
		m_Age = age;
	}

	Person(const Person& p) { 
    
		cout << "Person Copy constructor call for " << endl;
		m_Age = p.m_Age;
	}

	~Person() { 
    
		cout << "Person Destructor call for " << endl;
	}


	int m_Age;
};

void test01() { 
    
	Person p1;
	p1.m_Age = 18;

	Person p2(p1);

	cout << "p2 The age is ：" << p2.m_Age << endl;
}

int main(){ 
    
	test01();

	return 0;
}

Running results

Comment out the copy constructor

class Person { 
    
public:

	Person() { 
    
		cout << "Person The default constructor for calls " << endl;
	}

	Person(int age) { 
    
		cout << "Person The parameterized constructor of " << endl;
		m_Age = age;
	}

	/*Person(const Person& p) { cout << "Person Copy constructor call for " << endl; m_Age = p.m_Age; }*/

	~Person() { 
    
		cout << "Person Destructor call for " << endl;
	}


	int m_Age;
};

void test01() { 
    
	Person p1;
	p1.m_Age = 18;

	Person p2(p1);

	cout << "p2 The age is ：" << p2.m_Age << endl;
}

int main(){ 
    
	test01();

	return 0;
}

Running results

p2 Still show age , No Person Copy constructor for , There was no mistake , Therefore, the compiler provides a default copy constructor , namely m_Age = age;

The rules for calling constructors are as follows ：

1. If the user defines a parameter constructor ,c++ No default parameterless construction is provided , But the default copy structure will be provided
2. If you define a copy constructor ,c++ No more constructors

1. If the user defines a parameter constructor ,c++ No default parameterless construction is provided , But the default copy structure will be provided

// Provide a parameter constructor , But no default constructor is provided 
class Person { 
    
public:

	/*Person() { cout << "Person The default constructor for calls " << endl; }*/

	Person(int age) { 
    
		cout << "Person The parameterized constructor of " << endl;
		m_Age = age;
	}

	/*Person(const Person& p) { cout << "Person Copy constructor call for " << endl; m_Age = p.m_Age; }*/

	~Person() { 
    
		cout << "Person Destructor call for " << endl;
	}


	int m_Age;
};

void test02(){ 
    
	Person p3;
}

int main(){ 
    
	test02();

	return 0;
}

Error will be reported at this time

Because we provide a parameterized constructor , All compilers no longer provide default constructors , So there will be an error

// Provide a parameter constructor , But copy constructors are not provided 
class Person { 
    
public:

	/*Person() { cout << "Person The default constructor for calls " << endl; }*/

	Person(int age) { 
    
		cout << "Person The parameterized constructor of " << endl;
		m_Age = age;
	}

	/*Person(const Person& p) { cout << "Person Copy constructor call for " << endl; m_Age = p.m_Age; }*/

	~Person() { 
    
		cout << "Person Destructor call for " << endl;
	}


	int m_Age;
};

void test02(){ 
    
	Person p3(28);
	
	Person p4(p3);

	cout<< "p4 The age is ：" << p4.m_Age << endl;
}

int main(){ 
    
	test02();

	return 0;
}

Running results

Provide its own constructor with parameters , Even if we don't provide copy constructors , The compiler also provides a default copy constructor

2. If you define a copy constructor ,c++ No more constructors

// Provide copy constructor , But ordinary constructors are not provided 
class Person { 
    
public:

	/*Person() { cout << "Person The default constructor for calls " << endl; }*/

	/*Person(int age) { cout << "Person The parameterized constructor of " << endl; m_Age = age; }*/

	Person(const Person& p) { 
    
		cout << "Person Copy constructor call for " << endl;
		m_Age = p.m_Age;
	}

	~Person() { 
    
		cout << "Person Destructor call for " << endl;
	}


	int m_Age;
};

void test02(){ 
    
	Person p3;
}

int main(){ 
    
	test02();

	return 0;
}

Error will be reported at this time

Because we provide a copy constructor , But no default constructor is provided , The compiler also does not provide a default constructor , So it will report a mistake .

summary ： I wrote the parametric structure , The compiler does not provide a default construct , Will provide copy construction ; Wrote a copy structure , Default and parametric constructs are not provided .