STL Tutorial 9 deep copy and shallow copy of container elements

A case of shallow copy problem

Define a class
There is a pointer member below , This is prone to shallow copy problems

class Person {
    
public:
	char* name;
	int age;
public:
	Person(const char * name,int age) {
    
		this->name = new char[strlen(name) + 1];
		strcpy(this->name, name);
		this->age = age;
	}
	~Person() {
    
		if (this->name != NULL) {
    
			delete[] this->name;
		
		}
	}

};

Then define a class object , Put it in the container

int main() {
    
	const char* a = "name";
	Person p(a, 20);
	vector<Person>v;
	v.push_back(p);

}

Then report the error directly

The reason lies in the external objects and vector The object inside name The pointers all point to the same memory , At the end of the program , The outer object starts to destruct , After the completion of deconstruction vector The elements inside should also be destructed , In this way, the same memory is destructed twice , There will be errors

resolvent

The modified code

class Person {
    
public:
	char* name;
	int age;
public:
	Person(const char * name,int age) {
    
		this->name = new char[strlen(name) + 1];
		strcpy(this->name, name);
		this->age = age;
	}
	Person(const Person& p) {
    
		this->name = new char[strlen(p.name) + 1];
		strcpy(this->name, p.name);
		this->age = p.age;
	}
	Person& operator=(const Person& p) {
    
		if (this->name != NULL) {
    
			delete[] this->name;
		}
		this->name = new char[strlen(p.name) + 1];
		strcpy(this->name, p.name);
		this->age = p.age;
		return *this;
	}
	~Person() {
    
		if (this->name != NULL) {
    
			delete[] this->name;
		}
	}

};

Rerun without crash

STL Container commonality mechanism （ important ）

STL All containers provide value semantics , Rather than quoting semantics . When the container performs the insert element operation , Copy work is implemented internally , Put a copy of the elements we want to insert into the container for summary , Instead of putting the original data elements directly into the container , Therefore, the elements we provide must be able to be copied （ If there is a pointer, you need to construct it yourself ）.

Last

Use here strcpy There will be problems , Report errors

 severity 	 Code 	 explain 	 project 	 file 	 That's ok 	 Disable status 
 error 	C4996	'strcpy': This function or variable may be unsafe. Consider using strcpy_s instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS. See online help for details.	map	D:\store\server\leetcode\map\ Source .cpp	13	

Just add

#define _CRT_SECURE_NO_WARNINGS