Dynamic memory management

1.1 Various variables 、 Storage location of constants

When we code, we always define it in various places Define various constants 、 Variable , So do we know where they exist ？

Take a look at this code

#include<iostream>
using std::cout;
using std::endl;
using std::exception;
// Define global variables   Including global static variables 
int aa;
const int ad=0;
static int bb;
int main1()
{
    
    // Definition of various local variables 
	int a = 0;
	const int b = 0;
	static int c = 0;
	char arr[] = "abd";
	const char* p = "gneij";
	char* p1 = (char*)malloc(sizeof(char) * 10);
	// Print their address 
	cout << " The addresses of each variable are as follows :" << endl;
	cout << "aa:" << &aa << endl;
	cout << "ad:" << &ad << endl;
	cout << "bb:" << &bb << endl;
	cout << "c:" << &c << endl;
	cout << "p:" << (void*)p << endl;
	cout << endl;
	cout << "a:" << &a << endl;
	cout << "b:" << &b << endl;
	cout << "arr:" << (void*)arr << endl;
	
	cout << endl;
	cout << "p1:" << (void*)p1<<endl;

	return 0;
}

The operation results are as follows ：

** It can be observed through the operation results aa bb c Your address is similar It shows that they exist in the same area **

ad p Your address is similar The instructions are placed in the same area

a b arr Their addresses are similar They also exist in the same area

p1 There is an area alone

• among aa bb c in aa Global variable bb Is a global static variable c Is a local static variable Global ordinary or static variables are stored in Static zone （ Data segment ）

• ad Is a global constant p The printed address is actually a constant string “gneij" The address of Visible global constants and constant characters / String is stored in The constant area ( Code segment )

• a b It's a local variable arr Pointer variable refers to the data of the array on the stack All exist On the stack

• p1 Although it's a pointer It's pointing malloc Space created on the heap so p1 Point to space Pile it up

1.2c/c++ Distribution of memory

From the above, we know that the language memory includes The constant area Static zone The stack area Heap area etc.

2.1c++ Dynamic memory management

2.1.1c++ of use new and delete Operator for dynamic memory management

c++ Class... Is introduced Classes can contain various types Including built-in types and custom types It's still used at this time c Dynamic memory management is very inconvenient to manage For example, a class has multiple pointer variables pointing to multiple spaces stay c You have to use them one by one free Release however c++ There are destructors It can all be directly in the destructor free Each space Then call the destructor .

• therefore c++ Has its own dynamic memory management Namely new and delete The operator

​ new Used to create objects （ Including opening up space ） delete Used to clean up object resources and free up space

• among new A single space is direct type p=new type( Initialize parameters )*

• If it is new A continuous space is type p=new type[n];*

class Stack
{
    
public:
	Stack(int size = 4)
	{
    
		cout << "Stack(int size=4)" << endl;
		_top = 0;
		_capacity = size;
		_a = new int[size];
	}
	~Stack()
	{
    
		cout << "~Stack()" << endl;
		delete[] _a;
		_top = _capacity = 0;
	}
	void print()
	{
    
		cout << "void print()" << endl;
	}

private:
	int* _a;
	int _top;
	int _capacity;


};

int main()
{
    
    Stack* st=new Stack(10);
    Stack* p=new Stack[10];
    delete st;
    delste[] p;
    
    return 0;
}

The above code is executed as follows ：

Be careful ： In the above code Stack Class constructors and destructors have output flags The delegate is called

We new One Stack When the object It will output Stack The flag that the constructor of is called

delete When an object It will output Stack The flag that the destructor of is called

We can draw Conclusion ：new Creating objects is not just about opening up space The constructor of the class will also be called to initialize the object delete The destructor of the class will be called to clean up resources Release space after

In depth analysis of ：

Actually new It's a function operator new Functions and location new Combined

• among operator new It's encapsulation again malloc Got It includes more than malloc The function of opening up space It will also throw exceptions after the failure of opening up space Not with malloc Returns a null pointer

• location new Is responsible for explicitly calling the constructor Initialize the created object

Be careful ：operator new （） It's a function Not an operator overload ！！

delete At the bottom through operator delete overall situation
Function to free up space But the call operator delete The destructor will be called to clean up resources before

• And operator new（） Corresponding also operator delete（） It's just a function It's not overloading ！

location new The format of

// location new The format of 
//new( The address of the space )type（ Parameter initialization ）
// for example :
Stack* st= operator new(sizeof(Stack));// adopt operator new  Function opens up space  
new(st1)Stack(4);// location new  Initialize object  4 Is the parameter of the constructor 

location new Usage scenarios of

location new In practice, expressions are generally used with memory pools . Because the memory allocated by the memory pool is not initialized , So if it's custom
Object of type , Need to use new The definition expression is displayed and the constructor is initialized

• Extension ： Successive new delete The efficiency is not high because you have to apply to the operating system Pooling technology can be used to solve this problem Is to use memory pool technology Make every time not to ask for space from the operating system Instead, open up space in your own pool This will greatly improve efficiency It's like using your own things is very convenient It's not very convenient to borrow other people's things operator new and operator delete Can overload Then you can write one by yourself Use memory pool to realize ！！！！

2.1.2c of use malloc、calloc、realloc、free Realize the management of dynamic memory

void* malloc (size_t size);
void* calloc (size_t num, size_t size);
void* realloc (void* ptr, size_t size);

• malloc It is to apply to the operating system to open up a given space in bytes on the heap If the application is successful, the address pointing to the space will be returned Otherwise return null pointer

• calloc Also open up space Their ratio malloc One more parameter num Is the number of types of development size Is the size of the type If open 10 individual int The space of type is

int* p=(int*)calloc(10,sizeof(int));

• It is worth noting that calloc comparison malloc It's not just more than one parameter and calloc It will also initialize the opened space as 0 This is a malloc What you don't have ！

• realloc Is to continue to open up space behind a given space The first parameter is the address of the opened space The second parameter is the amount of space to be opened up

free： When we no longer use the open space, we can use free( Space address ) Of Way to free up space , It is safer to set the pointer to null

2.1.3 The difference between dynamic memory management and dynamic memory management

new delete and malloc free The difference between ：

1.new and delete It's the operator malloc and free Is the function
2.new Will initialize the opened space however malloc Can't
3.malloc When opening up space, you need to use sizeof The size of the type trouble new There is no need to Just follow the type directly If there are multiple objects, add one [] that will do
4.malloc The return type of is void After opening up, you need to force type conversion however new Unwanted
5.malloc If you fail to open a space, the null pointer is returned. You need to manually judge whether it is a null pointer and then know whether the space is successfully opened and new You don't have to If new Open up space
Failure will throw exceptions We just need to catch exceptions
6.new It will automatically call the constructor of the development object for initialization delete The destructor of the object will be called to clean up resources before space is released Free up space malloc free Will not be

3.1 What is memory leak

Memory leak is memory lost or pointer lost ？

• You should know this question The memory leak is the loss of the pointer Instead of losing memory Memory is not lost

• If we don't release after opening up space Make the address of this block space lost You cannot free space through this address This space will be occupied all the time without returning it to the operating system The memory of the operating system is getting smaller and smaller Long running programs process If there is a memory leak problem As time goes by Memory will be less and less Finally, the operating system may crash ！！！