C++ Basics

C ++ program memory division

Keypoints

• Memory division classfication

Memory division classfication

• Significance: to bring different life cycle for different division, for dexterous programming.

Text/Code division

• Store the binary code of the correspondence, managed by the operating system. Assigned before program execution.
• Shared: for frequently run code, it’s sufficient to store only once.
• Only readable: not changable.

Global division

• Store global variables and static variables and constants. Assigned before program execution.
• Global variable: out of main function.
• Static variable: its address is near global ones.

    static data_type variable_name = 10;
  

• Constant: string variable; const global variable. (constant local variable isn’t in Global division).

Stack division

• Automatically by compiler, to store function parameter and local variable.
• Don’t return address of local variable. Because stack data will be released after program execution.

    int * func(){
        int a = 10; 
        return &a;
    }
    int main(){
        int *p = func(); 
        cout << *p << endl; // return 10
        cout << *p << endl; // return mess
        cout << *p << endl; // return mess; because compiler store stack data only once, then it'll be released
    }
  

Heap division

• Assigned and released by programmer. When program is over, OS will release it.
• Use new to start new heap area.

    int * func(){
        int *p = new int(10); // pointer here is a local variable and in stack area; 
                              //new int (10) is not a value but an address
        return p;  // the data this pointer refers to is in heap area
    }
    int main(){
        int *p = func(); // right, because 10 is in heap area
        cout << *p << endl;
        cout << *p << endl;
        cout << *p << endl; // because programmer decide the release or not of heap data. It'll not be released automatically by compiler.
    }
  

• Use new to start array in heap area. Use delete to release heap area.

    int * func(){
        int *p = new int(10); //new int (10) is not a value but an address/pointer
        return p;
    }
    void test01(){
        int *p = func();
        cout << *p << endl;
        cout << *p << endl;
        cout << *p << endl;
        delete p; // release heap area, if try to reach p again, it'll report error
    }
    void test02(){
        int *arr = new int[10]; // [10] is array length
        for (int i = 0; i < 10; i++){
            arr[i] = i + 100;
        }
        for (int i = 0; i < 10; i++){
            cout << arr[i] << endl;
        }
        delete[] arr; // release heap area
    }
    int main(){
        test01();
        test02();
    }
  

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