[C++ speed pass notes] one (basic stage)

• common code

#include <iostream>
using namespace std;
int main() {
    cout << "Hello, World!" << endl;
    //字符串与变量一起输出时用<<分隔
    cout <<"一周总共有:"<< Day << "天"<< endl;
    //输入
    int a =0;
    cin >> a;

    return 0;
}

• constant

There are two ways to define constants in C++
1. #define macro constants: #define constant name constant star value.

Usually defined above the file to represent a constant. Pay attention to the format, there is no equal sign.
2. Variables modified by const: const data type constant name = constant value
Usually, the keyword const is added before the variable star definition to modify the variable as a constant and cannot be modified.

• character

1. When displaying character variables, enclose the characters in single quotes instead of double quotes

2. There can only be one character in single quotes, not a string
3. Character variables in C and C++ only occupy 1 byte.
4. The character variable does not store the character itself in the memory, but puts the corresponding ASCII code into the storage unit

• string

Two styles
1. C style string: char variable name []="string value" (note the square brackets)

2. C++ style string: string variable name = "string value" (note the introduction of the header file string)

• Operation

1. The result of dividing two integers is still an integer, and the decimal part is removed
2. Modulo operation cannot be performed on two decimal numbers
3. In C++, the ternary operator returns a variable , which can continue to be assigned

• Procedure flow chart

1. Disadvantages of switch: when judging, it can only be an integer or character type, not a range; advantages of switch: clear structure, high execution efficiency; if there is no break in the case, the program will continue to execute downward.

• pointer

Occupied space size:

Under the 32-bit operating system, the pointer occupies 4 bytes of space, no matter what data type

Under the 64-bit operating system, the pointer occupies 8 bytes of space

null pointer:

A null pointer is used for pointer initialization and is not accessible

const modification:

1. const modifier constant, pointer constant. int * const p

The pointer cannot be changed, but the value pointed to can be changed.

2. const modified pointer, constant pointer. const int *p

The pointer can be changed, but the pointed value cannot be changed. 

3. Modified at the same time, neither the pointer nor the value can be changed.

• memory partition model

Code area: store the binary code of the function body, managed by the operating system
Global area: store global variables, static variables and constants
Stack area: automatically allocated and released by the compiler, store function parameter values, local variables, etc.
Heap area: by the program If the programmer does not release, it will be recovered by the operating system at the end of the program