Summary of C++ review key points (1)

C++'s enhancement of C language - type enhancement

1. register keyword enhancement

register keyword requests the compiler to place variables directly in registers, which is fast.

In C language, variables modified by register cannot take addresses; in C++, they can take addresses.

2. Variable detection enhancement

In C language, it is legal to repeatedly define multiple global variables with the same name. In C++, it is not allowed to define multiple global variables with the same name.

3. All variables and functions in C++ must have types

In C language

int f(); means the return value is int and accepts any parameters Function

int f(void); represents a parameterless function whose return value is int

In C++

int f(); has the same characteristics as int f(void) means a parameterless function whose return value is int

4. Enhanced function of ternary operator

Example: a< b ? a : b )= 30;

Difference: C language returns the value of the variable; C++ language returns the variable itself

The ternary operator in C language returns the variable value and cannot be used as an lvalue

In C++ The ternary operator can directly return the variable itself, so it can appear anywhere in the program

Note: If one of the values ​​that the ternary operator may return is a constant value, it cannot be used as an lvalue ( a < b ? 1 :b )= 30;

How does the C language support features similar to C++?

====>The condition for lvalue: there must be memory space; the C++ compiler just helps the programmer get an address

1.const qualifier understanding

Function: Convert an object into a constant, read-only.

int main()
{
const int a;// 1 第1个第2个意思一样 代表一个常整形数
int const b;// 2
const int *c; // 3 第3个 c是一个指向常整形数的指针(所指向的内存数据不能被修改，但是本身可以修改)
int * const d; //4 第4个 d 常指针（指针变量不能被修改，但是它所指向内存空间可以被修改）
const int * const e ; //5 第5个 e一个指向常整形的常指针（指针和它所指向的内存空间，均不能被修改）
return 0;
}

Example:

const int bufSize=512；bufSize初始化之后就无法进行修改了。
1）在C++中，const符号表原理：存储方式是符号表即key value
key value
bufSize 512

2. How the C++ compiler handles const constants: When a constant is declared, it is placed in the symbol table; when a constant is used Replace directly with the value in the symbol table.

Conclusion: The difference between const variables in C language and C++

const variables in C language are read-only variables and have their own storage space

const constants in C++

Storage space may or may not be allocated

When the const constant is global and needs to be used in other files

When the & operator is used to obtain the const constant Address

2) The similarities and differences between const and #define

are the same: both can be used as constants

Difference: const constants are processed by the compiler, providing type detection and Scope checking, macro definitions are processed by the preprocessor, simple text replacement.

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