## 入门

### hello.cpp

    #include <iostream>
    int main() {
        std::cout << "Hello Quick Reference\n";
        return 0;
    }

编译运行

    $ g++ hello.cpp -o hello
    $ ./hello
    Hello Quick Reference

### 变量

    int number = 5;       // 整数
    float f = 0.95;       // 浮点数
    double PI = 3.14159;  // 浮点数
    char yes = 'Y';       // 特点
    std::string s = "ME"; // 字符串（文本）
    bool isRight = true;  // 布尔值
    // 常量
    const float RATE = 0.8;

    int age {25};      // 自 C++11
    std::cout << age;  // 打印 25

### 原始数据类型

数据类型 | 大小 | 范围
--- | --- | ---
`int` | 4 bytes | -231 到 231-1
`float` | 4 bytes | N/A
`double` | 8 bytes | N/A
`char` | 1 byte | -128 到 127
`bool` | 1 byte | `true` / `false`
`void` | N/A | N/A
`wchar_t` | 2 到 4 bytes | 1 个宽字符

### 用户输入

    int num;
    std::cout << "Type a number: ";
    std::cin >> num;
    std::cout << "You entered " << num;

### 交换

    int a = 5, b = 10;
    std::swap(a, b);
    // 输出: a=10, b=5
    std::cout << "a=" << a << ", b=" << b;

### 注释

    // C++中的单行注释
    /* 这是一个多行注释
        在 C++ 中 */

### If 语句

    if (a == 10) {
        // do something
    }

查看: 条件

### 循环

    for (int i = 0; i < 10; i++) {
        std::cout << i << "\n";
    }

查看: 循环 Loops

### 函数

    #include <iostream>

    void hello();   // 声明

    int main() {    // 主函数
        hello();    // 执行函数
    }

    void hello() { // 定义
      std::cout << "Hello Quick Reference!\n";
    }

查看: 函数 Functions

### 引用

    int i = 1;
    int& ri = i; // ri 是对 i 的引用
    ri = 2; // i 现在改为 2
    std::cout << "i=" << i;
    i = 3;   // i 现在改为 3
    std::cout << "ri=" << ri;

`ri` 和 `i` 指的是相同的内存位置

### 命名空间

    #include <iostream>
    namespace ns1 {int val(){return 5;}}
    int main()
    {
        std::cout << ns1::val();
    }

    #include <iostream>
    namespace ns1 {int val(){return 5;}}
    using namespace ns1;
    using namespace std;
    int main()
    {
        cout << val();
    }

名称空间允许名称下的全局标识符

## C++ 数组

### 定义

    std::array<int, 3> marks; // 定义
    marks[0] = 92;
    marks[1] = 97;
    marks[2] = 98;
    // 定义和初始化
    std::array<int, 3> = {92, 97, 98};
    // 有空成员
    std::array<int, 3> marks = {92, 97};
    std::cout << marks[2]; // 输出: 0

### 操控

    ┌─────┬─────┬─────┬─────┬─────┬─────┐
    | 92  | 97  | 98  | 99  | 98  | 94  |
    └─────┴─────┴─────┴─────┴─────┴─────┘
       0     1     2     3     4     5

    std::array<int, 6> marks = {
      92, 97, 98, 99, 98, 94
    };
    // 打印第一个元素
    std::cout << marks[0];
    // 将第 2 个元素更改为 99
    marks[1] = 99;
    // 从用户那里获取输入
    std::cin >> marks[2];

### 展示

    char ref[5] = {'R', 'e', 'f'};
    // 基于范围的for循环
    for (const int &n : ref) {
        std::cout << std::string(1, n);
    }
    // 传统的for循环
    for (int i = 0; i < sizeof(ref); ++i) {
        std::cout << ref[i];
    }

### 多维

         j0   j1   j2   j3   j4   j5
       ┌────┬────┬────┬────┬────┬────┐
    i0 | 1  | 2  | 3  | 4  | 5  | 6  |
       ├────┼────┼────┼────┼────┼────┤
    i1 | 6  | 5  | 4  | 3  | 2  | 1  |
       └────┴────┴────┴────┴────┴────┘

    int x[2][6] = {
        {1,2,3,4,5,6}, {6,5,4,3,2,1}
    };
    for (int i = 0; i < 2; ++i) {
        for (int j = 0; j < 6; ++j) {
            std::cout << x[i][j] << " ";
        }
    }
    // 输出: 1 2 3 4 5 6 6 5 4 3 2 1

## C++ 条件

### If Clause

    if (a == 10) {
        // do something
    }

    int number = 16;
    if (number % 2 == 0)
    {
        std::cout << "even";
    }
    else
    {
        std::cout << "odd";
    }
    // 输出: even

### Else if 语句

    int score = 99;
    if (score == 100) {
        std::cout << "Superb";
    }
    else if (score >= 90) {
        std::cout << "Excellent";
    }
    else if (score >= 80) {
        std::cout << "Very Good";
    }
    else if (score >= 70) {
        std::cout << "Good";
    }
    else if (score >= 60)
        std::cout << "OK";
    else
        std::cout << "What?";

### 运算符

#### 关系运算符

:-- | --
--- | ---
`a == b` | a 等于 b
`a != b` | a 不等于 b
`a < b` | a 小于 b
`a > b` | a 大于 b
`a <= b` | a 小于或等于 b
`a >= b` | a 大于或等于 b

#### 赋值运算符

范例 | 相当于
--- | ---
`a += b` | Aka `a = a + b`
`a -= b` | Aka `a = a - b`
`a *= b` | Aka `a = a * b`
`a /= b` | Aka `a = a / b`
`a %= b` | Aka `a = a % b`

#### 逻辑运算符

Example | Meaning
--- | ---
`exp1 && exp2` | Both are true (AND)
`exp1 |
`!exp` | `exp` is false (NOT)

#### 位运算符

Operator | Description
--- | ---
`a & b` | Binary AND
`a | b`
`a ^ b` | Binary XOR
`a ~ b` | Binary One's Complement
`a << b` | Binary Shift Left
`a >> b` | Binary Shift Right

### 三元运算符

               ┌── True ──┐
    Result = Condition ? Exp1 : Exp2;
               └───── False ─────┘

    int x = 3, y = 5, max;
    max = (x > y) ? x : y;
    // 输出: 5
    std::cout << max << std::endl;

    int x = 3, y = 5, max;
    if (x > y) {
        max = x;
    } else {
        max = y;
    }
    // 输出: 5
    std::cout << max << std::endl;

### switch 语句

    int num = 2;
    switch (num) {
        case 0:
            std::cout << "Zero";
            break;
        case 1:
            std::cout << "One";
            break;
        case 2:
            std::cout << "Two";
            break;
        case 3:
            std::cout << "Three";
            break;
        default:
            std::cout << "What?";
            break;
    }

## C++ 循环

### While

    int i = 0;
    while (i < 6) {
        std::cout << i++;
    }
    // 输出: 012345

### Do-while

    int i = 1;
    do {
        std::cout << i++;
    } while (i <= 5);
    // 输出: 12345

### Continue 语句

    for (int i = 0; i < 10; i++) {
        if (i % 2 == 0) {
            continue;
        }
        std::cout << i;
    } // 输出: 13579

### 无限循环

    while (true) { // true or 1
        std::cout << "无限循环";
    }

    for (;;) {
        std::cout << "无限循环";
    }

    for(int i = 1; i > 0; i++) {
        std::cout << "infinite loop";
    }

### for_each (C++11 起)

    #include <iostream>
    int main()
    {
        auto print = [](int num) {
          std::cout << num << std::endl;
        };
        std::array<int, 4> arr = {1, 2, 3, 4};
        std::for_each(arr.begin(), arr.end(), print);
        return 0;
    }

### 基于范围 (C++11 起)

    for (int n : {1, 2, 3, 4, 5}) {
        std::cout << n << " ";
    }
    // 输出: 1 2 3 4 5

    std::string hello = "Quick Reference.ME";
    for (char c: hello)
    {
        std::cout << c << " ";
    }
    // 输出: Q u i c k R e f . M E

### 中断语句

    int password, times = 0;
    while (password != 1234) {
        if (times++ >= 3) {
            std::cout << "Locked!\n";
            break;
        }
        std::cout << "Password: ";
        std::cin >> password; // input
    }

### Several variations

    for (int i = 0, j = 2; i < 3; i++, j--){
        std::cout << "i=" << i << ",";
        std::cout << "j=" << j << ";";
    }
    // 输出: i=0,j=2;i=1,j=1;i=2,j=0;

## C++ 函数

### 参数和返回

    #include <iostream>
    int add(int a, int b) {
        return a + b;
    }
    int main() {
        std::cout << add(10, 20);
    }

`add` 是一个接受 2 个整数并返回整数的函数

### 重载

    void fun(string a, string b) {
        std::cout << a + " " + b;
    }
    void fun(string a) {
        std::cout << a;
    }
    void fun(int a) {
        std::cout << a;
    }

### 内置函数

    #include <iostream>
    #include <cmath> // 导入库

    int main() {
        // sqrt() 来自 cmath
        std::cout << sqrt(9);
    }

## C++ 预处理器

### 预处理器

- if
- elif
- else
- endif
- ifdef
- ifndef
- define
- undef
- include
- line
- error
- pragma
- defined
- __has_include
- __has_cpp_attribute
- export
- import
- module

### Includes

    #include "iostream"
    #include <iostream>

### Defines

    #define FOO
    #define FOO "hello"
    #undef FOO

### If

    #ifdef DEBUG
      console.log('hi');
    #elif defined VERBOSE
      ...
    #else
      ...
    #endif

### Error

    #if VERSION == 2.0
      #error Unsupported
      #warning Not really supported
    #endif

### 宏

    #define DEG(x) ((x) * 57.29)

### 令牌连接

    #define DST(name) name##_s name##_t
    DST(object);   #=> object_s object_t;

### 字符串化

    #define STR(name) #name
    char * a = STR(object);   #=> char * a = "object";

### 文件和行

    #define LOG(msg) console.log(__FILE__, __LINE__, msg)
    #=> console.log("file.txt", 3, "hey")

## 各种各样的

### 转义序列

转义序列 | 说明
--- | ---
`\b` | 退格键
`\f` | 换页
`\n` | 换行
`\r` | 返回
`\t` | 水平制表符
`\v` | 垂直制表符
`\\` | 反斜杠
`\'` | 单引号
`\"` | 双引号
`\?` | 问号
`\0` | 空字符

### 关键字

- alignas
- alignof
- and
- and_eq
- asm
- atomic_cancel
- atomic_commit
- atomic_noexcept
- auto
- bitand
- bitor
- bool
- break
- case
- catch
- char
- char8_t
- char16_t
- char32_t
- class
- compl
- concept
- const
- consteval
- constexpr
- constinit
- const_cast
- continue
- co_await
- co_return
- co_yield
- decltype
- default
- delete
- do
- double
- dynamic_cast
- else
- enum
- explicit
- export
- extern
- false
- float
- for
- friend
- goto
- if
- inline
- int
- long
- mutable
- namespace
- new
- noexcept
- not
- not_eq
- nullptr
- operator
- or
- or_eq
- private
- protected
- public
- reflexpr
- register
- reinterpret_cast
- requires
- return
- short
- signed
- sizeof
- static
- static_assert
- static_cast
- struct
- switch
- synchronized
- template
- this
- thread_local
- throw
- true
- try
- typedef
- typeid
- typename
- union
- unsigned
- using
- virtual
- void
- volatile
- wchar_t
- while
- xor
- xor_eq
- final
- override
- transaction_safe
- transaction_safe_dynamic

### 预处理器

- if
- elif
- else
- endif
- ifdef
- ifndef
- define
- undef
- include
- line
- error
- pragma
- defined
- __has_include
- __has_cpp_attribute
- export
- import
- module

## 另见

- C++ Infographics & Cheat Sheets (hackingcpp.com)
- C++ reference (cppreference.com)
- C++ Language Tutorials (cplusplus.com)