c++ 学习笔记之语言基础

程序结构

注释

//line comment

/* block comment can occupy
multiple lines */

头文件

#include<iostream>

iostream

#inlcude<string>

#include<array>

命名空间

using namespace std;

cout<<"hello world!";

std::cout<<"hello world!";

::

main 函数

//my first program in c++
#include<iostream>
using namespace std;

int main() {
cout<<"hello world";
return 0;
}

变量和类型

标示符

alignas, alignof, and, and_eq, asm, auto, bitand, bitor,
bool, break, case, catch, char, char16_t, char32_t, class,
compl, const, constexpr, const_cast, continue, 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, register, reinterpret_cast, return,
short, signed, sizeof, static, static_assert, static_cast,
struct, switch, template, this, thread_local, throw, true,
try, typedef, typeid, typename, union, unsigned, using,
virtual, void, volatile, wchar_t, while, xor, xor_eq

基本数据类型

char

变量声明

int a;
int b;
int c;

int a, b, c;

变量初始化

//c-like initialization
int x = 0;

//constructor initialization
int x(0);

//uniform initialization
int x{0};

类型推导 auto 和 decltype

int foo = 0;
auto bar = foo; //the same as: int bar = foo;

int foo = 0;
decltype(foo) bar; //the same as: int bar;

string 类型

<string>

#include<iostream>
#include<string>
using namespace std;

int main(){
string mystring;
mystring = "hello world";
cout<<mystring;
}

string mystring = "hello";
string mystring("hello");
string mystring{"hello"};

常量

字面常量

整型常量：

75      //decimal
0113    //octal
0x4b    //hexadecimal

int

75      //int
75u     //unsigned int
75l     //long
75ul    //unsigned long
75lu    //unsigned long

浮点型常量:

3.14159    // 3.14159
6.02e23    // 6.02 x 10^23
1.6e-19    // 1.6 x 10^-19
3.0        // 3.0

double

3.14159L   // long double
6.02e23f   // float

字符和字符串型常量:

'm';
"hello";

\\

"Left \t Right"
"one \n two \n three"

"this forms" " a single"    " string "
"of characters"

//以上等于

"this forms a single string of characters"

x = "string expressed in \
two lines"

//以上等于

x = "string expressed in two lines"

R"sequence(

)sequence"

sequence

R"(string with \backslash)"
R"&%$(string with \backslash)&%$"

"string with \\backslash"

其他类型常量:

bool foo = true;
bool bar = false;
int* p = nullptr;

类型常量表达式

#include<iostream>
using namespace std;

//给常量值命名
const double pi = 3.14159;
const char newline = '\n';

int main(){
double r = 5.0;
double circle;
circle = 2 * pi * r;
cout<<circle;
cout<<newline;
}

预处理器定义(#define)

#define identifier replacement

identifier

replacement

replacement

#include<iostream>
using namespace std;

#define PI 3.14159
#define NEWLINE '\n'

int main(){
double r = 5.0;
double circle;
circle = 2 * PI * r;
cout<<circle;
cout<<NEWLINE;
}

操作符

赋值操作符(=)

y = 2 + (x = 5);

//以上等于

x = 5;
y = 2 + x;

x = y = z = 5;

数学操作符(+,-,*,/,%)

x = 11 % 3; //2

混合赋值操作符(+=,-=,*=,/=,%=,>>=,<<=,&=,^=,|=)

y += x; // 等于 y = y + x;
x -= 5; // 等于 x = x - 5;
x /= y; // 等于 x = x / y;
price *= units + 1; // 等于 price = price * (units + 1);

递增和递减操作符(++,–)

x = 3;
y = ++x; // x = 4, y = 4

x = 3;
y = x++; // x = 4, y = 3

比较操作符(==,!=,>,<,>=,<=)

a = 2; b = 3; c = 6;
a == 5; //false
a * b >= c; //true
b + 4 > a * c; //false
(b = 2) == a; //true

逻辑操作符(!,&&,||)

条件三元操作符(?)

result = codition ? result1 : result2;

//1. condition 为 true, result 为 result1.
//2. condition 为 false, result 为 result2.

顿号操作符(,)

a = (b = 3, b + 2);

//结果: a = 5, b = 3.

位操作符(&,|,^,~,<<,>>)

4 << 1; //8
4 >> 1; //2

显示类型转换操作

int i;
float f = 3.14;

i = (int) f; //c-like style
i = int (f); //function sytle

sizeof

x = sizeof(char); //x 的值是1,因为 char 是一个字节大小

操作符优先级

基本输入/输出

标准输出 cout

cout << "hello" << " world\n";

string str = "hello";
cout << str << " world" << endl;

//endl 除了起到换行的作用外，还会将缓冲区的流刷到输出设备上，
//频繁的请求输出设备可能会引起延迟或过载等风险。

标准输入 cin

cin >> a >> b;

//以上等于

cin >> a;
cin >> b;

cin 和 字符串

string

getline

cin

// cin with strings
#include <iostream>
#include <string>
using namespace std;

int main ()
{
string mystr;
cout << "What's your name? ";
getline (cin, mystr);
cout << "Hello " << mystr << ".\n";
cout << "What is your favorite team? ";
getline (cin, mystr);
cout << "I like " << mystr << " too!\n";
return 0;
}

stringstream

sstream

stringstream

// stringstreams
#include <iostream>
#include <string>
#include <sstream>
using namespace std;

int main ()
{
string mystr;
float price=0;
int quantity=0;

cout << "Enter price: ";
getline (cin,mystr);
stringstream(mystr) >> price;
cout << "Enter quantity: ";
getline (cin,mystr);
stringstream(mystr) >> quantity;
cout << "Total price: " << price*quantity << endl;
return 0;
}