3、基础类型

3.1、简单变量

变量的命名

carDrip和cardRip

或boat_sport和boats_port

此外，还有有前缀的命名，使用前缀表示数据类型。常见的前缀有:str（表示字符串）、n（表示整数值）、b（表示布尔值）、p（表示指针）、c（表示单个字符）和m（表示一个类成员值）

nMyWeight

整型长度

C++提供了一种灵活的标准，它确保了最小长度

short至少16位
int至少与short一样长
long至少32位，且至少与int一样长
longlong至少64位，且至少与long一样长

#include <iostream>
#include <climits>		//包含整数的限制信息int main() {using namespace std;short n_short = SHRT_MAX;int n_int = INT_MAX;long n_long = LONG_MAX;long long n_llong = LLONG_MAX;//sizeof operator yields size of type or of variablecout << "short is " << sizeof n_short << " bytes." << endl;cout << "int is " << sizeof(int) << " bytes." << endl;cout << "long is " << sizeof n_long << " bytes." << endl;cout << "long long is " << sizeof n_llong << " bytes." << endl << endl;cout << "Maxinum values:" << endl;cout << "short: " << n_short << endl;cout << "int: " << n_int << endl;cout << "long: " << n_long << endl;cout << "long long: " << n_llong << endl << endl;cout << "Minnum int value=" << INT_MIN << endl;cout << "Bits per byte =" << CHAR_BIT << endl;		//字节的位数return 0;
}
/*
short：2
int：4
long：4
long：8
*/

对于整数类型的限制，使用头文件climits（在老式中为limits.h）。即最大值、最小值等常量

在这里插入图片描述

对于int、long、long long

初始化

int owls=100;		//传统C语言初始化，
int wrens(432);		//可供选择的C++初始化

在C++98和C++11中，可以使用大括号对单值进行初始化

在C++98中，将大括号用于单值变量的情形还不多，但是对于C++11标准就多很多了。采用这种方式时，可以使用=，也可以不用
int emus{7};
int rheas={12};其次，大括号内可以不包括任何东西。在这种情况下，变量将被初始化为0.
int rocs ={};
int phy{};

无符号类型

short的表示范围是-32768到+32768

unsigned short 表示的符号是0-65535

// exceed.cpp -- exceeding some integer limits
#include <iostream>
#define ZERO 0      // makes ZERO symbol for 0 value
#include <climits>  // defines INT_MAX as largest int valueint main() {using namespace std;short sam = SHRT_MAX;     // initialize a variable to max valueunsigned short sue = sam;// okay if variable sam already definedcout << "Sam has " << sam << " dollars and Sue has " << sue;cout << " dollars deposited." << endl<< "Add $1 to each account." << endl << "Now ";sam = sam + 1;	//32767+1，溢出，-32768sue = sue + 1;cout << "Sam has " << sam << " dollars and Sue has " << sue;cout << " dollars deposited.\nPoor Sam!" << endl;sam = ZERO;sue = ZERO;		//0-1，溢出，65536cout << "Sam has " << sam << " dollars and Sue has " << sue;cout << " dollars deposited." << endl;cout << "Take $1 from each account." << endl << "Now ";sam = sam - 1;sue = sue - 1;cout << "Sam has " << sam << " dollars and Sue has " << sue;cout << " dollars deposited." << endl << "Lucky Sue!" << endl;// cin.get();return 0;
}

如果超越了限制，其值将为范围另一端的取值。C++确保了无符号类型类型的这种行为；但C++并不保证有符号整形超越限制（上溢出和下溢出）时不出错。

如果知道变量表示的数值可能大于16位整数的最大值，则使用long。【因为在移植到别的操作系统中，int可能由32位变成16位】。

如果short比int小，则使用short可以节省内存。如果节省内存很重要，则应使用short而不是int，计时它们的长度是一样的。例如，假设要将程序从int是16位的系统移植到int是32位的系统，则用于存储int的数组的内存量将加倍，但是short的数组是不受影响的。

如果只需要一个字节，可使用char。

进制

有8进制、10进制、16进制，默认的是10进制

// hexoct1.cpp -- shows hex and octal literals
#include <iostream>int main() {using namespace std;int chest = 42;     // decimal integer literalint waist = 0x42;   // hexadecimal integer literalint inseam = 042;   // octal integer literalcout << "Monsieur cuts a striking figure!\n";cout << "chest = " << chest << " (42 in decimal)\n";cout << "waist = " << waist << " (0x42 in hex)\n";cout << "inseam = " << inseam << " (042 in octal)\n";// cin.get();return 0;
}

对于C++，如果要以16进制或8进制方式显示值，则可以使用cout的一些特殊性。cout<<hex、cout<<oct

int main() {using namespace std;int chest = 42;int waist = 42;int inseam = 42;cout << "Monsieur cuts a striking figure!"  << endl;cout << "chest = " << chest << " (decimal for 42)" << endl;cout << hex;      // manipulator for changing number basecout << "waist = " << waist << " (hexadecimal for 42)" << endl;cout << oct;      // manipulator for changing number basecout << "inseam = " << inseam << " (octal for 42)" << endl;// cin.get();return 0;
}

标识符hex位于名称空间std中，而程序使用了该名称空间。

如果省略编译指令using，而使用std::cout、std::endl、std::hex、std::oct。

字符类型

char类型是专为存储字符（如数字和字母）而设计的。此外，char也可以看作是比short更小的整形

输入时，cin将键盘输入的M转换为77；输出时，cout将值77转换为所显示的字符M。

// chartype.cpp -- the char type
#include <iostream>
int main( )
{using namespace std;char ch;        // declare a char variablecout << "Enter a character: " << endl;cin >> ch;cout << "Hola! ";cout << "Thank you for the " << ch << " character." << endl;// cin.get();// cin.get();return 0;
}

下面程序说明了这一单。cin和cout的行为都是由变量类型引导的。如果将77存储在int变量中，则cout将把它显示为77。字符使用单引号’M’,对于字符串使用双引号。

最后，引入cout的一项特性——cout.put()函数

#include <iostream>int main() {using namespace std;char ch = 'M';       // assign ASCII code for M to chint i = ch;          // store same code in an intcout << "The ASCII code for " << ch << " is " << i << endl;cout << "Add one to the character code:" << endl;ch = ch + 1;          // change character code in chi = ch;               // save new character code in icout << "The ASCII code for " << ch << " is " << i << endl;// using the cout.put() member function to display a charcout << "Displaying char ch using cout.put(ch): ";cout.put(ch);// using cout.put() to display a char constantcout.put('!');cout << endl << "Done" << endl;// cin.get();return 0;
}/*
The ASCII code for M is 77
Add one to the character code:
The ASCII code for N is 78
Displaying char ch using cout.put(ch): N!
Done
*/

在Release2.0之后，C++将字符常量存储为char类型，而不是int类型。这意味着cout现在可以正确处理字符常量了。

在C++中，有一些转义字符，如下图所示。

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// bondini.cpp -- using escape sequences
#include <iostream>
int main()
{using namespace std;cout << "\aOperation \"HyperHype\" is now activated!\n";cout << "Enter your agent code:________\b\b\b\b\b\b\b\b";long code;cin >> code;cout << "\aYou entered " << code << "...\n";cout << "\aCode verified! Proceed with Plan Z3!\n";// cin.get();// cin.get();return 0; 
}
/*
Operation "HyperHype" is now activated!
Enter your agent code:23456___
You entered 23456...
Code verified! Proceed with Plan Z3!
*/

新增的类型

char16_t和char32_t。底层时一种内置的整形

char16_t ch1=u'q';
char32_t ch2=U'\U0000222B';

bool类型

C++标准中添加了bool的新类型。

在计算中，布尔变量的值可以是true或false。C++将非零解释为true，将零解释为false。

bool is_ready = true;
//字面值true和false都可以通过提升转换为int类型，true被转换位1，而false被转换为0
int ans=true;
int promise=false;
//任何数字值或指针值都可以被隐式转换（即不用显式强制转换）为bool值。任何非零值都被转换位true，而零被转换为false
bool start =-100;
bool stop = 0;

3.2、const

创建常量的通用格式如下：

const type name =value;const int val=10;

相较于#define语句，const有以下的好处

它能够明确指定类型
可以使用C++的作用域规则将定义限制在特定的函数或文件中
可以将const用于更复杂的类型中，如数组和结构

3.3、浮点数

C++有两种浮点数的表示方法

3.3.1、表示方法

1、标准小数点表示法

12.34
0.000023
8.0
45678.2345678

2、E表示法

2.52e+8
8.33e-4
7E5

3.3.2、浮点类型

float至少32位；double至少48位，且不少于float；long double不少于double

通常，float为32位；double为64为；long double为80、96或128位

对于不同类型的浮点数，小数点后的精确度不同

// floatnum.cpp -- floating-point types
#include <iostream>int main() {using namespace std;cout.setf(ios_base::fixed, ios_base::floatfield); // fixed-pointfloat tub = 10.0 / 3.0;     // good to about 6 placesdouble mint = 10.0 / 3.0;   // good to about 15 placesconst float million = 1.0e6;cout << "tub = " << tub;cout << ", a million tubs = " << million *tub;cout << ",\nand ten million tubs = ";cout << 10 * million *tub << endl;cout << "mint = " << mint << " and a million mints = ";cout << million *mint << endl;// cin.get();return 0;
}

3.3.3、浮点常量

如果希望常量为float类型，使用f或F后缀。如果时long double类型，可以使用l或L的后缀

1.234f
2.45E20F
2.456345E28		//double
2.2L			//long double

3.3.4、浮点数优缺点

两大优点

它们可以表示整数之间的值
由于有缩放因子，它们表示的范围大很多

另一方面，浮点运算的速度通常比整数运算慢，且精度将降低。

// fltadd.cpp -- precision problems with float
#include <iostream>int main() {using namespace std;float a = 2.34E+22f;float b = a + 1.0f;cout << "a = " << a << endl;cout << "b - a = " << b - a << endl;// cin.get();return 0;
}

float仅能表示数字的前6位或前7位。

类型的分类

类型signed char、short、int和long统称为符号整型；它们的无符号版本统称为无符号整型。

C++11新增了long long、bool、char、wchar_t。

符号整数和无符号整型统称为整型。C++11新增了char16_t和char32_t。

float、double和long double统称为浮点型。整数和浮点型统称算数类型。

3.4、C++算数运算符

// arith.cpp -- some C++ arithmetic
#include <iostream>
int main()
{using namespace std;float hats, heads;cout.setf(ios_base::fixed, ios_base::floatfield); // fixed-pointcout << "Enter a number: ";cin >> hats;cout << "Enter another number: ";cin >> heads;cout << "hats = " << hats << "; heads = " << heads << endl;cout << "hats + heads = " << hats + heads << endl;cout << "hats - heads = " << hats - heads << endl;cout << "hats * heads = " << hats * heads << endl;cout << "hats / heads = " << hats / heads << endl;// cin.get();// cin.get();return 0;
}
/*
Enter a number: 50.25
Enter another number: 11.17
hats = 50.250000; heads = 11.170000
hats + heads = 61.419998
hats - heads = 39.080002
hats * heads = 561.292480
hats / heads = 4.498657
*/

在C++中，对于float，仅保证6位或7位有效位

3.4.1、运算符优先级

C++的运算符优先级

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后续

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后续

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3.4.2、除法分支

// divide.cpp -- integer and floating-point division
#include <iostream>
int main()
{using namespace std;cout.setf(ios_base::fixed, ios_base::floatfield);cout << "Integer division: 9/5 = " << 9 / 5  << endl;cout << "Floating-point division: 9.0/5.0 = ";cout << 9.0 / 5.0 << endl;cout << "Mixed division: 9.0/5 = " << 9.0 / 5  << endl;cout << "double constants: 1e7/9.0 = ";cout << 1.e7 / 9.0 <<  endl;cout << "float constants: 1e7f/9.0f = ";cout << 1.e7f / 9.0f <<  endl;// cin.get();return 0;
}
/*
Integer division: 9/5 = 1
Floating-point division: 9.0/5.0 = 1.800000
Mixed division: 9.0/5 = 1.800000
double constants: 1e7/9.0 = 1111111.111111
float constants: 1e7f/9.0f = 1111111.125000
*/

最后两行的相对精度表明，如果两个操作数都是double类型，则结果为double类型；如果两个操作数都是float类型，则结果为float类型。浮点常量在默认的情况下，为double类型。

运算符重载

3.4.3、求模运算符

// modulus.cpp -- uses % operator to convert lbs to stone
#include <iostream>int main() {using namespace std;const int Lbs_per_stn = 14;int lbs;cout << "Enter your weight in pounds: ";cin >> lbs;int stone = lbs / Lbs_per_stn;      // whole stoneint pounds = lbs % Lbs_per_stn;     // remainder in poundscout << lbs << " pounds are " << stone<< " stone, " << pounds << " pound(s).\n";// cin.get();// cin.get();return 0;
}
/*
Enter your weight in pounds: 181
181 pounds are 12 stone, 13 pound(s).
*/

3.4.4、类型转换

C++中会有多种类型的转换

赋值时，类型不同，会进行转换
表达式中包含不同的类型时，C++将对值进行转换
将参数传递给函数时，C++将对值进行转换

对于类型的转换，可能会出现一些问题。

// assign.cpp -- type changes on assignment
#include <iostream>int main() {using namespace std;cout.setf(ios_base::fixed, ios_base::floatfield);float tree = 3;     // int converted to floatint guess = 3.9832; // float converted to intint debt = 7.2E12;  // result not defined in C++cout << "tree = " << tree << endl;cout << "guess = " << guess << endl;cout << "debt = " << debt << endl;// cin.get();return 0;
}/*
tree = 3.000000
guess = 3
debt = 2147483647
*/

在将整数变量初始化为浮点值时，有时编译器将会警告。int变量无法存储7.2E12,这导致C++没有对结果进行定义的情况发生。

整型级别

有符号整型按级别从高到低依次是：long long、long、int、short和signed char。

无符号整型的排列顺序与有符号整型相同。类型char、signed char和unsigned char的级别相同。类型bool的级别最低。wchar_t、char16_t和char32_t的级别与其底层类型相同。

强制类型转换

static_cast<>可用于将值从一种数值类型转换为另一种数值类型。

static_cast (value)

// typecast.cpp -- forcing type changes
#include <iostream>int main() {using namespace std;int auks, bats, coots;// the following statement adds the values as double,// then converts the result to intauks = 19.99 + 11.99;// these statements add values as intbats = (int) 19.99 + (int) 11.99;   // old C syntaxcoots = int (19.99) + int (11.99);  // new C++ syntaxcout << "auks = " << auks << ", bats = " << bats;cout << ", coots = " << coots << endl;char ch = 'Z';cout << "The code for " << ch << " is ";    // print as charcout << int(ch) << endl;                    // print as intcout << "Yes, the code is ";cout << static_cast<int>(ch) << endl;       // using static_cast// cin.get();return 0;
}
/*
auks = 31, bats = 30, coots = 30
The code for Z is 90
Yes, the code is 90
*/

3.4.5、auto声明

在初始化声明中，使用关键字auto，而不指定变量的类型，编译器将把变量的类型设置成与初始值相同

auto n=100;		//n is int
auto x=1.5;		//x is double
auto y=1.3e12L;	//y is long double

4、复合类型

4.1、数组

声明需要以下3点

存储在每个元素中的值的类型
数组名
数组中的元素数

声明数组的通用格式如下：

typeName arrayName[arraySize];

float loans[20];

loans的类型不是“数组”，而是“float数组”。这强调了loans数组是使用float类型创建的。为复合类型

// arrayone.cpp -- small arrays of integers
#include <iostream>int main() {using namespace std;int yams[3];    // creates array with three elementsyams[0] = 7;    // assign value to first elementyams[1] = 8;yams[2] = 6;int yamcosts[3] = {20, 30, 5}; // create, initialize array
// NOTE: If your C++ compiler or translator can't initialize
// this array, use static int yamcosts[3] instead of
// int yamcosts[3]cout << "Total yams = ";cout << yams[0] + yams[1] + yams[2] << endl;cout << "The package with " << yams[1] << " yams costs ";cout << yamcosts[1] << " cents per yam.\n";int total = yams[0] * yamcosts[0] + yams[1] * yamcosts[1];total = total + yams[2] * yamcosts[2];cout << "The total yam expense is " << total << " cents.\n";cout << "\nSize of yams array = " << sizeof yams;cout << " bytes.\n";cout << "Size of one element = " << sizeof yams[0];cout << " bytes.\n";// cin.get();return 0;
}
/*
Total yams = 21
The package with 8 yams costs 30 cents per yam.
The total yam expense is 410 cents.Size of yams array = 12 bytes.
Size of one element = 4 bytes.
*/

sizeof运算符返回类型或数据对象的长度(单位为字节)。注意，如果将sizeof运算符用于数组名，得到的将是整个数组的字节数。但如果将sizeof用于数组元素，则得到的将是元素的长度(单位为字节)。

初始化的方法

初始化禁止缩窄转换。

long plifs[]={25,92,3.0};		//not allowed
char slifs[4]{'h','i',12208,'\0'};	//not allowed
char tlifs[4]{'h','i',112,'\0'};	//allowed

第1条语句不能通过编译，因为将浮点数转换为整型是缩窄操作，即使浮点数的小数点后面为零。

第2条语句也不能通过编译，因为12208超出了char变量的取值范围（这里假设char变量的长度为8位）。

第3条语句可通过编译，因为虽然112是一个int值，但它在char变量的取值范围内。

C++标准模板库（STL）提供了一种数组替代品——模板类vector，而C++11新增了模板类array。

4.2、字符串

可以使用字符数组初始化为字符串，但需要使用大量单引号，且必须加上空字符。可以使用括号括起字符串，这种字符串被称为字符串常量或字符串字面值。

char bird[11]="Mr. Cheeps";
char fish[]="Bubbles";

**注意：**字符常量（使用单引号）和字符串常量（使用双引号）不能互换。

char shirt_size ='s';	//this is fine
char shirt_size ="S";	//illegal type mismatch

字符串的拼接

**注意：**拼接时不会在被连接的字符串之间添加空格，第二个字符串的第一个字符将紧跟在第一个字符串的最后一个字符(不考虑\0)后面。第一个字符串中的\0字符将被第二个字符串的第一个字符取代。

// strings.cpp -- storing strings in an array
#include <iostream>
#include <cstring>  // for the strlen() functionint main() {using namespace std;const int Size = 15;char name1[Size];               // empty arraychar name2[Size] = "C++owboy";  // initialized array// NOTE: some implementations may require the static keyword// to initialize the array name2cout << "Howdy! I'm " << name2;cout << "! What's your name?\n";cin >> name1;cout << "Well, " << name1 << ", your name has ";cout << strlen(name1) << " letters and is stored\n";cout << "in an array of " << sizeof(name1) << " bytes.\n";cout << "Your initial is " << name1[0] << ".\n";name2[3] = '\0';                // set to null charactercout << "Here are the first 3 characters of my name: ";cout << name2 << endl;// cin.get();// cin.get();return 0;
}
/*
Howdy! I'm C++owboy! What's your name?
Basicman
Well, Basicman, your name has 8 letters and is stored
in an array of 15 bytes.
Your initial is B.
Here are the first 3 characters of my name: C++
*/

sizeof运算符指出整个数组的长度:15字节；strlen()函数返回的是存储在数组中的字符串的长度，而不是数组本身的长度。另外,strlen()只计算可见的字符，而不把空字符计算在内，因此为8。

可以将name2[3]设置为空字符。这使得字符号在第3个字符后即结束，达到截断字符串的效果。

在这里插入图片描述

字符串的输入

// instr1.cpp -- reading more than one string
#include <iostream>
int main()
{using namespace std;const int ArSize = 20;char name[ArSize];char dessert[ArSize];cout << "Enter your name:\n";cin >> name;cout << "Enter your favorite dessert:\n";cin >> dessert;cout << "I have some delicious " << dessert;cout << " for you, " << name << ".\n";// cin.get();// cin.get();return 0; 
}
/*
Enter your name:
Ali dreeb
Enter your favorite dessert:
I have some delicious dreeb for you, Ali.
*/

如下图所示：
在这里插入图片描述

面向行的输入:getline()

cin.getline(name,20)

// instr2.cpp -- reading more than one word with getline
#include <iostream>
int main()
{using namespace std;const int ArSize = 20;char name[ArSize];char dessert[ArSize];cout << "Enter your name:\n";cin.getline(name, ArSize);  // reads through newlinecout << "Enter your favorite dessert:\n";cin.getline(dessert, ArSize);cout << "I have some delicious " << dessert;cout << " for you, " << name << ".\n";// cin.get();return 0; 
}
/*
Enter your name:
Mai Par
Enter your favorite dessert:
Chocolate Mousse
I have some delicious Chocolate Mousse for you, Mai Par.
*/

面向行的输入：get()

cin.get(name,Arsize);
cin.get();
cin.get(dessert,Arsize);//另一种方式
cin.get(name,Arsize).get();

#include <iostream>int main() {using namespace std;const int ArSize = 20;char name[ArSize];char dessert[ArSize];cout << "Enter your name:\n";cin.get(name, ArSize).get();    // read string, newlinecout << "Enter your favorite dessert:\n";cin.get(dessert, ArSize).get();cout << "I have some delicious " << dessert;cout << " for you, " << name << ".\n";// cin.get();return 0;
}/*
Enter your name:
Mai Per
Enter your favorite dessert:
Chocolate Mousse
I have some delicious Chocolate Mousse for you, Mai Per.
*/

混合输入字符串和数字

// numstr.cpp -- following number input with line input
#include <iostream>int main() {using namespace std;cout << "What year was your house built?\n";int year;cin >> year;	//可与下面结合(cin>>year).get()或(cin>>year).get(ch)
//	cin.get();  或cin.get(ch)cout << "What is its street address?\n";char address[80];cin.getline(address, 80);cout << "Year built: " << year << endl;cout << "Address: " << address << endl;cout << "Done!\n";// cin.get();return 0;
}

4.3、string类

// strtype1.cpp -- using the C++ string class
#include <iostream>
#include <string>               // make string class availableint main() {using namespace std;char charr1[20];            // create an empty arraychar charr2[20] = "jaguar"; // create an initialized arraystring str1;                // create an empty string objectstring str2 = "panther";    // create an initialized stringcout << "Enter a kind of feline: ";cin >> charr1;cout << "Enter another kind of feline: ";cin >> str1;                // use cin for inputcout << "Here are some felines:\n";cout << charr1 << " " << charr2 << " "<< str1 << " " << str2 // use cout for output<< endl;cout << "The third letter in " << charr2 << " is "<< charr2[2] << endl;cout << "The third letter in " << str2 << " is "<< str2[2] << endl;    // use array notation// cin.get();// cin.get();return 0;
}
/*
Enter a kind of feline: ocelot
Enter another kind of feline: tiger
Here are some felines:
ocelot jaguar tiger panther
The third letter in jaguar is g
The third letter in panther is n
*/

赋值、拼接

使用string类时，某些操作比使用数组时更简单。例如，不能将一个数组赋给另一个数组，但可以将一个string对象赋给另一个string对象。

string str3;
str3=str1+str2;
str1+=str2;

例子

int main() {using namespace std;string s1 = "penguin";string s2, s3;cout << "You can assign one string object to another: s2 = s1\n";s2 = s1;cout << "s1: " << s1 << ", s2: " << s2 << endl;cout << "You can assign a C-style string to a string object.\n";cout << "s2 = \"buzzard\"\n";s2 = "buzzard";cout << "s2: " << s2 << endl;cout << "You can concatenate strings: s3 = s1 + s2\n";s3 = s1 + s2;cout << "s3: " << s3 << endl;cout << "You can append strings.\n";s1 += s2;cout << "s1 += s2 yields s1 = " << s1 << endl;s2 += " for a day";cout << "s2 += \" for a day\" yields s2 = " << s2 << endl;//cin.get();return 0;
}
/*
You can assign one string object to another: s2 = s1
s1: penguin, s2: penguin
You can assign a C-style string to a string object.
s2 = "buzzard"
s2: buzzard
You can concatenate strings: s3 = s1 + s2
s3: penguinbuzzard
You can append strings.
s1 += s2 yields s1 = penguinbuzzard
s2 += " for a day" yields s2 = buzzard for a day
*/

其他操作

strcpy(charr1,charr2); //copy charr2 to charr1

strcat(charr1,charr2); //append contents of charr2 to char1

// strtype3.cpp -- more string class features
#include <iostream>
#include <string>               // make string class available
#include <cstring>              // C-style string library
int main()
{using namespace std;char charr1[20]; char charr2[20] = "jaguar"; string str1;  string str2 = "panther";// assignment for string objects and character arraysstr1 = str2;                // copy str2 to str1strcpy(charr1, charr2);     // copy charr2 to charr1// appending for string objects and character arraysstr1 += " paste";           // add paste to end of str1strcat(charr1, " juice");   // add juice to end of charr1// finding the length of a string object and a C-style stringint len1 = str1.size();     // obtain length of str1int len2 = strlen(charr1);  // obtain length of charr1cout << "The string " << str1 << " contains "<< len1 << " characters.\n";cout << "The string " << charr1 << " contains "<< len2 << " characters.\n";// cin.get();return 0; 
}
/*
The string panther paste contains 13 characters.
The string jaguar juice contains 12 characters.
*/

string类I/O

#include <iostream>
#include <string>               // make string class available
#include <cstring>              // C-style string libraryint main() {using namespace std;char charr[20];string str;cout << "Length of string in charr before input: "<< strlen(charr) << endl;cout << "Length of string in str before input: "<< str.size() << endl;cout << "Enter a line of text:\n";cin.getline(charr, 20);     // indicate maximum lengthcout << "You entered: " << charr << endl;cout << "Enter another line of text:\n";getline(cin, str);          // cin now an argument; no length specifiercout << "You entered: " << str << endl;cout << "Length of string in charr after input: "<< strlen(charr) << endl;cout << "Length of string in str after input: "<< str.size() << endl;// cin.get();return 0;
}
/*
Length of string in charr before input: 1
Length of string in str before input: 0
Enter a line of text:
puagj letter
You entered: puagj letter
Enter another line of text:
fghjdfgh hjd
You entered: fghjdfgh hjd
Length of string in charr after input: 12
Length of string in str after input: 12
*/

其他形式的字符串字面值

除了char类型，C++还有类型wchar_t；而C++11新增了类型char16_t和char32_t。可以分别使用前缀L、u和U表示

wchar_t title[]=L"Chief Ast";
char16_t name[]=u"FFF WANG"
char32_t car[]=U"Humber Super snipe"

Q

01、sizeof()&strlen()

char str[20]="0123456789";
int a=strlen(str);         // a=10; >>>> strlen 计算字符串的长度，以结束符 0x00 为字符串结束。
int b=sizeof(str);         // 而 b=20; >>>> sizeof 计算的则是分配的数组 str[20] 所占的内存空间的大小，不受里面存储的内容改变。

对于指针

char* ss = "0123456789";
sizeof(ss) 结果 4 ＝＝＝》ss 是指向字符串常量的字符指针，sizeof 获得的是一个指针的之所占的空间,应该是长整型的，所以是 4。
sizeof(*ss) 结果 1 ＝＝＝》*ss 是第一个字符 其实就是获得了字符串的第一位 '0' 所占的内存空间，是 char 类型的，占了 1 位
strlen(ss)= 10      ＝＝＝》 如果要获得这个字符串的长度，则一定要使用 strlen。strlen 用来求字符串的长度；而 sizeof 是用来求指定变量或者变量类型等所占内存大小。

() << endl;
// cin.get();
return 0;
}
/*
Length of string in charr before input: 1
Length of string in str before input: 0
Enter a line of text:
puagj letter
You entered: puagj letter
Enter another line of text:
fghjdfgh hjd
You entered: fghjdfgh hjd
Length of string in charr after input: 12
Length of string in str after input: 12
*/


**其他形式的字符串字面值**除了char类型，C++还有类型wchar_t；而C++11新增了类型char16_t和char32_t。可以分别使用前缀L、u和U表示

wchar_t title[]=L"Chief Ast";
char16_t name[]=u"FFF WANG"
char32_t car[]=U"Humber Super snipe"

# Q## 01、sizeof()&strlen()```c++
char str[20]="0123456789";
int a=strlen(str);         // a=10; >>>> strlen 计算字符串的长度，以结束符 0x00 为字符串结束。
int b=sizeof(str);         // 而 b=20; >>>> sizeof 计算的则是分配的数组 str[20] 所占的内存空间的大小，不受里面存储的内容改变。

对于指针

char* ss = "0123456789";
sizeof(ss) 结果 4 ＝＝＝》ss 是指向字符串常量的字符指针，sizeof 获得的是一个指针的之所占的空间,应该是长整型的，所以是 4。
sizeof(*ss) 结果 1 ＝＝＝》*ss 是第一个字符 其实就是获得了字符串的第一位 '0' 所占的内存空间，是 char 类型的，占了 1 位
strlen(ss)= 10      ＝＝＝》 如果要获得这个字符串的长度，则一定要使用 strlen。strlen 用来求字符串的长度；而 sizeof 是用来求指定变量或者变量类型等所占内存大小。