[C++]cpp小笔记3 --- C++ String and char

以你之姓@ 2022-09-18 15:53 113阅读 0赞

字符串实际上就是在内存中在连续位置上保存的字符，字符串结尾以'\\0' null字符表示结束。

String在C++中有两种表现形式。1. 用C style; 2. 用String class.

### 1. C Style String ###

Store String in the form of an array of chars. The last bit of the char array MUST BE '\\0' to indicate the end of the string.

char dog[8] = { 'b', 'e', 'a', 'u', 'x', ' ', 'I', 'I'}; // not a string!， continuous stored but not ended with '\0'
    char cat[8] = {'f', 'a', 't', 'e', 's', 's', 'a', '\0'}; // a string!

char val[] = {"this is also a string"};                  //a string! '\0' is added implicitly for double quotation""

cout <<sizeof(wchar_t) <<endl;
    	cout<<sizeof(char32_t) <<endl;
    	cout<<sizeof(char16_t) <<endl;
    	wchar_t wchar[] = {L" a 4-Byte long string"};
    	char32_t char32[] = {"a 4-Byte long string"};
    	char16_t char16[] = {"a 2-Byte long string"};Result is:
     
     4 4 2

除了显式的指定'\\0'的位置，还可以用"" 双引号来给char array初始化：

char bird[11] = "Mr. Cheeps";  // the \0 is understood
    char fish[] = "Bubbles";   // let the compiler count

Quoted strings always include the terminating null character implicitly。要注意的一点是，正因为 " " 双引号会自动添加 \\0, 所以"s"这样的一个字符实际上包含两个字符， 而's'近包含一个字符。

char shirt_size = 'S';   // this is fine
    char shirt_size = ”S“;   // this is wrong

### cin只能读取一个word, 任何空白符--space, tab, newline将结束cin读取，cin自动在读取的内容后添加'\\0'并将内容保存在character array中。 ###

例如：

#include <iostream>
    #include "Chapter4.h"
    using namespace std;
    
    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";
    	return 0;
    }

![Image 1][]  
结果：

Enter your name:
    Alistair Dreeb
    Enter your favorite dessert:
    I have some delicious Dreeb for you, Alistair.

### 2. C++ string class style string and its initialization ###

#include <string>
    
    using namespace std;
    
    string str1;
    string str2 = "string";

string str3 = {"also a string"};

string str4 {"string too!"};

### 3. C-style and string style functions. ###

C-style

strcpy(dest, src);                                       //copy src char arry  to dest char array. 
    strncpy(dest, src);                                     //copy "n" src char arry  to dest char array. 
    strlen(chararray1);                                   //calulate the length of chars in chararray1
    strcat(chararray1, chararray2);               //concate chararray2 to the end of chararray1

In C-style string,

char char1[] = {'1','2','3'};
    char char2[3];
    char2 = char1;    //wrong, char copy need to use strcpy().

string ctyle functions:

string str = "xyz";

str.size();                          //size of str

#include <iostream>
    #include "Chapter4.h"
    #include <string>
    #include <cstring>  //C-style string library
     using namespace std;
    
    int main() {
    	using namespace std;
    	const int ArSize = 20;
    	char name[ArSize] = "bbai";
    	char dessert[ArSize];
    	string namestr = "bbai";
    
    	//strcpy name to dessert
    	strcpy(dessert, name);
    	cout <<" copy from " << name << " to "<<dessert<<endl;
    	int n = 2;
    	char dessert2[ArSize];
    
    	//strncpy n characters from name to dessert2
    	strncpy(dessert2, name, n);
    	cout <<" copy first "<< n << "characters from " << name << " to "<<dessert2<<endl;
    
    	//strcat
    	strcat(dessert2, name);  //add name to the end of dessert2
    
    	//strlen
    	cout<< "length of name is " << strlen(name) << " length of dessert2 is " << strlen(dessert2) <<"  and lenght of string is " << namestr.size()<<endl;
    	return 0;
    }

Result:

copy from bbai to bbai
     copy first 2characters from bbai to bb
    length of name is 4 length of dessert2 is 6  and lenght of string is 4

### Raw string representation. PS, tired with R but got error... ###

raw strings use  "( and  )" as delimiters, and they use anR prefix to identify them as rawstrings:

cout << R"(Jim "King" Tutt uses "\n" instead of endl.)" << '\n';

This would display the following:

Jim "King" Tutt uses \n instead of endl.

The standard string literal equivalent would be this:

cout << "Jim \"King\" Tutt uses \" \\n\" instead of endl." << '\n';

###  ###

### cin.get(), cin.get(char) and cin>>ch ###

### cin>>ch 会自动过滤whitespaces, new lines, and tabs, 所以一遇到这些字符就结束输入； ###