C++相关闲碎记录(15)

1、string字符串

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

int main (int argc, char** argv)
{
   const string delims(" \t,.;");
   string line;

   // for every line read successfully
   while (getline(cin,line)) {
       string::size_type begIdx, endIdx;

       // search beginning of the first word
       begIdx = line.find_first_not_of(delims);

       // while beginning of a word found
       while (begIdx != string::npos) {
           // search end of the actual word
           endIdx = line.find_first_of (delims, begIdx);
           if (endIdx == string::npos) {
               // end of word is end of line
               endIdx = line.length();
           }

           // print characters in reverse order
           for (int i=endIdx-1; i>=static_cast<int>(begIdx); --i) {
               cout << line[i];
           }
           cout << ' ';

           // search beginning of the next word
           begIdx = line.find_first_not_of (delims, endIdx);
       }
       cout << endl;
   }
}
输入：ajsdk12345e.asfa        \jkawefa
输出：e54321kdsja afsa afewakj\ 

（1）string各项操作

?（2）构造函数和析构函数

?data() 和 c_str()以字符数组的形式返回string内容，并且在该数组位置[size()]上有一个'\0'结束字符。copy()将string内容复制到调用者提供的字符数组中，其末尾不添加'\0'字符，注意，data()和c_str()返回的字符数组由该string拥有，也就是说，调用者不可以改动它或者释放其内存。

例如：

std::string s("12345");
atoi(s.c_str())           
f(s.data, s.length())

char buffer[100];
s.copy(buffer, 100);    //copy at most 100 characters os s into buffer
s.copy(buffer, 200, 2); //copy at most 100 characters of s into buffer
                        //starting with the third character of s

一般而言，整个程序你应该坚持使用string，直到你必须将其内容转化为char*时，注意c_str()和data()的返回值的有效期在下一次调用non-const 成员函数时即告终止。

std::string s;
...
foo(s.c_str());  //s.c_str() is valid during the whole statement

const char* p;
p = s.c_str();   //p refers to the contents of s as a C-string
foo(p);          //OK (p is still valid)
s += "ext";      //invalidates p
foo(p);          //ERROR: argument p is not valid

（3）赋值操作

const std::string aString("othello");
std::string s;
s = aString;
s = "two\nlines";
a = ' ';
s.assign(aString);
s.assign(aString, 1, 3);
s.assign(aString, 2, std::string::npos);
s.assign("two\nlines");
s.assign("nico", 5);
s.assign(5, 'x');

（4）安插和移除字符

const std::string aString("othello");
std::string s;
s += aString;
s += "two\nlines";
s += '\n';
s += {'o', 'k'};
s.append(aString);
s.append(aString, 1, 3);   //append "the"
s.append(aString, 2, std::string::npos);    // append "hello"
s.append("two\nlines");
s.append("nico", 5);         //append character array 'n' 'i' 'c' '0' '\0'
s.append(5, 'x');            //append five characters 'x' 'x' 'x' 'x' 'x'
s.push_back('\n');       

s.insert(1, aString);
//注意，成员函数insert()不接受索引+单字符的实参组合，必须传入一个string或者加上一个额外数字
s.insert(0, ' ');  //ERROR
s.insert(0, " ");  //OK
//你也可以这样尝试
s.insert(0, 1,' ');   // ERROR : ambiguous
// 由于insert()具有以下重载形式，上一行会导致令人厌烦的歧义
insert(size_type idx, size_type num, charT c);  //position is index
insert(iterator pos, size_type num, charT c);   //position is iterator
string的size_type通常被定义为unsigned，string的iterator通常被定义为char*。
于是第一实参0有两种转换可能，不分优劣，为了获得正确的操作，必须使用如下:
s.insert((std::string::size_tpye)0, 1, ' ');  //OK

std::string s = "i18n";                 //s:i18n
s.replace(1, 2, "nternationalizatio");  //s:internationalization
s.erase(13);                            //s:international
s.erase(7, 5);                          //s:internal
s.pop_back();                           //s:interna(since C++11)
s.replace(0, 2, "ex");                  //s:externa

（5）子字符串和字符串拼接

std::string s("interchangeability");
s.substr()                  //returns a copy of s
s.substr(11)                //returns string("ability")
s.substr(5, 6);             //returns string("change")
s.substr(s.find('c'));      //returns string("changeability")

（6）getline()

读取一行字符，包括前导空白字符，遇到换行或者end-of-file，分行符会被读取出来，但是不会添加到结果上，默认分行符是换行符号，也可以自定义任意符号作为分行符。

while(std::getline(std::cin,s)) {}
while(std::getline(std::cin, s, ':')){}

?如果自定义了分行符，则换行符就被当做普通字符。

（7）搜索查找

std::string s("Hi Bill, I'm ill, so please pay the bill");
s.find("il");                    //returns 4
s.find("il", 10);                //returns 13
s.rfind("il");                   //returns 37
s.find_first_of("il");           //returns 1
s.find_last_of("il");            //returns 39
s.find_first_not_of("il");       //returns 0
s.find_last_not_of("il");        //returns 36
s.find("hi");                    //returns npos

（8）npos的意义

如果查找失败，会返回string::npos，使用string的npos值及其类型时要格外小心，若要检查函数返回值，一定要使用类型string::size_type,不能使用int或unsigned作为返回值类型，否则返回值与string::npos之间的比较可能无法正确执行，这是因为npos被设置为-1。

事实上(unsigned long)-1与(unsigned short)-1不同，因此对于下列表达式：

idx == std::string::npos，如果idx的值为-1，由于idx和string::npos类型不同，比较结果可能会是false。

（9）数值转换

自C++11起，C++标准库提供了一些便捷函数，用来将string转换为数值或者反向转换，但是只适用于类型string或者wstring类型，不适用于u16string和u32string。

#include <string>
#include <iostream>
#include <limits>
#include <exception>

int main()
{
  try {
    // convert to numeric type
    std::cout << std::stoi ("  77") << std::endl;
    std::cout << std::stod ("  77.7") << std::endl;
    std::cout << std::stoi ("-0x77") << std::endl;

    // use index of characters not processed
    std::size_t idx;
    std::cout << std::stoi ("  42 is the truth", &idx) << std::endl;
    std::cout << " idx of first unprocessed char: " << idx << std::endl;

    // use bases 16 and 8
    std::cout << std::stoi ("  42", nullptr, 16) << std::endl;
    std::cout << std::stol ("789", &idx, 8) << std::endl;
    std::cout << " idx of first unprocessed char: " << idx << std::endl;

    // convert numeric value to string
    long long ll = std::numeric_limits<long long>::max();
    std::string s = std::to_string(ll);  // converts maximum long long to string
    std::cout << s << std::endl;

    // try to convert back
    std::cout << std::stoi(s) << std::endl;  // throws out_of_range
  }
  catch (const std::exception& e) {
    std::cout << e.what() << std::endl;
  }
}
输出：
77
77.7
0
42
 idx of first unprocessed char: 4
66
7
 idx of first unprocessed char: 1
9223372036854775807
stoi

?std::stoi("-0x77")只会解析-0，std::stol("789", &idx, 8)只解析7，因为8在8进制中是一个无效字符。

（10）string iterator使用实例

#include <string>
#include <iostream>
#include <algorithm>
#include <cctype>
#include <regex>
using namespace std;


int main()
{
    // create a string
    string s("The zip code of Braunschweig in Germany is 38100");
    cout << "original: " << s << endl;

    // lowercase all characters
    transform (s.cbegin(), s.cend(),  // source
               s.begin(),             // destination
               [] (char c) {          // operation
                   return tolower(c);
               });
    cout << "lowered:  " << s << endl;

    // uppercase all characters
    transform (s.cbegin(), s.cend(),  // source
               s.begin(),             // destination
               [] (char c) {          // operation
                   return toupper(c);
               });
    cout << "uppered:  " << s << endl;

    // search case-insensitive for Germany
    string g("Germany");
    string::const_iterator pos;
    pos = search (s.cbegin(),s.cend(),     // source string in which to search
                  g.cbegin(),g.cend(),     // substring to search
                  [] (char c1, char c2) {  // comparison criterion
                      return toupper(c1) == toupper(c2);
                 });
    if (pos != s.cend()) {
        cout << "substring \"" << g << "\" found at index "
             << pos - s.cbegin() << endl;
    }
}
输出：
original: The zip code of Braunschweig in Germany is 38100
lowered:  the zip code of braunschweig in germany is 38100
uppered:  THE ZIP CODE OF BRAUNSCHWEIG IN GERMANY IS 38100
substring "Germany" found at index 32

（11）为string打造trait class，允许以大小写无关的方式操作字符

#ifndef ICSTRING_HPP
#define ICSTRING_HPP

#include <string>
#include <iostream>
#include <cctype>

// replace functions of the standard char_traits<char>
// so that strings behave in a case-insensitive way
struct ignorecase_traits : public std::char_traits<char> {
    // return whether c1 and c2 are equal
    static bool eq(const char& c1, const char& c2) {
        return std::toupper(c1)==std::toupper(c2);
    }
    // return whether c1 is less than c2
    static bool lt(const char& c1, const char& c2) {
        return std::toupper(c1)<std::toupper(c2);
    }
    // compare up to n characters of s1 and s2
    static int compare(const char* s1, const char* s2,
                       std::size_t n) {
        for (std::size_t i=0; i<n; ++i) {
            if (!eq(s1[i],s2[i])) {
                return lt(s1[i],s2[i])?-1:1;
            }
        }
        return 0;
    }
    // search c in s
    static const char* find(const char* s, std::size_t n,
                            const char& c) {
        for (std::size_t i=0; i<n; ++i) {
            if (eq(s[i],c)) {
                return &(s[i]);
            }
        }
        return 0;
    }
};

// define a special type for such strings
typedef std::basic_string<char,ignorecase_traits> icstring;

// define an output operator
// because the traits type is different from that for std::ostream
inline
std::ostream& operator << (std::ostream& strm, const icstring& s)
{
    // simply convert the icstring into a normal string
    return strm << std::string(s.data(),s.length());
}

#endif    // ICSTRING_HPP

#include "icstring.hpp"

int main()
{
    using std::cout;
    using std::endl;

    icstring s1("hallo");
    icstring s2("otto");
    icstring s3("hALLo");
    
    cout << std::boolalpha;
    cout << s1 << " == " << s2 << " : " << (s1==s2) << endl;
    cout << s1 << " == " << s3 << " : " << (s1==s3) << endl;

    icstring::size_type idx = s1.find("All");
    if (idx != icstring::npos) {
        cout << "index of \"All\" in \"" << s1 << "\": "
             << idx << endl;
    }
    else {
        cout << "\"All\" not found in \"" << s1 << endl;
    }
}
输出：
hallo == otto : false
hallo == hALLo : true
index of "All" in "hallo": 1