【C/C++】通过下面的工作来改进String类声明（即将String1.h升级为String2.h）。 a. 对+运算符进行重载，使之可将两个字符串合并成一个。 b. 提供一个Stringlow()

通过下面的工作来改进String类声明（即将String1.h升级为String2.h）。

a. 对+运算符进行重载，使之可将两个字符串合并成一个。

b. 提供一个Stringlow()成员函数，将字符串中所有的字母字符转换为小写（别忘了cctype系列字符函数）。

c. 提供String()成员函数，将字符串中所有字母字符转换成大写。

d. 提供一个这样的成员函数，它接受一个char参数，返回该字符在字符串中出现的次数。

使用下面的程序来测试您的工作：

// pe12_2.cpp
#include <iostream>
using namespace std;
#include "string2.h"
int main()
{
    String s1(" and I am a C++ student.");
    String s2 = "Please enter your name: ";
    String s3;
    cout << s2; // overloaded << operator
    cin >> s3; // overloaded >> operator
    s2 = "My name is " + s3; // overloaded =, + operators
    cout << s2 << ".\n";
    s2 = s2 + s1;
    s2.stringup(); // converts string to uppercase
    cout << "The string\n" << s2 << "\ncontains " << s2.has('A')
        << " 'A' characters in it.\n";
    s1 = "red"; // String(const char *),
    // then String & operator=(const String&)
    String rgb[3] = { String(s1), String("green"), String("blue")};
    cout << "Enter the name of a primary color for mixing light: ";
    String ans;
    bool success = false;
    while (cin >> ans)
    {
        ans.stringlow(); // converts string to lowercase
        for (int i = 0; i < 3; i++)
        {
            if (ans == rgb[i]) // overloaded == operator
            {
                cout << "That's right!\n";
                success = true;
                break;
            }
        }
        if (success)
            break;
        else
            cout << "Try again!\n";
    }
    cout << "Bye\n";
    return 0;
}

输出应与下面相似：

Please enter your name: Fretta Farbo
My name is Fretta Farbo.
The string
MY NAME IS FRETTA FARBO AND I AM A C++ STUDENT.
contains 6 'A' characters in it.
Enter the name of a primary color for mixing light: yellow
Try again!
BLUE
That's right!
Bye

这道题是C++ Primer Plus习题及答案-第十二章的第二题，以下为自己写的答案，仅以此作为记录，若有不足欢迎指出：

string2.h:

#pragma once
#include <iostream>
#include <cctype>
#include <string>
#include <fstream>
#include <istream>
using namespace std;
class String {
public:
  String();
  String(const char* str);
  String(string& str);
  void stringup();
  void stringlow();
  int has(const char str)const;
  String operator+(String& object);
private:
  string str;
  int len = 0;
private:
  friend ostream& operator<<(ostream& os, String& object);
  friend istream& operator>>(istream& is, String& object);
  friend String operator+(const char* str1, String& object);
  friend bool operator==(const String& object1, const String& object2);
};

string2.cpp:

#include "String.h"

String::String() {
  str = "未知";
  len = str.length();
}

String::String(const char* str) {
  this->str = str;
  len = strlen(str);
}

String::String(string& str) {
  this->str = str;
  len = str.length();
}

void String::stringup() {
  for (int i = 0; i < len; i++) {
    str[i] = toupper(str[i]);
  }
}

void String::stringlow() {
  for (int i = 0; i < len; i++) {
    str[i] = tolower(str[i]);
  }
}

int String::has(char str) const {
  int count = 0;
  for (int i = 0; i < len; i++) {
    if (this->str[i] == str) {
      count++;
    }
  }
  return count;
}

String String::operator+(String& object) {
  str = str.append(object.str);
  return String(str);
}

ostream& operator<<(ostream& os, String& object) {
  os << object.str;
  return os;
}

istream& operator>>(istream& is, String& object) {
  getline(is, object.str);//可以包括空格的输入
  //is >> object.str;
  return is;
}

String operator+(const char* str1, String& object) {
  string tmp = string(str1) + object.str;

  return String(tmp);
}

bool operator==(const String& object1, const String& object2) {
  if (object1.str == object2.str && object1.str.length() == object2.str.length()) {
    return true;
  }
  return false;
}

main.cpp:

#include <iostream>
#include "String2.h"
using namespace std;
int main() {
  String s1(" and I am a C++ student.");
  String s2 = "Please enter your name:";
  String s3;
  cout << s2;
  cin >> s3;
  s2 = "My name is " + s3;
  cout << s2 << ".\n";
  s2 = s2 + s1;
  s2.stringup();
  cout << "The string\n" << s2 << "\n contanins " << s2.has('A') << " 'A' characters in it.\n";
  s1 = "red";
  String rgb[3] = { String(s1),String("green"),String("blue") };
  cout << "Enter the name of a primary color for mixing light:";
  String ans;
  bool success = false;
  //cin.get();
  while (cin >> ans) {
    ans.stringlow();
    for (int i = 0; i < 3; i++) {
      if (ans == rgb[i]) {
        cout << "That's right!\n";
        success = true;
        break;
      }
    }
    if (success) {
      break;
    }
    else {
      cout << "Try again!\n";
    }
  }
  cout << "Bye\n";
  return 0;
}

运行结果：