通过完成下面的工作来改进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1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 <iostreeam>
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;
} 输出应与下面的相似：
Please enter your name: Pretta Farbo
My name is Fretta Farbo.
The string
MY NAME IS P PRETTA FARBO AND I AM A C++ STUDENT.
coutains 6 'A' characters in it.
Enter the name of a primary color for mixing light: yellow
Try again!
BLUE
That's right!
Bye
My name is Fretta Farbo.
The string
MY NAME IS P PRETTA FARBO AND I AM A C++ STUDENT.
coutains 6 'A' characters in it.
Enter the name of a primary color for mixing light: yellow
Try again!
BLUE
That's right!
Bye
