string大小写转换

将一个string转换成大写或者小写，是项目中经常需要做的事情，但string类里并没有提供这个方法。自己写个函数来实现，说起来挺简单，但做起来总让人觉得不方便。打个比方：早上起来想吃个汉堡，冰箱里有生牛肉，有面粉，也有微波炉，是可以自己做的，但是实在是太费事，没几个人愿意做。但是，打个电话给肯德基宅急送，10分钟后就有热乎乎的汉堡送上门了，大大节省了时间（时间就是金钱，你可以将时间用在更重要的开发工作上），并且味道也不差，何乐而不为呢？

STL的algorithm库确实给我们提供了这样的便利，使用模板函数transform可以轻松解决这个问题，开发人员只需要提供一个函数对象，例如将char转成大写的toupper函数或者小写的函数tolower函数。

transform原型：

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1. template < class InputIterator,class OutputIterator, class UnaryOperator >
2. OutputIterator transform ( InputIterator first1, InputIterator last1,
3. OutputIterator result, UnaryOperator op );
5. template < class InputIterator1, class InputIterator2,
6. class OutputIterator, class BinaryOperator >
7. OutputIterator transform ( InputIterator1 first1, InputIterator1 last1,
8. InputIterator2 first2, OutputIterator result,
9. BinaryOperator binary_op );

template < class InputIterator, class OutputIterator, class UnaryOperator >  OutputIterator transform ( InputIterator first1, InputIterator last1,                             OutputIterator result, UnaryOperator op );template < class InputIterator1, class InputIterator2,           class OutputIterator, class BinaryOperator >  OutputIterator transform ( InputIterator1 first1, InputIterator1 last1,                             InputIterator2 first2, OutputIterator result,                             BinaryOperator binary_op );

测试代码：

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1. #include <string>
2. #include <algorithm>
3. using namespace std;
5. int main()
6. {
7. string strA = "[e-mail]";
8. string strB = "[e-mail]";
9. printf("Before transform:\n");
10. printf("strA:%s \n", strA.c_str());
11. printf("strB:%s \n\n", strB.c_str());
13. transform(strA.begin(), strA.end(), strA.begin(), ::toupper);
14. transform(strB.begin(), strB.end(), strB.begin(), ::toupper);
15. printf("After transform to toupper:\n");
16. printf("strA:%s \n", strA.c_str());
17. printf("strB:%s \n\n", strB.c_str());
19. transform(strA.begin(), strA.end(), strA.begin(), ::tolower);
20. transform(strB.begin(), strB.end(), strB.begin(), ::tolower);
21. printf("After transform to lower:\n");
22. printf("strA:%s \n", strA.c_str());
23. printf("strB:%s \n\n", strB.c_str());
24. return 0;
25. }

#include <string>#include <algorithm>using namespace std;int main(){    string strA = "[e-mail]";    string strB = "[e-mail]";    printf("Before transform:\n");    printf("strA:%s \n", strA.c_str());    printf("strB:%s \n\n", strB.c_str());    transform(strA.begin(), strA.end(), strA.begin(), ::toupper);    transform(strB.begin(), strB.end(), strB.begin(), ::toupper);    printf("After transform to toupper:\n");    printf("strA:%s \n", strA.c_str());    printf("strB:%s \n\n", strB.c_str());    transform(strA.begin(), strA.end(), strA.begin(), ::tolower);    transform(strB.begin(), strB.end(), strB.begin(), ::tolower);    printf("After transform to lower:\n");    printf("strA:%s \n", strA.c_str());    printf("strB:%s \n\n", strB.c_str());    return 0;}

运行结果：

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1. strA:[e-mail]
2. strB:[e-mail]
4. After transform to toupper:
5. strA:[e-mail]
6. strB:[e-mail]
8. After transform to lower:
9. strA:[e-mail]
10. strB:[e-mail]

strA:[e-mail] strB:[e-mail] After transform to toupper:strA:[e-mail] strB:[e-mail] After transform to lower:strA:[e-mail] strB:[e-mail] 

最后补一句：STL algorithm功能实在是太强劲了，非常推荐。