C++ STL string 大小写转换时的 no matching function for call to ‘transform 错误

参考：http://forums.codeguru.com/showthread.php?489969-no-matching-function-transform

这里介绍了 C++ STL string 大小写转换的代码，但是要注意，可能有些机器用下面的代码编译不过

#include 	// toupper, tolower
#include 
#include 
#include     // transform
using namespace std;

int main()
{
   string str = "abcdADcdeFDde!@234";
   transform(str.begin(), str.end(), str.begin(), toupper);
   cout << str << endl;
   transform(str.begin(), str.end(), str.begin(), tolower);
   cout << str << endl;
   return 0;
}

可能的错误提示如下：

error: no matching function for call to ‘transform(__gnu_cxx::__normal_iterator, std::allocator > >, __gnu_cxx::__normal_iterator, std::allocator > >, __gnu_cxx::__normal_iterator, std::allocator > >, )’

这里 说明了原因：

The problem is that the version of std::tolower inherited from the C standard library is a non-template function, but there are other versions of std::tolower that are function templates, and it is possible for them to be included depending on the standard library implementation. You actually want to use the non-template function, but there is ambiguity when just tolower is provided as the predicate.

翻译过来就是说，既有C版本的toupper/tolower函数，又有STL模板函数toupper/tolower，二者存在冲突。

解决办法：

在toupper/tolower前面加::，强制指定是C版本的（这时也不要include 了）：

#include 
#include 
#include     // transform
using namespace std;

int main()
{
   string str = "abcdADcdeFDde!@234";
   transform(str.begin(), str.end(), str.begin(), ::toupper);
   cout << str << endl;
   transform(str.begin(), str.end(), str.begin(), ::tolower);
   cout << str << endl;
   return 0;
}