C++ STL string 大小寫轉換時的 no matching function for call to ‘transform 錯誤

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

這裡介紹了 C++ STL string 大小寫轉換的代碼，但是要注意，可能有些機器用下面的代碼編譯不過

#include <cctype>	// toupper, tolower
#include <iostream>
#include <string>
#include <algorithm>    // transform
using namespace std;

int main()
{
   string str = "[email protected]";
   transform(str.begin(), str.end(), str.begin(), toupper);
   cout << str << endl;
   transform(str.begin(), str.end(), str.begin(), tolower);
   cout << str << endl;
   return 0;
}
           

可能的錯誤提示如下：

error: no matching function for call to ‘transform(__gnu_cxx::__normal_iterator<char*, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, __gnu_cxx::__normal_iterator<char*, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, __gnu_cxx::__normal_iterator<char*, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, <unknown type>)’
           

這裡 說明了原因：

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.

翻譯過來就是說，既有C版本的toupper/tolower函數，又有STL模闆函數toupper/tolower，二者存在沖突。

解決辦法：

在toupper/tolower前面加::，強制指定是C版本的（這時也不要include <cctype>了）：

#include <iostream>
#include <string>
#include <algorithm>    // transform
using namespace std;

int main()
{
   string str = "[email protected]";
   transform(str.begin(), str.end(), str.begin(), ::toupper);
   cout << str << endl;
   transform(str.begin(), str.end(), str.begin(), ::tolower);
   cout << str << endl;
   return 0;
}