no matching function for call to 'transform [duplicate] - c++

Question

This question already has answers here:
std::transform() and toupper(), no matching function
(3 answers)
Closed 9 years ago.
can anyone tell me what is the mistake in this program
#include <iostream>
#include <algorithm>
using namespace std;
int main()
{
string str = "Now";
transform(str.begin(), str.end(), str.begin(), toupper);
cout<<str;
return 0;
}
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> > >, <unresolved overloaded function type>)'
compilation terminated due to -Wfatal-errors."

Answer 1

There are two functions with name toupper. One from cctype header:
int toupper( int ch );
And second from locale header:
charT toupper( charT ch, const locale& loc );
Compiler can't deduce which function should be used, since you allow namespace std. You should use scope resolution operator(::) to choose function defined in global space:
transform(str.begin(), str.end(), str.begin(), ::toupper);
Or, better: Do not use using namespace std.
Thanks to #Praetorian -
This is probably the cause of the error, but adding :: may not always
work. If you include cctype toupper is not guaranteed to exist in the
global namespace. A cast can provide the necessary disambiguation
static_cast<int(*)(int)>(std::toupper)
So, the call should look like:
std::transform
(
str.begin(), str.end(),
str.begin(),
static_cast<int(*)(int)>(std::toupper)
);

Answer 2

In order to use toupper, you need to include header file:
#include <cctype>
You also need to include header file:
#include <string>
The problem is the std::toupper takes int as parameter, while std::transform will pass char into the function, therefore, it has a problem (by courtesy of #juanchopanza).
You may try to use:
#include <functional>
std::transform(s.begin(), s.end(), s.begin(), std::ptr_fun<int, int>(std::toupper));
See example code from std::transform
Or you can implement your own toupper that takes char as argument.

Answer 3

As the compiler has buried in its error message, the real problem is that toupper is an overloaded function, and the compiler can't figure out which one you want. There's the C toupper(int) function which may or may not be a macro (may not in C++, but does the C library care?), and there's std::toupper(char, locale) from (pulled in by without doubt), which you made available globally with your using namespace std;.
Tony's solution works because he accidentally resolved the overloading issue with his separate function.