C++ find_if with predicate isalpha gives error - c++

Question

I can't understand why I have this error, apparently the isalpha function is redeclared twice (my editor marks me: "2 more overloads") but I don't understand why, this is my code:
#include <iostream>
#include <set>
#include <algorithm>
#include <ctype.h>
#include <string>
using namespace std;
int main(){
cin.tie(nullptr);ios_base::sync_with_stdio(false);
string line;
string::iterator prev, act;
while(cin>>line){
// act = find_if(line.begin(), line.end(), [](int x) {return isalpha(x);}); // GOOD
act = find_if(line.begin(), line.end(), isalpha); // ERROR
cout<<*act<<endl;
}
}
I use g++ (MinGW.org GCC-6.3.0-1) 6.3.0
Error:
1215.cpp: In function 'int main()':
1215.cpp:14:56: error: no matching function for call to 'find_if(std::__cxx11::basic_string<char>::iterator, std::__cxx11::basic_string<char>::iterator, <unresolved overloaded function type>)'
act = find_if(line.begin(), line.end(), isalpha); // ERROR
^
In file included from c:\mingw\lib\gcc\mingw32\6.3.0\include\c++\algorithm:62:0,
from 1215.cpp:3:
c:\mingw\lib\gcc\mingw32\6.3.0\include\c++\bits\stl_algo.h:3808:5: note: candidate: template<class _IIter, class _Predicate> _IIter std::find_if(_IIter, _IIter, _Predicate)
find_if(_InputIterator __first, _InputIterator __last,
^~~~~~~
c:\mingw\lib\gcc\mingw32\6.3.0\include\c++\bits\stl_algo.h:3808:5: note: template argument deduction/substitution failed:
1215.cpp:14:56: note: couldn't deduce template parameter '_Predicate'
act = find_if(line.begin(), line.end(), isalpha); // ERROR
Note: if I don't use the "using namespace std" the code compiles and executes correctly.
Thanks in advance for any answers or advice

Answer 1

This is what might be happening.
There's an isalpha() in the <cctype> header under the std namespace and another isalpha() in the <locale> header under the very same std namespace. using namespace std pulls both into the your scope and thus the ambiguity while deducing the template.
Moral of the story: Avoid using using namespace std.

Answer 2

I can't understand why I have this error, apparently the isalpha function is redeclared twice (my editor marks me: "2 more overloads") but I don't understand why
Two overloads are declared, because the standard says that there are two overloads.
A good solution is to use a lambda, as you did in the comment. Unlike creating a function pointer, calling the function in the lambda allows the call to be resolved to the correct overload based on the passed arguments.
Your lambda should be improved though: Change the type of the parameter to unsigned char. This is needed because char may be a signed type, and if the value of a char is negative, then it will have a value outside the range of unsigned char, and passing a value outside the range of unsigned char into islpha will result in undefined behaviour. Fixed example:
auto isalpha = [](unsigned char c){
return std::isalpha(c);
};
act = find_if(line.begin(), line.end(), isalpha);
Also, remove the line using namespace std.

Answer 3

To complete # Zoso's answer:
Inside <iostream> are included <cctype> and locale_facets.h both define different std::isalpha functions .
Also, inside <cctype> includes <ctype.h> (which is in fact explicitly included in the example) and <ctype.h> defines an isalpha function (without the std namespace).
So one way to solve the problem is to call the isalpha function of<ctype.h>, for this it is necessary to exit the current namespace (std) with :: operator.
This code works:
#include <iostream>
#include <algorithm>
#include <ctype.h>
#include <string>
using namespace std;
int main(){
cin.tie(nullptr);ios_base::sync_with_stdio(false);
string line;
string::iterator prev, act;
while(cin>>line){
act = find_if(line.begin(), line.end(), ::isalpha);
cout<<*act<<endl;
}
}