find_if() problem with string

[SSteven]

I get a compilation error when using find_if() with a string:

#include <algorithm>            /// find_if()
#include <iostream>
#include <cctype>               /// islower()

using namespace std;


void first_lower(string& s)
{
    auto p = find_if(s.begin(), s.end(),
                     islower);
}


int main()
{

    return 0;
}

link: http://cpp.sh/64vet

The compilation error is:

In function 'void first_lower(std::string&)':
no matching function for call to 'find_if(std::basic_string<char>::iterator, std::basic_string<char>::iterator, <unresolved overloaded function type>)'
note: candidate is:
note: template<class _IIter, class _Predicate> _IIter std::find_if(_IIter, _IIter, _Predicate)
     find_if(_InputIterator __first, _InputIterator __last,
     ^
note:   template argument deduction/substitution failed:
11:29: note:   couldn't deduce template parameter '_Predicate'

I am unable to identify the problem here.

Any clues?

Thanks.

[JLBorges]

There are two visible overloads of std::islower; and template argument deduction fails.

<cctyype> http://en.cppreference.com/w/cpp/string/byte/islower
<locale> http://en.cppreference.com/w/cpp/locale/islower

The second overload could have been indirectly included by <iostream>

[Peter87]

The problem is that there are two versions of std::islower.

http://en.cppreference.com/w/cpp/string/byte/islower
http://en.cppreference.com/w/cpp/locale/islower

You can either use a lambda, in which case the correct function will be determined by the arguments that you pass to the function.

auto p = find_if(s.begin(), s.end(),
                 [](char c){ return islower(c); });

Or you can cast to a function pointer of the correct type.

auto p = find_if(s.begin(), s.end(),
                 static_cast<int(*)(int)>(islower));

[SSteven]

Thanks for the reply.

I've tried it and it works:

#include <algorithm>            /// find_if()
#include <iostream>
#include <cctype>                /// islower()

using namespace std;


void first_lower(const string& s)
{
    auto p = find_if(s.begin(), s.end(),
                     [] (char c)
                     {
                         return islower(c);
                     }
                    );
    if (p != s.end())                     
        cout << *p << endl;
}


int main()
{
    first_lower(string {"Hello"});
    first_lower(string {"dear"});
}

http://cpp.sh/4inws

[gunnerfunner]

Using scope-resolution (::) with the islower function object also makes the OP work:

#include <algorithm>            /// find_if()
#include <iostream>
#include <cctype>                /// islower()

using namespace std;


void first_lower(const string& s)
{
    auto p = find_if(s.begin(), s.end(),::islower);
    if (p != s.end())
        cout << *p << endl;
}

int main()
{
    first_lower(string {"Hello"});
    first_lower(string {"dear"});
}

http://coliru.stacked-crooked.com/a/25392f96e5c7eb53

[JLBorges]

There is no guarantee that the header <cctype> would provide its declarations and definitions within the global namespace. (It may do so, but it is not required to do so.)

To use use ::islower in a portable manner, #include <ctype.h>. This header assuredly provides its declarations and definitions within the global namespace. (It may also provide these names within the namespace std, but it is not required to do so.)

[gunnerfunner]

This header assuredly provides its declarations and definitions within the global namespace.

This is interesting, is there a list of namespaces associated with the various header files? Thanks

[JLBorges]

See 20.5.1.1 and 20.5.1.2 (Library-wide requirements) http://eel.is/c++draft/requirements
and Annex D.5 (deprecated features, normative) http://eel.is/c++draft/depr.c.headers

[Bopaki]

Thank you all. My program runs fine now.