Why can't “transform(s.begin(),s.end(),s.begin

问题:

Given the code:

#include <iostream>
#include <cctype>
#include <string>
#include <algorithm>
using namespace std;

int main()
{
     string s("ABCDEFGHIJKL");
     transform(s.begin(),s.end(),s.begin(),tolower);
     cout<<s<<endl;
}

I get the 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>)

What does "unresolved overloaded function type" mean?

If I replace the tolower with a function I wrote, it no longer errors.

回答1:

Try using ::tolower. This fixed the problem for me.

回答2:

The problem most probably relates with multiple overloads of tolower and the compiler is unable to select one for you. You can try qualifying it to select an specific version of it, or you might need to provide a function pointer cast to disambiguate. The tolower function can be present (multiple different overloads) in the <locale> header, as well as in <cctype>.

Try:

int (*tl)(int) = tolower; // Select that particular overload
transform(s.begin(),s.end(),s.begin(),tl );

That can be done in a single line with a cast, but it is probably harder to read:

transform(s.begin(),s.end(),s.begin(),(int (*)(int))tolower );

回答3:

Let's look at a list of options starting with the worst and moving to the best. We'll list them here and discuss them below:

1. transform(cbegin(s), cend(s), begin(s), ::tolower)
2. transform(cbegin(s), cend(s), begin(s), static_cast<int(*)(int)>(tolower))
3. transform(cbegin(s), cend(s), begin(s), [](const unsigned char i){ return tolower(i); })

---

The code in your question, transform(s.begin(), s.end(), s.begin(), tolower) will produce an error like:

No matching function for call to transform(std::basic_string<char>::iterator, std::basic_string<char>::iterator, std::basic_string<char>::iterator, <unresolved overloaded function type>)

The reason that you were getting an "unresolved overloaded function type" is there are 2 tolowers in the std namespace:

1. The locale library defines template <typename T> T tolower(T, const locale&)
2. The cctype library defines int tolower(int)

1 is the solution offered by davka. It addresses your error by leveraging the fact that locale's tolower is not defined in the global namespace.

Depending upon your situation locale's tolower may merit consideration. You can find a comparison of the tolowers here: Which tolower in C++?

---

Unfortunately 1 depends upon cctype's tolower being defined in the global namespace. Let's look at why that may not be the case:

You are rightly using #include <cctype>, as doing #include <ctype.h> has been deprecated in C++: http://en.cppreference.com/w/cpp/header

But the C++ standard states in D.3[depr.c.headers]2 of the declarations in the headers:

It is unspecified whether these names are first declared or defined within namespace scope (3.3.6) of the namespace std and are then injected into the global namespace scope by explicit using-declarations (7.3.3)

So the only way that we can guarantee our code is implementation independent is to use a tolower from namespace std. 2 is the solution offered by David Rodríguez - dribeas. It leverages the fact that static_cast can:

Be used to disambiguate function overloads by performing a function-to-pointer conversion to specific type

Before we move on, let me comment that if you find int (*)(int) to be a bit confusing you can read more on function pointer syntax here.

---

Sadly there is one other issue with tolower's input argument, if it:

Is not representable as unsigned char and does not equal EOF, the behavior is undefined

You are using a string which uses elements of type: char. The standard states of char specifically 7.1.6.2[dcl.type.simple]3:

It is implementation-defined whether objects of char type are represented as signed or unsigned quantities. The signed specifier forces char objects to be signed

So if the implementation defined a char to mean a signed char then both 1 and 2 would result in Undefined Behavior for all characters corresponding to negative numbers. (If an ASCII character encoding is being used the characters corresponding to negative numbers are Extended ASCII.)

The Undefined Behavior can be avoided by converting the input to an unsigned char before passing it to tolower. 3 accomplishes that using a lambda that accepts an unsigned char by value, then passes it to tolower implicitly converting to int.

To guarantee Defined Behavior on all compliant implementations, independent of character encoding, you'll need to use transform(cbegin(s), cend(s), begin(s), [](const unsigned char i){ return tolower(i); }) or something similar.

回答4:

David already identified the issue, namely a conflict between:

• <cctype>'s int tolower(int c)
• <locale>'s template <typename charT> charT tolower(charT c, locale const& loc)

Using the first is much easier, but is undefined behavior (unfortunately) as soon as you deal with anything else than lower-ascii (0-127) in signed chars. By the way, I do recommend defining char as unsigned.

The template version would be nice, but you would have to use bind to provide the second parameter, and it's bound to be ugly...

So, may I introduce the Boost String Algorithm library ?

And more importantly: boost::to_lower :)

boost::to_lower(s);

Expressiveness is desirable.

回答5:

Browsing my <ctype> header from gcc 4.2.1, I see this:

// -*- C++ -*- forwarding header.

// Copyright (C) 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005
// Free Software Foundation, Inc.

...

#ifndef _GLIBCXX_CCTYPE
#define _GLIBCXX_CCTYPE 1

#pragma GCC system_header

#include <bits/c++config.h>
#include <ctype.h>

// Get rid of those macros defined in <ctype.h> in lieu of real functions.
#undef isalnum
#undef isalpha

...

#undef tolower
#undef toupper

_GLIBCXX_BEGIN_NAMESPACE(std)

  using ::isalnum;
  using ::isalpha;

...

  using ::tolower;
  using ::toupper;

_GLIBCXX_END_NAMESPACE

#endif

So it looks like tolower exists in both the std (from <cctype>) and root (from <ctype.h>) namespaces. I'm not sure what the #pragma does.