Added C-String support for 'is_palindrome'

Updated doc, example and tests.

Author: zamazan4ik

doc/is_palindrome.qbk

@@ -9,23 +9,26 @@ Distributed under the Boost Software License, Version 1.0.
 (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 ]
 
-The header file 'is_palindrome.hpp' contains four variants of a single algorithm, is_palindrome. 
+The header file 'is_palindrome.hpp' contains six variants of a single algorithm, is_palindrome. 
 The algorithm tests the sequence and returns true if the sequence is a palindrome; i.e, it is identical when traversed either backwards or frontwards.
 
 The routine `is_palindrome` takes a sequence and, optionally, a predicate. It will return true if the predicate returns true for tested elements by algorithm in the sequence. 
 
-The routine come in 4 forms; the first one takes two iterators to define the range. The second form takes two iterators to define the range and a predicate. 
-The third form takes a single range parameter, and uses Boost.Range to traverse it. And the fourth form takes a single range parameter ( uses Boost.Range to traverse it) and a predicate.
+The routine come in 6 forms; the first one takes two iterators to define the range. The second form takes two iterators to define the range and a predicate. 
+The third form takes a single range parameter, and uses Boost.Range to traverse it. The fourth form takes a single range parameter ( uses Boost.Range to traverse it) and a predicate.
+The fifth form takes a single C-string and a predicate. The sixth form takes a single C-string.
 
 
 [heading interface]
 
 The function `is_palindrome` returns true if the predicate returns true any tested by algorithm items in the sequence.  
-There are four versions: 
+There are six versions: 
 1) takes two iterators.
 2) takes two iterators and a predicate.
 3) takes a range.
 4) takes a range and a predicate.
+5) takes a C-string and a predicate.
+6) takes a C-string.
 
 ``
 template<typename BidirectionalIterator>
@@ -36,6 +39,9 @@ template<typename Range>
 	bool is_palindrome ( const Range &r );
 template<typename Range, typename Predicate> 
 	bool is_palindrome ( const Range &r, Predicate p );
+template<typename Predicate>
+	bool is_palindrome ( const char* str, Predicate p );
+bool is_palindrome(const char* str);
 ``
 
 
@@ -59,6 +65,9 @@ is_palindrome(std::begin(oddNonPalindrome), std::end(oddNonPalindrome), funcComp
 is_palindrome(std::begin(oddPalindrome), std::end(oddPalindrome)) --> true
 is_palindrome(evenPalindrome, std::equal_to<int>())) --> true
 is_palindrome(std::begin(evenNonPalindrome), std::end(evenNonPalindrome)) --> false
+is_palindrome(nullptr) --> true
+is_palindrome("a") --> true
+is_palindrome("aba", std::equal_to<char>()) --> true
 ``
 
 [heading Iterator Requirements]
@@ -71,15 +80,15 @@ All of the variants of `is_palindrome` run in ['O(N)] (linear) time; that is, th
 
 [heading Exception Safety]
 
-All of the variants of `is_palindrome` take their parameters by value or const reference, and do not depend upon any global state. Therefore, all the routines in this file provide the strong exception guarantee.
+All of the variants of `is_palindrome` take their parameters by value, const pointer or const reference, and do not depend upon any global state. Therefore, all the routines in this file provide the strong exception guarantee.
 
 [heading Notes]
 
-* `is_palindrome` returns true for empty ranges and for single element ranges. 
+* `is_palindrome` returns true for empty ranges, null pointers and for single element ranges. 
 
-* If you use version of 'is_palindrome' without custom predicate, 'is_palindrome' uses default 'operator==' for elements. If you want use custom predicate, you must redefine 'operator=='.
+* If you use version of 'is_palindrome' without custom predicate, 'is_palindrome' uses default 'operator==()' for elements.
 
-* Don't use 'const char*' with 'is_palindrome', because 'const char*' is always non-palindromic ('\0' at the end). If you will try to compile 'is_palindrome' with 'const char*', you will get an error.
+* Be careful with using not null pointer 'const char*' without '\0' - if you use it with 'is_palindrome', it's a undefined behaviour.
 
 [endsect]
 

example/is_palindrome_example.cpp

@@ -89,6 +89,11 @@ int main ( int /*argc*/, char * /*argv*/ [] )
         std::cout << "This container is palindrome" << std::endl;
     else
         std::cout << "This container is not palindrome" << std::endl;
-
+    
+    //You can use C-strings
+    if(ba::is_palindrome("aba"))
+	std::cout << "This C-string is palindrome" << std::endl;
+    else
+        std::cout << "This C-string is not palindrome" << std::endl;
     return 0;
 }

include/boost/algorithm/is_palindrome.hpp

@@ -17,6 +17,8 @@
 
 #include <iterator>
 #include <functional>
+#include <type_traits>
+#include <cstring>
 
 #include <boost/range/begin.hpp>
 #include <boost/range/end.hpp>
@@ -34,7 +36,7 @@ namespace boost {  namespace algorithm {
 ///     For other sequences function will return false.
 ///     Complexity: O(N).
 template <typename BidirectionalIterator, typename Predicate>
-bool is_palindrome(BidirectionalIterator begin, BidirectionalIterator end, Predicate p)
+bool is_palindrome(BidirectionalIterator begin, BidirectionalIterator end, Predicate p )
 {
     if(begin == end)
     {
@@ -70,8 +72,26 @@ bool is_palindrome(BidirectionalIterator begin, BidirectionalIterator end, Predi
 template <typename BidirectionalIterator>
 bool is_palindrome(BidirectionalIterator begin, BidirectionalIterator end)
 {
-    return is_palindrome(begin, end,
-                         std::equal_to<typename std::iterator_traits<BidirectionalIterator>::value_type> ());
+    if(begin == end)
+    {
+        return true;
+    }
+
+    --end;
+    while(begin != end)
+    {
+        if(!(*begin == *end))
+        {
+            return false;
+        }
+        ++begin;
+        if(begin == end)
+        {
+            break;
+        }
+        --end;
+    }
+    return true;
 }
 
 /// \fn is_palindrome ( const R& range )
@@ -82,7 +102,7 @@ bool is_palindrome(BidirectionalIterator begin, BidirectionalIterator end)
 /// \note This function will return true for empty sequences and for palindromes.
 ///     For other sequences function will return false.
 ///     Complexity: O(N).
-template <typename R>
+template <typename R, typename std::enable_if<!std::is_integral<R>::value_type>::type>
 bool is_palindrome(const R& range)
 {
     return is_palindrome(boost::begin(range), boost::end(range));
@@ -97,26 +117,59 @@ bool is_palindrome(const R& range)
 /// \note This function will return true for empty sequences and for palindromes.
 ///     For other sequences function will return false.
 ///     Complexity: O(N).
-template <typename R, typename Predicate>
+template <typename R, typename Predicate, typename std::enable_if<!std::is_integral<R>::value_type>::type>
 bool is_palindrome(const R& range, Predicate p)
 {
     return is_palindrome(boost::begin(range), boost::end(range), p);
 }
 
-//Disable is_palindrome for const char* because it work not properly.
-//Please use string_view for const char* cases.
-//Here we use dirty hack to disable 'is_palindrome' with 'const char*'
-//URL: http://stackoverflow.com/questions/14637356/static-assert-fails-compilation-even-though-template-function-is-called-nowhere
-template<typename T>
-struct foobar : std::false_type
-{ };
 
-template<typename T = int>
+/// \fn is_palindrome ( const char* str )
+/// \return true if the entire sequence is palindrome
+///
+/// \param str C-string to be tested.
+///
+/// \note This function will return true for empty sequences and for palindromes.
+///     For other sequences function will return false.
+///     Complexity: O(N).
 bool is_palindrome(const char* str)
 {
-    static_assert(foobar<T>::value, "Using 'is_palindrome' for 'const char*' is dangerous, because result is always false"
-                         "(reason: '\0' in the end of the string). You can use string_view in this case");
-    return false;
+    if(str == nullptr)
+    {
+	return true;
+    }
+    return is_palindrome(str, str + strlen(str));
+}
+
+
+/// \fn is_palindrome ( const char* str, Predicate p )
+/// \return true if the entire sequence is palindrome
+///
+/// \param str C-string to be tested.
+/// \param p   A predicate used to compare the values.
+///
+/// \note This function will return true for empty sequences and for palindromes.
+///     For other sequences function will return false.
+///     Complexity: O(N).
+template<typename Predicate>
+bool is_palindrome(const char* str, Predicate p)
+{
+    if(str == nullptr)
+    {
+	return true;
+    }
+    return is_palindrome(str, str + strlen(str), p);
+}
+
+bool is_palindrome (nullptr_t)
+{ 
+    return true;
+}
+
+template<typename T>
+bool is_palindrome (T)
+{ 
+    return true;
 }
 
 }}

test/is_palindrome_test.cpp

@@ -47,6 +47,7 @@ void test_is_palindrome()
     const int oddPalindrome[] = {1,2,3,2,1};
     const int evenPalindrome[] = {1,2,2,1};
     int evenNonPalindrome[] = {1,4,8,8};
+    const char* stringNullPtr = nullptr;
 
     // Test a default operator==
     BOOST_CHECK ( ba::is_palindrome(empty));
@@ -60,6 +61,16 @@ void test_is_palindrome()
     BOOST_CHECK ( ba::is_palindrome(empty.begin(), empty.end(), functorComparator()));
     BOOST_CHECK (!ba::is_palindrome(std::begin(oddNonPalindrome), std::end(oddNonPalindrome), funcComparator<int>));
     BOOST_CHECK ( ba::is_palindrome(evenPalindrome, std::equal_to<int>()));
+    
+    //Test C-strings like cases
+    BOOST_CHECK ( ba::is_palindrome(nullptr));
+    BOOST_CHECK ( ba::is_palindrome(0));
+    BOOST_CHECK ( ba::is_palindrome(stringNullPtr));
+    BOOST_CHECK ( ba::is_palindrome(""));
+    BOOST_CHECK ( ba::is_palindrome("a"));
+    BOOST_CHECK ( ba::is_palindrome("abacaba"), std::equal_to<char>());
+    BOOST_CHECK ( ba::is_palindrome("abba"));
+    BOOST_CHECK ( ba::is_palindrome("acab"));
 }
 
 BOOST_AUTO_TEST_CASE( test_main )