Add std::not_fn.

Author: AnthonyCalandra

CPP17.md

@@ -36,6 +36,7 @@ C++17 includes the following new library features:
 - [std::reduce](#stdreduce)
 - [prefix sum algorithms](#prefix-sum-algorithms)
 - [gcd and lcm](#gcd-and-lcm)
+- [std::not_fn](#stdnot_fn)
 
 ## C++17 Language Features
 
@@ -600,6 +601,19 @@ std::gcd(p, q); // == 3
 std::lcm(p, q); // == 9
 ```
 
+### std::not_fn
+Utility function that returns the negation of the result of the given function.
+```c++
+const std::ostream_iterator<int> ostream_it{ std::cout, " " };
+const auto is_even = [](const auto n) { return n % 2 == 0; };
+std::vector<int> v{ 0, 1, 2, 3, 4 };
+
+// Print all even numbers.
+std::copy_if(std::cbegin(v), std::cend(v), ostream_it, is_even); // 0 2 4
+// Print all odd (not even) numbers.
+std::copy_if(std::cbegin(v), std::cend(v), ostream_it, std::not_fn(is_even)); // 1 3
+```
+
 ## Acknowledgements
 * [cppreference](http://en.cppreference.com/w/cpp) - especially useful for finding examples and documentation of new library features.
 * [C++ Rvalue References Explained](http://thbecker.net/articles/rvalue_references/section_01.html) - a great introduction I used to understand rvalue references, perfect forwarding, and move semantics.

README.md

@@ -66,6 +66,7 @@ C++17 includes the following new library features:
 - [std::reduce](#stdreduce)
 - [prefix sum algorithms](#prefix-sum-algorithms)
 - [gcd and lcm](#gcd-and-lcm)
+- [std::not_fn](#stdnot_fn)
 
 C++14 includes the following new language features:
 - [binary literals](#binary-literals)
@@ -1264,6 +1265,19 @@ std::gcd(p, q); // == 3
 std::lcm(p, q); // == 9
 ```
 
+### std::not_fn
+Utility function that returns the negation of the result of the given function.
+```c++
+const std::ostream_iterator<int> ostream_it{ std::cout, " " };
+const auto is_even = [](const auto n) { return n % 2 == 0; };
+std::vector<int> v{ 0, 1, 2, 3, 4 };
+
+// Print all even numbers.
+std::copy_if(std::cbegin(v), std::cend(v), ostream_it, is_even); // 0 2 4
+// Print all odd (not even) numbers.
+std::copy_if(std::cbegin(v), std::cend(v), ostream_it, std::not_fn(is_even)); // 1 3
+```
+
 ## C++14 Language Features
 
 ### Binary literals