Add can_increment and can_decrement (#43)

### Why 
It can simplify the use of stronk types with custom methods.

### What was changed
- **Add can_increment/decrement**
- **Version bump**

include/stronk/can_decrement.h

@@ -0,0 +1,23 @@
+#pragma once
+
+namespace twig
+{
+
+template<typename StronkT>
+struct can_decrement
+{
+    constexpr auto operator--() noexcept -> StronkT&
+    {
+        --static_cast<StronkT&>(*this).template unwrap<StronkT>();
+        return static_cast<StronkT&>(*this);
+    }
+
+    constexpr auto operator--(int) noexcept -> StronkT
+    {
+        auto copy = static_cast<StronkT&>(*this);
+        --static_cast<StronkT&>(*this).template unwrap<StronkT>();
+        return copy;
+    }
+};
+
+}  // namespace twig

include/stronk/can_increment.h

@@ -0,0 +1,23 @@
+#pragma once
+
+namespace twig
+{
+
+template<typename StronkT>
+struct can_increment
+{
+    constexpr auto operator++() noexcept -> StronkT&
+    {
+        ++static_cast<StronkT&>(*this).template unwrap<StronkT>();
+        return static_cast<StronkT&>(*this);
+    }
+
+    constexpr auto operator++(int) noexcept -> StronkT
+    {
+        auto copy = static_cast<StronkT&>(*this);
+        ++static_cast<StronkT&>(*this).template unwrap<StronkT>();
+        return copy;
+    }
+};
+
+}  // namespace twig

tests/CMakeLists.txt

@@ -19,6 +19,8 @@ find_package(absl CONFIG REQUIRED)
 # ---- Tests ----
 add_executable(
     stronk_test
+    src/can_decrement_tests.cpp
+    src/can_increment_tests.cpp
     src/extensions_tests.cpp
     src/specializers_tests.cpp
     src/stronk_tests.cpp

tests/src/can_decrement_tests.cpp

@@ -0,0 +1,22 @@
+#include <gtest/gtest.h>
+#include <stronk/can_decrement.h>
+#include <stronk/stronk.h>
+
+namespace twig
+{
+
+struct an_int_decrement_test_type : stronk<an_int_decrement_test_type, int, can_decrement, can_equate>
+{
+    using stronk::stronk;
+};
+
+TEST(can_decrement, can_decrement_stronk_integer)
+{
+    auto a = an_int_decrement_test_type {1};
+
+    EXPECT_EQ(--a, an_int_decrement_test_type {0});
+    EXPECT_EQ(a--, an_int_decrement_test_type {0});
+    EXPECT_EQ(a, an_int_decrement_test_type {-1});
+}
+
+}  // namespace twig

tests/src/can_increment_tests.cpp

@@ -0,0 +1,38 @@
+#include <cstdint>
+
+#include <gtest/gtest.h>
+#include <stronk/can_increment.h>
+#include <stronk/stronk.h>
+
+namespace twig
+{
+
+struct an_int_increment_test_type : stronk<an_int_increment_test_type, int, can_increment, can_equate>
+{
+    using stronk::stronk;
+};
+
+TEST(can_increment, can_increment_stronk_integer)
+{
+    auto a = an_int_increment_test_type {-1};
+
+    EXPECT_EQ(++a, an_int_increment_test_type {0});
+    EXPECT_EQ(a++, an_int_increment_test_type {0});
+    EXPECT_EQ(a, an_int_increment_test_type {1});
+}
+
+struct a_uint64_t_increment_test_type : stronk<a_uint64_t_increment_test_type, uint64_t, can_increment, can_equate>
+{
+    using stronk::stronk;
+};
+
+TEST(can_increment, can_increment_stronk_uint64_t)
+{
+    auto a = a_uint64_t_increment_test_type {0};
+
+    EXPECT_EQ(++a, a_uint64_t_increment_test_type {1});
+    EXPECT_EQ(a++, a_uint64_t_increment_test_type {1});
+    EXPECT_EQ(a, a_uint64_t_increment_test_type {2});
+}
+
+}  // namespace twig

version.py

@@ -1 +1 @@
-__version__ = "0.3.7"
+__version__ = "0.3.8"