add EnTT example (#78)

examples/entt/CMakeLists.txt

@@ -0,0 +1,24 @@
+cmake_minimum_required(VERSION 3.14 FATAL_ERROR)
+
+# ---- Dependencies ----
+
+include(../../cmake/CPM.cmake)
+
+CPMAddPackage(
+  NAME EnTT
+  VERSION 3.1.1
+  GITHUB_REPOSITORY skypjack/entt
+  # EnTT's CMakeLists screws with configuration options
+  DOWNLOAD_ONLY True
+)
+
+if (EnTT_ADDED)
+  add_library(EnTT INTERFACE)
+  target_include_directories(EnTT INTERFACE ${EnTT_SOURCE_DIR}/src)
+endif()
+
+# ---- Executable ----
+
+add_executable(CPMEnTTExample "main.cpp")
+set_target_properties(CPMEnTTExample PROPERTIES CXX_STANDARD 17)        
+target_link_libraries(CPMEnTTExample EnTT)

examples/entt/main.cpp

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+#include <entt/entt.hpp>
+#include <cstdint>
+
+struct position {
+    float x;
+    float y;
+};
+
+struct velocity {
+    float dx;
+    float dy;
+};
+
+void update(entt::registry &registry) {
+    auto view = registry.view<position, velocity>();
+
+    for(auto entity: view) {
+        // gets only the components that are going to be used ...
+
+        auto &vel = view.get<velocity>(entity);
+
+        vel.dx = 0.;
+        vel.dy = 0.;
+
+        // ...
+    }
+}
+
+void update(std::uint64_t dt, entt::registry &registry) {
+    registry.view<position, velocity>().each([dt](auto &pos, auto &vel) {
+        // gets all the components of the view at once ...
+
+        pos.x += vel.dx * dt;
+        pos.y += vel.dy * dt;
+
+        // ...
+    });
+}
+
+int main() {
+    entt::registry registry;
+    std::uint64_t dt = 16;
+
+    for(auto i = 0; i < 10; ++i) {
+        auto entity = registry.create();
+        registry.assign<position>(entity, i * 1.f, i * 1.f);
+        if(i % 2 == 0) { registry.assign<velocity>(entity, i * .1f, i * .1f); }
+    }
+
+    update(dt, registry);
+    update(registry);
+
+    // ...
+}