SanderMertens#1440 Add multiple_queries C++ example

examples/cpp/systems/multiple_queries/include/multiple_queries.h

@@ -0,0 +1,16 @@
+#ifndef MULTIPLE_QUERIES_H
+#define MULTIPLE_QUERIES_H
+
+/* This generated file contains includes for project dependencies */
+#include "multiple_queries/bake_config.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
+

examples/cpp/systems/multiple_queries/include/multiple_queries/bake_config.h

@@ -0,0 +1,24 @@
+/*
+                                   )
+                                  (.)
+                                  .|.
+                                  | |
+                              _.--| |--._
+                           .-';  ;`-'& ; `&.
+                          \   &  ;    &   &_/
+                           |"""---...---"""|
+                           \ | | | | | | | /
+                            `---.|.|.|.---'
+
+ * This file is generated by bake.lang.c for your convenience. Headers of
+ * dependencies will automatically show up in this file. Include bake_config.h
+ * in your main project file. Do not edit! */
+
+#ifndef MULTIPLE_QUERIES_BAKE_CONFIG_H
+#define MULTIPLE_QUERIES_BAKE_CONFIG_H
+
+/* Headers of public dependencies */
+#include <flecs.h>
+
+#endif
+

examples/cpp/systems/multiple_queries/project.json

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+{
+    "id": "multiple_queries",
+    "type": "application",
+    "value": {
+        "use": [
+            "flecs"
+        ],
+        "language": "c++",
+        "private": false
+    }
+}

examples/cpp/systems/multiple_queries/src/main.cpp

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+#include <multiple_queries.h>
+#include <iostream>
+
+// This example shows how to pass one or more existing queries to a system, 
+// which allows for systems that iterate queries multiple times and/or iterate
+// multiple queries.
+
+struct Position { 
+    float x, y;
+};
+
+struct Bee {
+    static const int MaxRange = 10;
+    flecs::entity flower;
+};
+
+struct Flower { };
+
+float randf(int n) {
+    return static_cast<float>(rand() % n);
+}
+
+float pow2(float n) {
+    return n * n;
+}
+
+int main(int, char *[]) {
+    flecs::world ecs;
+
+    // Create two queries. Since they'll stick around until the end of the ECS
+    // world (or until we delete them) give them a name which makes them easier
+    // to find in the explorer.
+    // Giving the query a name also causes the caching policy to default to 
+    // cached. Named queries are (very) likely candidates for being evaluated
+    // many times, which is the sweet spot for caching.
+
+    flecs::query<const Position, Bee> bees = 
+        ecs.query<const Position, Bee>("BeeQuery");
+
+    flecs::query<const Position> flowers = 
+        ecs.query_builder<const Position>("FlowerQuery")
+            .with<Flower>()
+            .build();
+
+    // Find the closest in range flower for each bee.
+    ecs.system("FlowersAndTheBees")
+        // Capture queries by value. This prevents common bugs caused by local
+        // variables going out of scope before the system runs.
+        .run([bees, flowers](flecs::iter&) {
+            bees.each([&](flecs::entity bee, const Position& p_bee, Bee& b) {
+                float dsqr_min = 2 * pow2(Bee::MaxRange);
+
+                b.flower = flecs::entity();
+
+                flowers.each([&](flecs::entity flower, const Position& p_flower) {
+                    float dsqr = pow2(p_bee.x - p_flower.x) + 
+                                 pow2(p_bee.y - p_flower.y);
+
+                    if (dsqr < dsqr_min) {
+                        dsqr_min = dsqr;
+                        b.flower = flower;
+                    }
+                });
+
+                if (b.flower) {
+                    std::cout << "Bee " << bee.path() 
+                              << " picked flower " << b.flower.path() 
+                              << std::endl;
+                }
+            });
+        });
+
+    // Find bees that picked the same flower.
+    ecs.system("BumpingBees")
+        .run([bees](flecs::iter&) {
+            bees.each([&](flecs::entity bee1, const Position&, Bee& b1) {
+                bees.each([&](flecs::entity bee2, const Position&, Bee& b2) {
+                    if (bee1 > bee2) { // Don't handle the same collision twice
+                        if (b1.flower && b1.flower == b2.flower) {
+                            std::cout << "Bee " << bee1.path() 
+                                      << " and bee " << bee2.path()
+                                      << " bumped into each other"
+                                      << std::endl;
+                        }
+                    }
+                });
+            });
+        });
+
+    // Create flowers and bees
+    for (int i = 0; i < 5; i ++) {
+        ecs.entity()
+            .add<Bee>() // Add with default value
+            .set(Position{ randf(20), randf(20) });
+    }
+
+    for (int i = 0; i < 10; i ++) {
+        ecs.entity()
+            .add<Flower>()
+            .set(Position{ randf(20), randf(20) });
+    }
+
+    ecs.progress();
+
+    // Output
+    //  Bee #544 picked flower #552
+    //  Bee #545 picked flower #555
+    //  Bee #546 picked flower #552
+    //  Bee #547 picked flower #555
+    //  Bee #548 picked flower #554
+    //  Bee #546 and bee #544 bumped into each other
+    //  Bee #547 and bee #545 bumped into each other
+}