[commands] Add C++ Requirements struct (#5504)

wpilibNewCommands/src/main/native/include/frc2/command/Requirements.h

@@ -0,0 +1,46 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#pragma once
+
+#include <initializer_list>
+#include <span>
+#include <vector>
+
+#include "frc2/command/Subsystem.h"
+
+namespace frc2 {
+
+/**
+ * Represents requirements for a command, which is a set of (pointers to)
+ * subsystems. This class is implicitly convertible from std::initializer_list
+ * and std::span.
+ */
+class Requirements {
+ public:
+  // NOLINTNEXTLINE
+  /*implicit*/ Requirements(std::initializer_list<Subsystem*> requirements)
+      : m_subsystems{requirements.begin(), requirements.end()} {}
+
+  // NOLINTNEXTLINE
+  /*implicit*/ Requirements(std::span<Subsystem* const> requirements)
+      : m_subsystems{requirements.begin(), requirements.end()} {}
+
+  Requirements() = default;
+
+  Requirements(const Requirements&) = default;
+
+  std::vector<Subsystem*>::const_iterator begin() const {
+    return m_subsystems.begin();
+  }
+
+  std::vector<Subsystem*>::const_iterator end() const {
+    return m_subsystems.end();
+  }
+
+ private:
+  std::vector<Subsystem*> m_subsystems;
+};
+
+}  // namespace frc2