From 38f7318a85136b73a461c9f86c5136fc309b316b Mon Sep 17 00:00:00 2001
From: Roland Reichwein <Roland.Reichwein@bmw.de>
Date: Thu, 16 Jul 2026 10:15:24 +0200
Subject: [PATCH] Add etl::apply (#1509)

Fixes: #1272

Co-authored-by: John Wellbelove <jwellbelove@users.noreply.github.com>
---
 docs/types/tuple.md | 18 ++++++++++
 include/etl/tuple.h | 30 +++++++++++++++++
 test/test_tuple.cpp | 82 +++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 130 insertions(+)

diff --git a/docs/types/tuple.md b/docs/types/tuple.md
index 5981e34c..666719fa 100644
--- a/docs/types/tuple.md
+++ b/docs/types/tuple.md
@@ -249,3 +249,21 @@ template <typename T, typename... Types>
 ETL_CONSTEXPR14
 const T&& get(const tuple<Types...>&&);
 ```
+
+---
+
+```cpp
+template <typename TFunction, typename TTuple>
+ETL_CONSTEXPR14
+auto apply(TFunction&& f, TTuple&& t);
+```
+
+Invokes the callable object `f` (via `etl::invoke`) with the elements of the tuple `t` expanded as its arguments. Equivalent to `std::apply`.
+
+```cpp
+int add(int a, int b, int c) { return a + b + c; }
+
+auto t = etl::make_tuple(1, 2, 3);
+int sum = etl::apply(add, t); // sum == 6
+```
+
diff --git a/include/etl/tuple.h b/include/etl/tuple.h
index 5b7257ec..a20def79 100644
--- a/include/etl/tuple.h
+++ b/include/etl/tuple.h
@@ -39,6 +39,7 @@ SOFTWARE.
   #endif
 
   #include "functional.h"
+  #include "invoke.h"
   #include "nth_type.h"
   #include "type_list.h"
   #include "type_traits.h"
@@ -934,6 +935,35 @@ namespace etl
     return tuple<TTypes&&...>(etl::forward<TTypes>(args)...);
   }
 
+  namespace private_tuple
+  {
+    //**********************************
+    // Helper for apply.
+    // Invokes the callable with the tuple elements expanded via the index sequence.
+    // Uses an unqualified 'get' so that argument-dependent lookup finds the
+    // appropriate overload for etl::tuple, etl::array, std::tuple, std::pair, etc.
+    //**********************************
+    template <typename TFunction, typename TTuple, size_t... Indices>
+    ETL_CONSTEXPR14 auto apply_impl(TFunction&& f, TTuple&& t, etl::index_sequence<Indices...>)
+      -> decltype(etl::invoke(etl::forward<TFunction>(f), get<Indices>(etl::forward<TTuple>(t))...))
+    {
+      return etl::invoke(etl::forward<TFunction>(f), get<Indices>(etl::forward<TTuple>(t))...);
+    }
+  } // namespace private_tuple
+
+  //***************************************************************************
+  /// Invokes the callable object 'f' with the elements of the tuple 't' as
+  /// arguments. Equivalent to std::apply.
+  //***************************************************************************
+  template <typename TFunction, typename TTuple>
+  ETL_CONSTEXPR14 auto apply(TFunction&& f, TTuple&& t)
+    -> decltype(private_tuple::apply_impl(etl::forward<TFunction>(f), etl::forward<TTuple>(t),
+                                          etl::make_index_sequence<etl::tuple_size<etl::remove_reference_t<TTuple>>::value>{}))
+  {
+    return private_tuple::apply_impl(etl::forward<TFunction>(f), etl::forward<TTuple>(t),
+                                     etl::make_index_sequence<etl::tuple_size<etl::remove_reference_t<TTuple>>::value>{});
+  }
+
   namespace private_tuple
   {
     //**********************************
diff --git a/test/test_tuple.cpp b/test/test_tuple.cpp
index d3816d6f..18b10bdf 100644
--- a/test/test_tuple.cpp
+++ b/test/test_tuple.cpp
@@ -76,6 +76,32 @@ namespace
     int i;
   };
 
+  //*********************************
+  int apply_sum(int a, int b, int c)
+  {
+    return a + b + c;
+  }
+
+  //*********************************
+  struct Summer
+  {
+    int operator()(int a, int b) const
+    {
+      return a + b;
+    }
+  };
+
+  //*********************************
+  struct Accumulator
+  {
+    int total;
+
+    void add(int a, int b)
+    {
+      total = a + b;
+    }
+  };
+
   SUITE(test_tuple)
   {
     //*************************************************************************
@@ -930,5 +956,61 @@ namespace
       CHECK_EQUAL(1, i);
       CHECK_EQUAL(std::string("2"), d.value);
     }
+
+    //*************************************************************************
+    TEST(test_apply_free_function)
+    {
+      etl::tuple<int, int, int> t(1, 2, 3);
+
+      CHECK_EQUAL(6, etl::apply(apply_sum, t));
+    }
+
+    //*************************************************************************
+    TEST(test_apply_function_object)
+    {
+      etl::tuple<int, int> t(10, 20);
+
+      CHECK_EQUAL(30, etl::apply(Summer(), t));
+    }
+
+    //*************************************************************************
+#if ETL_USING_CPP11
+    TEST(test_apply_lambda)
+    {
+      etl::tuple<int, int, int> t(2, 3, 4);
+
+      int result = etl::apply([](int a, int b, int c) { return a * b * c; }, t);
+
+      CHECK_EQUAL(24, result);
+    }
+#endif
+
+    //*************************************************************************
+    TEST(test_apply_member_function)
+    {
+      Accumulator acc = {0};
+
+      etl::apply(&Accumulator::add, etl::make_tuple(&acc, 5, 7));
+
+      CHECK_EQUAL(12, acc.total);
+    }
+
+    //*************************************************************************
+    TEST(test_apply_rvalue_tuple)
+    {
+      CHECK_EQUAL(6, etl::apply(apply_sum, etl::make_tuple(1, 2, 3)));
+    }
+
+    //*************************************************************************
+#if ETL_USING_CPP11
+    TEST(test_apply_empty_tuple)
+    {
+      etl::tuple<> t;
+
+      int result = etl::apply([]() { return 42; }, t);
+
+      CHECK_EQUAL(42, result);
+    }
+#endif
   }
 } // namespace