introduce overload class

and use it with std::visit when inspecting std::alternative

4 files changed, 44 insertions, 0 deletions

diff --git a/vespalib/CMakeLists.txt b/vespalib/CMakeLists.txt
index 14c14fe85ca..a76b8848b47 100644
--- a/vespalib/CMakeLists.txt
+++ b/vespalib/CMakeLists.txt
@@ -78,6 +78,7 @@ vespa_define_module(
     src/tests/net/tls/transport_options
     src/tests/objects/nbostream
     src/tests/optimized
+    src/tests/overload
     src/tests/portal
     src/tests/portal/handle_manager
     src/tests/portal/http_request
diff --git a/vespalib/src/tests/overload/CMakeLists.txt b/vespalib/src/tests/overload/CMakeLists.txt
new file mode 100644
index 00000000000..67aa6230225
--- /dev/null
+++ b/vespalib/src/tests/overload/CMakeLists.txt
@@ -0,0 +1,9 @@
+# Copyright 2020 Oath Inc. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+vespa_add_executable(vespalib_overload_test_app TEST
+    SOURCES
+    overload_test.cpp
+    DEPENDS
+    vespalib
+    gtest
+)
+vespa_add_test(NAME vespalib_overload_test_app COMMAND vespalib_overload_test_app)
diff --git a/vespalib/src/tests/overload/overload_test.cpp b/vespalib/src/tests/overload/overload_test.cpp
new file mode 100644
index 00000000000..ceae29ac02f
--- /dev/null
+++ b/vespalib/src/tests/overload/overload_test.cpp
@@ -0,0 +1,19 @@
+// Copyright 2020 Oath Inc. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+
+#include <vespa/vespalib/util/overload.h>
+#include <vespa/vespalib/gtest/gtest.h>
+#include <variant>
+#include <string>
+
+using namespace vespalib;
+
+TEST(OverloadTest, visit_with_overload_works) {
+    std::variant<std::string,int> a = 10;
+    std::variant<std::string,int> b = "foo";
+    std::visit(overload{[](int v){ EXPECT_EQ(v,10); },
+                        [](const std::string &){ FAIL() << "invalid visit"; }}, a);
+    std::visit(overload{[](int){ FAIL() << "invalid visit"; },
+                        [](const std::string &v){ EXPECT_EQ(v, "foo"); }}, b);
+}
+
+GTEST_MAIN_RUN_ALL_TESTS()
diff --git a/vespalib/src/vespa/vespalib/util/overload.h b/vespalib/src/vespa/vespalib/util/overload.h
new file mode 100644
index 00000000000..d5af9dee2d3
--- /dev/null
+++ b/vespalib/src/vespa/vespalib/util/overload.h
@@ -0,0 +1,15 @@
+// Copyright 2020 Oath Inc. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+
+#pragma once
+
+namespace vespalib {
+
+/**
+ * Simple overload lambda composition class. To be replaced by
+ * standard overload functionality when available. (C++20)
+ **/
+
+template<class... Ts> struct overload : Ts... { using Ts::operator()...; };
+template<class... Ts> overload(Ts...) -> overload<Ts...>;
+
+}