Extract transform iterator. (#8498)

tests/cpp/common/test_transform_iterator.cc

@@ -0,0 +1,20 @@
+/**
+ * Copyright 2022 by XGBoost Contributors
+ */
+#include <gtest/gtest.h>
+
+#include <cstddef>  // std::size_t
+
+#include "../../../src/common/transform_iterator.h"
+
+namespace xgboost {
+namespace common {
+TEST(IndexTransformIter, Basic) {
+  auto sqr = [](std::size_t i) { return i * i; };
+  auto iter = MakeIndexTransformIter(sqr);
+  for (std::size_t i = 0; i < 4; ++i) {
+    ASSERT_EQ(iter[i], sqr(i));
+  }
+}
+}  // namespace common
+}  // namespace xgboost