From 2dc22e7aad197236479b5e84240c0120073fd59e Mon Sep 17 00:00:00 2001
From: Rong Ou <rong.ou@gmail.com>
Date: Fri, 3 Mar 2023 08:24:13 -0800
Subject: [PATCH] Take advantage of C++17 features (#8858)

---------

Co-authored-by: Hyunsu Philip Cho <chohyu01@cs.washington.edu>
Co-authored-by: Jiaming Yuan <jm.yuan@outlook.com>
---
 R-package/src/Makevars.in  |  2 +-
 R-package/src/Makevars.win |  2 +-
 src/common/common.h        | 25 -------------------------
 src/learner.cc             | 12 ++++++------
 4 files changed, 8 insertions(+), 33 deletions(-)

diff --git a/R-package/src/Makevars.in b/R-package/src/Makevars.in
index 5cbfd0684..ed3f10571 100644
--- a/R-package/src/Makevars.in
+++ b/R-package/src/Makevars.in
@@ -3,7 +3,7 @@ PKGROOT=../../
 ENABLE_STD_THREAD=1
 # _*_ mode: Makefile; _*_
 
-CXX_STD = CXX14
+CXX_STD = CXX17
 
 XGB_RFLAGS = -DXGBOOST_STRICT_R_MODE=1 -DDMLC_LOG_BEFORE_THROW=0\
            -DDMLC_ENABLE_STD_THREAD=$(ENABLE_STD_THREAD) -DDMLC_DISABLE_STDIN=1\
diff --git a/R-package/src/Makevars.win b/R-package/src/Makevars.win
index e8e1579f7..024ba1aa1 100644
--- a/R-package/src/Makevars.win
+++ b/R-package/src/Makevars.win
@@ -3,7 +3,7 @@ PKGROOT=../../
 ENABLE_STD_THREAD=0
 # _*_ mode: Makefile; _*_
 
-CXX_STD = CXX14
+CXX_STD = CXX17
 
 XGB_RFLAGS = -DXGBOOST_STRICT_R_MODE=1 -DDMLC_LOG_BEFORE_THROW=0\
            -DDMLC_ENABLE_STD_THREAD=$(ENABLE_STD_THREAD) -DDMLC_DISABLE_STDIN=1\
diff --git a/src/common/common.h b/src/common/common.h
index 6bc96dfc9..35c807bef 100644
--- a/src/common/common.h
+++ b/src/common/common.h
@@ -195,31 +195,6 @@ template <typename Indexable>
 XGBOOST_DEVICE size_t LastOf(size_t group, Indexable const &indptr) {
   return indptr[group + 1] - 1;
 }
-
-/**
- * \brief A CRTP (curiously recurring template pattern) helper function.
- *
- * https://www.fluentcpp.com/2017/05/19/crtp-helper/
- *
- * Does two things:
- * 1. Makes "crtp" explicit in the inheritance structure of a CRTP base class.
- * 2. Avoids having to `static_cast` in a lot of places.
- *
- * \tparam T The derived class in a CRTP hierarchy.
- */
-template <typename T>
-struct Crtp {
-  T &Underlying() { return static_cast<T &>(*this); }
-  T const &Underlying() const { return static_cast<T const &>(*this); }
-};
-
-/**
- * \brief C++17 std::as_const
- */
-template <typename T>
-typename std::add_const<T>::type &AsConst(T &v) noexcept {  // NOLINT(runtime/references)
-  return v;
-}
 }  // namespace common
 }  // namespace xgboost
 #endif  // XGBOOST_COMMON_COMMON_H_
diff --git a/src/learner.cc b/src/learner.cc
index dfcab281d..0e47c694c 100644
--- a/src/learner.cc
+++ b/src/learner.cc
@@ -21,7 +21,7 @@
 #include <sstream>
 #include <stack>
 #include <string>
-#include <utility>
+#include <utility>  // for as_const
 #include <vector>
 
 #include "collective/communicator-inl.h"
@@ -257,11 +257,11 @@ LearnerModelParam::LearnerModelParam(Context const* ctx, LearnerModelParamLegacy
     : LearnerModelParam{user_param, t} {
   std::swap(base_score_, base_margin);
   // Make sure read access everywhere for thread-safe prediction.
-  common::AsConst(base_score_).HostView();
+  std::as_const(base_score_).HostView();
   if (!ctx->IsCPU()) {
-    common::AsConst(base_score_).View(ctx->gpu_id);
+    std::as_const(base_score_).View(ctx->gpu_id);
   }
-  CHECK(common::AsConst(base_score_).Data()->HostCanRead());
+  CHECK(std::as_const(base_score_).Data()->HostCanRead());
 }
 
 linalg::TensorView<float const, 1> LearnerModelParam::BaseScore(int32_t device) const {
@@ -287,9 +287,9 @@ void LearnerModelParam::Copy(LearnerModelParam const& that) {
   base_score_.Reshape(that.base_score_.Shape());
   base_score_.Data()->SetDevice(that.base_score_.DeviceIdx());
   base_score_.Data()->Copy(*that.base_score_.Data());
-  common::AsConst(base_score_).HostView();
+  std::as_const(base_score_).HostView();
   if (that.base_score_.DeviceIdx() != Context::kCpuId) {
-    common::AsConst(base_score_).View(that.base_score_.DeviceIdx());
+    std::as_const(base_score_).View(that.base_score_.DeviceIdx());
   }
   CHECK_EQ(base_score_.Data()->DeviceCanRead(), that.base_score_.Data()->DeviceCanRead());
   CHECK(base_score_.Data()->HostCanRead());