Prevent copying SimpleDMatrix. (#5453)

* Set default dtor for SimpleDMatrix to initialize default copy ctor, which is
deleted due to unique ptr.

* Remove commented code.
* Remove warning for calling host function (std::max).
* Remove warning for initialization order.
* Remove warning for unused variables.

src/common/common.h

@@ -64,6 +64,11 @@ inline std::vector<std::string> Split(const std::string& s, char delim) {
   return ret;
 }
 
+template <typename T>
+XGBOOST_DEVICE T Max(T a, T b) {
+  return a < b ? b : a;
+}
+
 // simple routine to convert any data to string
 template<typename T>
 inline std::string ToString(const T& data) {

src/common/compressed_iterator.h

@@ -8,6 +8,8 @@
 #include <cstddef>
 #include <algorithm>
 
+#include "common.h"
+
 #ifdef __CUDACC__
 #include "device_helpers.cuh"
 #endif  // __CUDACC__
@@ -29,12 +31,12 @@ inline void ClearBit(CompressedByteT *byte, int bit_idx) {
   *byte &= ~(1 << bit_idx);
 }
 static const int kPadding = 4;  // Assign padding so we can read slightly off
-                               // the beginning of the array
+                                // the beginning of the array
 
 // The number of bits required to represent a given unsigned range
 inline XGBOOST_DEVICE size_t SymbolBits(size_t num_symbols) {
   auto bits = std::ceil(log2(static_cast<double>(num_symbols)));
-  return std::max(static_cast<size_t>(bits), size_t(1));
+  return common::Max(static_cast<size_t>(bits), size_t(1));
 }
 }  // namespace detail
 

src/common/hist_util.h

@@ -297,9 +297,9 @@ struct Index {
   std::vector<uint8_t> data_;
   std::vector<uint32_t> offset_;  // size of this field is equal to number of features
   void* data_ptr_;
-  uint32_t* offset_ptr_;
-  size_t p_;
   BinTypeSize binTypeSize_;
+  size_t p_;
+  uint32_t* offset_ptr_;
   Func func_;
 };
 

src/common/quantile.h

@@ -710,7 +710,7 @@ class QuantileSketchTemplate {
     // check invariant
     size_t n = (1ULL << nlevel);
     CHECK(n * limit_size >= maxn) << "invalid init parameter";
-    CHECK(nlevel <= std::max(1, static_cast<int>(limit_size * eps)))
+    CHECK(nlevel <= std::max(static_cast<size_t>(1), static_cast<size_t>(limit_size * eps)))
         << "invalid init parameter";
   }