Change cudf::scalar copy and move constructors to protected (#8857)

An instance of the `cudf::scalar` base class can currently through it's public copy constructor using the following example:
```
cudf::scalar b = cudf::numeric_scalar<int32_t>(8);
```

This line builds the numeric scalar instance, passes it to the base scalar's copy-ctor, and then destroys it.
(Simpler example here to show that this is happening: https://godbolt.org/z/hGEGvsqjq )
This means that the `b` instance is actually a plain `cudf::scalar` with no derived type.

Downcasting with dynamic_cast can runtime-check the object but we could have this line fail at compile time by making the `cudf::scalar` copy-ctor protected.

We could make the ctor explicit but I think making it protected would more clearly communicate the intention of the class.

This PR makes the `cudf::scalar` copy and move constructors protected to prevent instantiating a `cudf::scalar` at compile time.

Authors:
  - David Wendt (https://github.com/davidwendt)

Approvers:
  - Karthikeyan (https://github.com/karthikeyann)
  - Vyas Ramasubramani (https://github.com/vyasr)

URL: #8857

cpp/include/cudf/scalar/scalar.hpp

@@ -45,23 +45,10 @@ namespace cudf {
  */
 class scalar {
  public:
-  virtual ~scalar()      = default;
-  scalar(scalar&& other) = default;
-
+  virtual ~scalar() = default;
   scalar& operator=(scalar const& other) = delete;
   scalar& operator=(scalar&& other) = delete;
 
-  /**
-   * @brief Construct a new scalar object by deep copying another.
-   *
-   * @param other The scalar to copy.
-   * @param stream CUDA stream used for device memory operations.
-   * @param mr Device memory resource to use for device memory allocation.
-   */
-  scalar(scalar const& other,
-         rmm::cuda_stream_view stream        = rmm::cuda_stream_default,
-         rmm::mr::device_memory_resource* mr = rmm::mr::get_current_device_resource());
-
   /**
    * @brief Returns the scalar's logical value type.
    */
@@ -103,6 +90,19 @@ class scalar {
 
   scalar() = delete;
 
+  scalar(scalar&& other) = default;
+
+  /**
+   * @brief Construct a new scalar object by deep copying another.
+   *
+   * @param other The scalar to copy.
+   * @param stream CUDA stream used for device memory operations.
+   * @param mr Device memory resource to use for device memory allocation.
+   */
+  scalar(scalar const& other,
+         rmm::cuda_stream_view stream        = rmm::cuda_stream_default,
+         rmm::mr::device_memory_resource* mr = rmm::mr::get_current_device_resource());
+
   /**
    * @brief Construct a new scalar object.
    *

cpp/tests/binaryop/binop-fixture.hpp

@@ -74,7 +74,7 @@ struct BinaryOperationTest : public cudf::test::BaseFixture {
     cudf::test::UniformRandomGenerator<uint8_t> rand_gen(r_min, r_max);
     uint8_t size = rand_gen.generate();
     std::string str{"ஔⒶbc⁂∰ൠ \tنж水✉♪✿™"};
-    return cudf::scalar_type_t<T>(string_view(str.data(), size));
+    return cudf::scalar_type_t<T>(str.substr(0, size));
   }
 };