Replaced std::string with std::string_view and removed excessive copi…

…es in cudf::io (#17734)

As part of the improvement effort discussed in #15907, this merge request removes some of the excessive `std::string` copies and uses `std::string_view` in place of `std::string` when the lifetime semantics are clear.

`std::string` is only replaced in this MR in linear functions and constructors, but not in structs as there's no established ownership or lifetime semantics to guarantee the `string_view`s will not outlive their source.
There were also some cases of excessive copies, i.e. consider:

```cpp
struct source_info{
source_info(std::string const& s) : str{s}{}

private:
std::string str;
};
```

In the above example, the string is likely to be allocated twice if a temporary/string-literal is used to construct "s": one for the temporary and one for the copy constructor for `str`

```cpp
struct source_info{
source_info(std::string  s) : str{std::move(s)}{}

private:
std::string str;
};
```

The string is only allocated once in all scenarios.
This also applies to `std::vector` and is arguably worse as there's no small-vector-optimization (i.e. `std::string`'s small-string-optimization/SSO).

Authors:
  - Basit Ayantunde (https://github.com/lamarrr)
  - Muhammad Haseeb (https://github.com/mhaseeb123)
  - Vyas Ramasubramani (https://github.com/vyasr)

Approvers:
  - Tianyu Liu (https://github.com/kingcrimsontianyu)
  - Muhammad Haseeb (https://github.com/mhaseeb123)
  - David Wendt (https://github.com/davidwendt)

URL: #17734

cpp/benchmarks/json/json.cu

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021-2024, NVIDIA CORPORATION.
+ * Copyright (c) 2021-2025, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -190,10 +190,10 @@ static void bench_query(nvbench::state& state)
 {
   srand(5236);
 
-  auto const num_rows      = static_cast<cudf::size_type>(state.get_int64("num_rows"));
-  auto const desired_bytes = static_cast<cudf::size_type>(state.get_int64("bytes"));
-  auto const query         = state.get_int64("query");
-  auto const json_path     = queries[query];
+  auto const num_rows              = static_cast<cudf::size_type>(state.get_int64("num_rows"));
+  auto const desired_bytes         = static_cast<cudf::size_type>(state.get_int64("bytes"));
+  auto const query                 = state.get_int64("query");
+  std::string_view const json_path = queries[query];
 
   auto const stream = cudf::get_default_stream();
   auto input        = build_json_string_column(desired_bytes, num_rows);

cpp/benchmarks/string/join_strings.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2023-2024, NVIDIA CORPORATION.
+ * Copyright (c) 2023-2025, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -41,8 +41,8 @@ static void bench_join(nvbench::state& state)
   state.add_global_memory_reads<nvbench::int8_t>(chars_size);   // all bytes are read;
   state.add_global_memory_writes<nvbench::int8_t>(chars_size);  // all bytes are written
 
-  std::string separator(":");
-  std::string narep("null");
+  std::string_view separator(":");
+  std::string_view narep("null");
   state.exec(nvbench::exec_tag::sync, [&](nvbench::launch& launch) {
     auto result = cudf::strings::join_strings(input, separator, narep);
   });

cpp/benchmarks/string/like.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022-2024, NVIDIA CORPORATION.
+ * Copyright (c) 2022-2025, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -34,7 +34,7 @@ static void bench_like(nvbench::state& state)
   auto input = cudf::strings_column_view(col->view());
 
   // This pattern forces reading the entire target string (when matched expected)
-  auto pattern = std::string("% 5W4_");  // regex equivalent: ".* 5W4.$"
+  auto pattern = std::string_view("% 5W4_");  // regex equivalent: ".* 5W4.$"
 
   state.set_cuda_stream(nvbench::make_cuda_stream_view(cudf::get_default_stream().value()));
   // gather some throughput statistics as well

cpp/benchmarks/string/replace_re.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021-2024, NVIDIA CORPORATION.
+ * Copyright (c) 2021-2025, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -49,7 +49,7 @@ static void bench_replace(nvbench::state& state)
       cudf::strings::replace_with_backrefs(input, *program, replacement);
     });
   } else {
-    auto replacement = std::string("77");
+    auto replacement = std::string_view("77");
     state.exec(nvbench::exec_tag::sync, [&](nvbench::launch& launch) {
       cudf::strings::replace_re(input, *program, replacement);
     });

cpp/examples/strings/libcudf_apis.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022-2023, NVIDIA CORPORATION.
+ * Copyright (c) 2022-2025, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -53,7 +53,7 @@ std::unique_ptr<cudf::column> redact_strings(cudf::column_view const& names,
 
   auto const last_initial_first = cudf::table_view({last_initial->view(), first});
 
-  auto result = cudf::strings::concatenate(last_initial_first, std::string(" "));
+  auto result = cudf::strings::concatenate(last_initial_first, std::string_view(" "));
 
   cudaStreamSynchronize(0);
 

cpp/include/cudf/io/csv.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020-2024, NVIDIA CORPORATION.
+ * Copyright (c) 2020-2025, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -916,7 +916,7 @@ class csv_reader_options_builder {
    */
   csv_reader_options_builder& prefix(std::string pfx)
   {
-    options._prefix = pfx;
+    options._prefix = std::move(pfx);
     return *this;
   }
 
@@ -1450,7 +1450,7 @@ class csv_writer_options {
    *
    * @return string to used for null entries
    */
-  [[nodiscard]] std::string get_na_rep() const { return _na_rep; }
+  [[nodiscard]] std::string const& get_na_rep() const { return _na_rep; }
 
   /**
    * @brief Whether to write headers to csv.
@@ -1471,7 +1471,7 @@ class csv_writer_options {
    *
    * @return Character used for separating lines
    */
-  [[nodiscard]] std::string get_line_terminator() const { return _line_terminator; }
+  [[nodiscard]] std::string const& get_line_terminator() const { return _line_terminator; }
 
   /**
    * @brief Returns character used for separating column values.
@@ -1485,14 +1485,14 @@ class csv_writer_options {
    *
    * @return string used for values != 0 in INT8 types
    */
-  [[nodiscard]] std::string get_true_value() const { return _true_value; }
+  [[nodiscard]] std::string const& get_true_value() const { return _true_value; }
 
   /**
    * @brief Returns string used for values == 0 in INT8 types.
    *
    * @return string used for values == 0 in INT8 types
    */
-  [[nodiscard]] std::string get_false_value() const { return _false_value; }
+  [[nodiscard]] std::string const& get_false_value() const { return _false_value; }
 
   /**
    * @brief Returns the quote style for the writer.
@@ -1519,7 +1519,7 @@ class csv_writer_options {
    *
    * @param val String to represent null value
    */
-  void set_na_rep(std::string val) { _na_rep = val; }
+  void set_na_rep(std::string val) { _na_rep = std::move(val); }
 
   /**
    * @brief Enables/Disables headers being written to csv.
@@ -1540,7 +1540,7 @@ class csv_writer_options {
    *
    * @param term Character to represent line termination
    */
-  void set_line_terminator(std::string term) { _line_terminator = term; }
+  void set_line_terminator(std::string term) { _line_terminator = std::move(term); }
 
   /**
    * @brief Sets character used for separating column values.
@@ -1554,14 +1554,14 @@ class csv_writer_options {
    *
    * @param val String to represent values != 0 in INT8 types
    */
-  void set_true_value(std::string val) { _true_value = val; }
+  void set_true_value(std::string val) { _true_value = std::move(val); }
 
   /**
    * @brief Sets string used for values == 0 in INT8 types.
    *
    * @param val String to represent values == 0 in INT8 types
    */
-  void set_false_value(std::string val) { _false_value = val; }
+  void set_false_value(std::string val) { _false_value = std::move(val); }
 
   /**
    * @brief (Re)sets the table being written.

cpp/include/cudf/io/text/detail/trie.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021-2024, NVIDIA CORPORATION.
+ * Copyright (c) 2021-2025, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -27,6 +27,7 @@
 #include <algorithm>
 #include <queue>
 #include <string>
+#include <string_view>
 #include <unordered_map>
 #include <vector>
 
@@ -128,7 +129,7 @@ struct trie {
     /**
      * @brief Insert the string in to the trie tree, growing the trie as necessary
      */
-    void insert(std::string s) { insert(s.c_str(), s.size(), 0); }
+    void insert(std::string_view s) { insert(s.data(), s.size(), 0); }
 
    private:
     trie_builder_node& insert(char const* s, uint16_t size, uint8_t depth)
@@ -164,12 +165,12 @@ struct trie {
    * @param mr Memory resource to use for the device memory allocation
    * @return The trie.
    */
-  static trie create(std::string const& pattern,
+  static trie create(std::string pattern,
                      rmm::cuda_stream_view stream,
                      rmm::device_async_resource_ref mr)
 
   {
-    return create(std::vector<std::string>{pattern}, stream, mr);
+    return create(std::vector<std::string>{std::move(pattern)}, stream, mr);
   }
 
   /**

cpp/include/cudf/io/text/multibyte_split.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021-2024, NVIDIA CORPORATION.
+ * Copyright (c) 2021-2025, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -25,6 +25,7 @@
 
 #include <memory>
 #include <optional>
+#include <string_view>
 
 namespace CUDF_EXPORT cudf {
 namespace io {
@@ -90,7 +91,7 @@ struct parse_options {
  */
 std::unique_ptr<cudf::column> multibyte_split(
   data_chunk_source const& source,
-  std::string const& delimiter,
+  std::string_view delimiter,
   parse_options options             = {},
   rmm::cuda_stream_view stream      = cudf::get_default_stream(),
   rmm::device_async_resource_ref mr = cudf::get_current_device_resource_ref());

cpp/include/cudf/io/types.hpp

@@ -353,8 +353,8 @@ struct source_info {
    *
    * @param file_paths Input files paths
    */
-  explicit source_info(std::vector<std::string> const& file_paths)
-    : _type(io_type::FILEPATH), _filepaths(file_paths)
+  explicit source_info(std::vector<std::string> file_paths)
+    : _type(io_type::FILEPATH), _filepaths(std::move(file_paths))
   {
   }
 
@@ -363,8 +363,8 @@ struct source_info {
    *
    * @param file_path Single input file
    */
-  explicit source_info(std::string const& file_path)
-    : _type(io_type::FILEPATH), _filepaths({file_path})
+  explicit source_info(std::string file_path)
+    : _type(io_type::FILEPATH), _filepaths({std::move(file_path)})
   {
   }
 
@@ -534,8 +534,8 @@ struct sink_info {
    *
    * @param file_paths Output files paths
    */
-  explicit sink_info(std::vector<std::string> const& file_paths)
-    : _type(io_type::FILEPATH), _num_sinks(file_paths.size()), _filepaths(file_paths)
+  explicit sink_info(std::vector<std::string> file_paths)
+    : _type(io_type::FILEPATH), _num_sinks(file_paths.size()), _filepaths(std::move(file_paths))
   {
   }
 
@@ -544,8 +544,8 @@ struct sink_info {
    *
    * @param file_path Single output file path
    */
-  explicit sink_info(std::string const& file_path)
-    : _type(io_type::FILEPATH), _filepaths({file_path})
+  explicit sink_info(std::string file_path)
+    : _type(io_type::FILEPATH), _filepaths({std::move(file_path)})
   {
   }
 
@@ -554,8 +554,8 @@ struct sink_info {
    *
    * @param buffers Output host buffers
    */
-  explicit sink_info(std::vector<std::vector<char>*> const& buffers)
-    : _type(io_type::HOST_BUFFER), _num_sinks(buffers.size()), _buffers(buffers)
+  explicit sink_info(std::vector<std::vector<char>*> buffers)
+    : _type(io_type::HOST_BUFFER), _num_sinks(buffers.size()), _buffers(std::move(buffers))
   {
   }
   /**
@@ -571,7 +571,9 @@ struct sink_info {
    * @param user_sinks Output user-implemented sinks
    */
   explicit sink_info(std::vector<cudf::io::data_sink*> const& user_sinks)
-    : _type(io_type::USER_IMPLEMENTED), _num_sinks(user_sinks.size()), _user_sinks(user_sinks)
+    : _type(io_type::USER_IMPLEMENTED),
+      _num_sinks(user_sinks.size()),
+      _user_sinks(std::move(user_sinks))
   {
   }
 
@@ -821,7 +823,7 @@ class column_in_metadata {
    *
    * @return The name of this column
    */
-  [[nodiscard]] std::string get_name() const noexcept { return _name; }
+  [[nodiscard]] std::string const& get_name() const noexcept { return _name; }
 
   /**
    * @brief Get whether nullability has been explicitly set for this column.

cpp/include/cudf/scalar/scalar.hpp

@@ -27,6 +27,8 @@
 #include <rmm/device_buffer.hpp>
 #include <rmm/device_scalar.hpp>
 
+#include <string_view>
+
 /**
  * @file
  * @brief Class definitions for cudf::scalar
@@ -454,7 +456,7 @@ class string_scalar : public scalar {
    * @param stream CUDA stream used for device memory operations.
    * @param mr Device memory resource to use for device memory allocation.
    */
-  string_scalar(std::string const& string,
+  string_scalar(std::string_view string,
                 bool is_valid                     = true,
                 rmm::cuda_stream_view stream      = cudf::get_default_stream(),
                 rmm::device_async_resource_ref mr = cudf::get_current_device_resource_ref());

cpp/src/io/avro/avro.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2019-2024, NVIDIA CORPORATION.
+ * Copyright (c) 2019-2025, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -297,7 +297,7 @@ enum attrtype_e {
  *
  * @returns true if successful, false if error
  */
-bool schema_parser::parse(std::vector<schema_entry>& schema, std::string const& json_str)
+bool schema_parser::parse(std::vector<schema_entry>& schema, std::string_view json_str)
 {
   // Empty schema
   if (json_str == "[]") return true;
@@ -306,7 +306,7 @@ bool schema_parser::parse(std::vector<schema_entry>& schema, std::string const&
   int depth = 0, parent_idx = -1, entry_idx = -1;
   json_state_e state = state_attrname;
   std::string str;
-  std::unordered_map<std::string, type_kind_e> const typenames = {
+  std::unordered_map<std::string_view, type_kind_e> const typenames = {
     {"null", type_null},
     {"boolean", type_boolean},
     {"int", type_int},
@@ -329,17 +329,17 @@ bool schema_parser::parse(std::vector<schema_entry>& schema, std::string const&
     {"local-timestamp-millis", type_local_timestamp_millis},
     {"local-timestamp-micros", type_local_timestamp_micros},
     {"duration", type_duration}};
-  std::unordered_map<std::string, attrtype_e> const attrnames = {
+  std::unordered_map<std::string_view, attrtype_e> const attrnames = {
     {"type", attrtype_type},
     {"name", attrtype_name},
     {"fields", attrtype_fields},
     {"symbols", attrtype_symbols},
     {"items", attrtype_items},
     {"logicalType", attrtype_logicaltype}};
   attrtype_e cur_attr = attrtype_none;
-  m_base              = json_str.c_str();
+  m_base              = json_str.begin();
   m_cur               = m_base;
-  m_end               = m_base + json_str.length();
+  m_end               = json_str.end();
   while (more_data()) {
     int const c = *m_cur++;
     switch (c) {
@@ -487,7 +487,8 @@ std::string schema_parser::get_str()
     ;
   auto len = static_cast<int32_t>(cur - start - 1);
   m_cur    = cur;
-  return s.assign(start, std::max(len, 0));
+  s.assign(start, std::max(len, 0));
+  return s;
 }
 
 }  // namespace avro