Dynamic arguments storage. Implementation of enhancement from issue

#1170.

First try...

include/fmt/core.h

@@ -1005,6 +1005,10 @@ template <typename Context> class basic_format_arg {
   friend FMT_CONSTEXPR basic_format_arg<ContextType> internal::make_arg(
       const T& value);
 
+  template <typename ContextType>
+  friend FMT_CONSTEXPR basic_format_arg<ContextType> internal::make_arg(
+      const named_arg_base<typename ContextType::char_type>& value);
+
   template <typename Visitor, typename Ctx>
   friend FMT_CONSTEXPR auto visit_format_arg(Visitor&& vis,
                                              const basic_format_arg<Ctx>& arg)
@@ -1177,6 +1181,15 @@ template <bool IS_PACKED, typename Context, typename T,
 inline basic_format_arg<Context> make_arg(const T& value) {
   return make_arg<Context>(value);
 }
+
+template <typename Context>
+FMT_CONSTEXPR basic_format_arg<Context> make_arg(
+        const named_arg_base<typename Context::char_type>& value) {
+  basic_format_arg<Context> arg;
+  arg.type_ = type::named_arg_type;
+  arg.value_ = value;
+  return arg;
+}
 }  // namespace internal
 
 // Formatting context.
@@ -1286,6 +1299,8 @@ inline format_arg_store<Context, Args...> make_format_args(
   return {args...};
 }
 
+template <typename Context, typename... Args> class dynamic_format_arg_store;
+
 /**
   \rst
   A view of a collection of formatting arguments. To avoid lifetime issues it
@@ -1357,6 +1372,17 @@ template <typename Context> class basic_format_args {
     set_data(store.data_);
   }
 
+  /**
+   \rst
+   Constructs a `dynamic_basic_format_args` object from `~fmt::format_arg_store`.
+   \endrst
+   */
+  template <typename... Args>
+  basic_format_args(const dynamic_format_arg_store<Context, Args...>& store)
+      : types_(store.get_types()) {
+    set_data(store.data_.data());
+  }
+
   /**
    \rst
    Constructs a `basic_format_args` object from a dynamic set of arguments.

include/fmt/dyn-args.h

@@ -0,0 +1,199 @@
+// Copyright (c) 2020 Vladimir Solontsov
+// SPDX-License-Identifier: MIT Licence
+
+#ifndef FMT_DYN_ARGS_H_
+#define FMT_DYN_ARGS_H_
+
+#include <vector>
+#include <forward_list>
+#include <functional>
+
+#include "core.h"
+
+#if (defined(FMT_HAS_VARIANT) || __cplusplus >= 201703L)
+#  include <variant>
+#  ifndef FMT_HAS_VARIANT
+#    define FMT_HAS_VARIANT
+#  endif
+#endif
+
+FMT_BEGIN_NAMESPACE
+
+namespace internal {
+
+template<typename T, typename Char>
+struct is_string_view : std::false_type{};
+
+template<typename Char>
+struct is_string_view<basic_string_view<Char>, Char>
+: std::true_type{};
+
+#ifdef FMT_USE_STRING_VIEW
+template<typename Traits, typename Char>
+struct is_string_view<std::basic_string_view<Char, Traits>, Char>
+: std::true_type{};
+#endif
+
+#ifdef FMT_USE_EXPERIMENTAL_STRING_VIEW
+template<typename Traits, typename Char>
+struct is_string_view<std::expeimental::basic_string_view<Char, Traits>, Char>
+: std::true_type{};
+#endif
+
+template<typename T>
+struct is_ref_wrapper : std::false_type{};
+
+template<typename T>
+struct is_ref_wrapper<std::reference_wrapper<T>> : std::true_type{};
+
+template<typename T, typename Context>
+struct need_dyn_copy{
+  using mapped_type = mapped_type_constant<T, Context>;
+  static_assert(mapped_type::value != internal::type::named_arg_type,
+    "Bug indicator. Named arguments must be processed separately");
+
+  using type = std::integral_constant<bool,!(
+      is_ref_wrapper<T>::value ||
+      is_string_view<T, typename Context::char_type>::value ||
+      (
+        mapped_type::value != internal::type::cstring_type &&
+        mapped_type::value != internal::type::custom_type &&
+        mapped_type::value != internal::type::string_type
+      )
+  )>;
+};
+
+template<typename T, typename Context> using need_dyn_copy_t =
+    typename need_dyn_copy<T, Context>::type;
+
+template<typename T, typename StorageValue>
+const T& get(const StorageValue& v){
+  return v;
+}
+
+}  // namespace internal
+
+/**
+  \rst
+  A dynamic version of `fmt::format_arg_store<>`.
+  It's equipped with a storage to potentially temporary objects which lifetime
+  could be shorter than the format arguments object.
+
+  It can be implicitly converted into `~fmt::basic_format_args` for passing
+  into type-erased formatting functions such as `~fmt::vformat`.
+  \endrst
+ */
+template <typename Context, typename... Args>
+class dynamic_format_arg_store
+#if FMT_GCC_VERSION < 409
+    // Workaround a GCC template argument substitution bug.
+    : public basic_format_args<Context>
+#endif
+{
+ private:
+  using char_type = typename Context::char_type;
+  static const bool is_packed = false;
+
+  static const bool has_custom_args = (sizeof...(Args) > 0);
+  using string_type = std::basic_string<char_type>;
+#ifdef FMT_HAS_VARIANT
+  using storage_item_type =
+      conditional_t<has_custom_args,
+                    std::variant<string_type, Args...>,
+                    string_type>;
+#else
+  static_assert(!has_custom_args, "std::variant<> is required to support "
+      "custom types in dynamic_format_arg_store");
+  using storage_item_type = string_type;
+#endif
+  using value_type = basic_format_arg<Context>;
+  using named_value_type = internal::named_arg_base<char_type>;
+
+  template<typename T>
+  using storaged_type =
+      conditional_t<internal::is_string<T>::value, string_type, T>;
+
+  // Storage of basic_format_arg must be contiguous
+  // Required by basic_format_args::args_ which is just a pointer
+  std::vector<value_type> data_;
+
+  // Storage of arguments not fitting into basic_format_arg must grow
+  // without relocation because items in data_ refer to it.
+
+  std::forward_list<storage_item_type> storage_;
+
+  // Storage of serialized name_args. Must grow without relocation
+  // because items in data_ refer to it.
+  std::forward_list<named_value_type> named_args_;
+
+  friend class basic_format_args<Context>;
+
+  template<typename T>
+  const T& get_last_pushed() const{
+    using internal::get;
+    return get<T>(storage_.front());
+  }
+
+  unsigned long long get_types() const {
+    return internal::is_unpacked_bit | data_.size();
+  }
+
+  template<typename T> const T& stored_value(const T& arg, std::false_type) {
+    return arg;
+  }
+
+  template<typename T> const T& stored_value(
+      const std::reference_wrapper<T>& arg, std::false_type) {
+    return arg.get();
+  }
+
+  template<typename T> const storaged_type<T>& stored_value(const T& arg,
+                                                            std::true_type) {
+    using type = storaged_type<T>;
+    storage_.emplace_front(type{arg});
+    return get_last_pushed<type>();
+  }
+
+  template<typename T> void emplace_arg(const T& arg) {
+    data_.emplace_back(internal::make_arg<Context>(arg));
+  }
+
+ public:
+  dynamic_format_arg_store() FMT_NOEXCEPT = default;
+  ~dynamic_format_arg_store() FMT_NOEXCEPT = default;
+
+  dynamic_format_arg_store(const dynamic_format_arg_store&) = delete;
+  dynamic_format_arg_store& operator=(const dynamic_format_arg_store&) = delete;
+
+  dynamic_format_arg_store(dynamic_format_arg_store&&) = default;
+  dynamic_format_arg_store& operator=(dynamic_format_arg_store&&) = default;
+
+  template <typename T> void push_back(const T& arg) {
+    emplace_arg(stored_value(arg, internal::need_dyn_copy_t<T, Context>{}));
+  }
+
+  template <typename T> void push_back(std::reference_wrapper<T> arg) {
+    emplace_arg(arg.get());
+  }
+
+  template <typename T> void push_back(
+      const internal::named_arg<T, char_type>& arg) {
+    // Named argument is tricky. It's returned by value from fmt::arg()
+    // and then pointer to it is stored in basic_format_arg<>.
+    // So after end of expression the pointer becomes dangling.
+    storage_.emplace_front(string_type{arg.name.data(), arg.name.size()});
+    basic_string_view<char_type> name = get_last_pushed<string_type>();
+    const auto& val = stored_value(
+        arg.value, internal::need_dyn_copy_t<T, Context>{});
+
+    auto named_with_stored_parts = fmt::arg(name, val);
+    // Serialize value into base
+    internal::arg_mapper<Context>().map(named_with_stored_parts);
+    named_args_.push_front(named_with_stored_parts);
+    data_.emplace_back(internal::make_arg<Context>(named_args_.front()));
+  }
+};
+
+FMT_END_NAMESPACE
+
+#endif /* FMT_DYN_ARGS_H_ */

test/CMakeLists.txt

@@ -95,6 +95,7 @@ add_fmt_test(grisu-test)
 target_compile_definitions(grisu-test PRIVATE FMT_USE_GRISU=1)
 add_fmt_test(gtest-extra-test)
 add_fmt_test(format-test mock-allocator.h)
+add_fmt_test(format-dyn-args-test)
 if (MSVC)
   target_compile_options(format-test PRIVATE /bigobj)
 endif ()

test/format-dyn-args-test.cc

@@ -0,0 +1,124 @@
+// Copyright (c) 2020 Vladimir Solontsov
+// SPDX-License-Identifier: MIT Licence
+
+#include <fmt/dyn-args.h>
+
+#include "gtest-extra.h"
+
+
+TEST(FormatDynArgsTest, Basic) {
+  fmt::dynamic_format_arg_store<fmt::format_context> store;
+  store.push_back(42);
+  store.push_back("abc1");
+  store.push_back(1.5f);
+
+  std::string result = fmt::vformat(
+      "{} and {} and {}",
+      store);
+
+  EXPECT_EQ("42 and abc1 and 1.5", result);
+}
+
+TEST(FormatDynArgsTest, StringsAndRefs) {
+  // Unfortunately the tests are compiled with old ABI
+  // So strings use COW.
+  fmt::dynamic_format_arg_store<fmt::format_context> store;
+  char str[]{"1234567890"};
+  store.push_back(str);
+  store.push_back(std::cref(str));
+  store.push_back(std::string_view{str});
+  str[0] = 'X';
+
+  std::string result = fmt::vformat(
+      "{} and {} and {}",
+      store);
+
+  EXPECT_EQ("1234567890 and X234567890 and X234567890", result);
+}
+
+struct Custom{ int i{0}; };
+template <>
+struct fmt::formatter<Custom> {
+  constexpr auto parse(format_parse_context& ctx) {
+    return ctx.begin();
+  }
+
+  template <typename FormatContext>
+    auto format(const Custom& p, FormatContext& ctx) {
+      // ctx.out() is an output iterator to write to.
+      return format_to(
+          ctx.out(),
+          "cust={}", p.i);
+    }
+};
+
+#ifdef FMT_HAS_VARIANT
+
+TEST(FormatDynArgsTest, CustomFormat) {
+  fmt::dynamic_format_arg_store<fmt::format_context, Custom> store;
+  Custom c{};
+  store.push_back(c);
+  ++c.i;
+  store.push_back(c);
+  ++c.i;
+  store.push_back(std::cref(c));
+  ++c.i;
+
+  std::string result = fmt::vformat(
+      "{} and {} and {}",
+      store);
+
+  EXPECT_EQ("cust=0 and cust=1 and cust=3", result);
+}
+
+#endif // FMT_HAS_VARIANT
+
+TEST(FormatDynArgsTest, NamedArgByRef) {
+  fmt::dynamic_format_arg_store<fmt::format_context> store;
+  auto a1 = fmt::arg("a1_", 42);
+  store.push_back(std::cref(a1));
+
+  std::string result = fmt::vformat(
+      "{a1_}", // and {} and {}",
+      store);
+
+  EXPECT_EQ("42", result);
+}
+
+TEST(FormatDynArgsTest, NamedInt) {
+  fmt::dynamic_format_arg_store<fmt::format_context> store;
+  store.push_back(fmt::arg("a1", 42));
+  std::string result = fmt::vformat("{a1}", store);
+  EXPECT_EQ("42", result);
+}
+
+TEST(FormatDynArgsTest, NamedStrings) {
+  fmt::dynamic_format_arg_store<fmt::format_context> store;
+  char str[]{"1234567890"};
+  store.push_back(fmt::arg("a1", str));
+  store.push_back(fmt::arg("a2", std::cref(str)));
+  str[0] = 'X';
+
+  std::string result = fmt::vformat(
+      "{a1} and {a2}",
+      store);
+
+  EXPECT_EQ("1234567890 and X234567890", result);
+}
+
+TEST(FormatDynArgsTest, NamedCustomFormat) {
+  fmt::dynamic_format_arg_store<fmt::format_context, Custom> store;
+  Custom c{};
+  store.push_back(fmt::arg("a1", c));
+  ++c.i;
+  store.push_back(fmt::arg("a2", c));
+  ++c.i;
+  store.push_back(fmt::arg("a3", std::cref(c)));
+  ++c.i;
+
+  std::string result = fmt::vformat(
+      "{a1} and {a2} and {a3}",
+      store);
+
+  EXPECT_EQ("cust=0 and cust=1 and cust=3", result);
+}