Dynamic arguments storage. Implementation of enhancement from issue #1170. (#1584)

test/format-dyn-args-test.cc

@@ -0,0 +1,87 @@
+// Copyright (c) 2020 Vladimir Solontsov
+// SPDX-License-Identifier: MIT Licence
+
+#include <fmt/core.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(fmt::string_view{str});
+  str[0] = 'X';
+
+  std::string result = fmt::vformat("{} and {} and {}", store);
+
+  EXPECT_EQ("1234567890 and X234567890 and X234567890", result);
+}
+
+struct custom_type {
+  int i{0};
+};
+FMT_BEGIN_NAMESPACE
+
+template <> struct formatter<custom_type> {
+  auto parse(format_parse_context& ctx) const -> decltype(ctx.begin()) {
+    return ctx.begin();
+  }
+
+  template <typename FormatContext>
+  auto format(const custom_type& p, FormatContext& ctx) -> decltype(format_to(
+      ctx.out(), std::declval<typename FormatContext::char_type const*>())) {
+    return format_to(ctx.out(), "cust={}", p.i);
+  }
+};
+FMT_END_NAMESPACE
+
+TEST(FormatDynArgsTest, CustomFormat) {
+  fmt::dynamic_format_arg_store<fmt::format_context> store;
+  custom_type 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);
+}
+
+TEST(FormatDynArgsTest, NamedArgByRef) {
+  fmt::dynamic_format_arg_store<fmt::format_context> store;
+
+  // Note: fmt::arg() constructs an object which holds a reference
+  // to its value. It's not an aggregate, so it doesn't extend the
+  // reference lifetime. As a result, it's a very bad idea passing temporary
+  // as a named argument value. Only GCC with optimization level >0
+  // complains about this.
+  //
+  // A real life usecase is when you have both name and value alive
+  // guarantee their lifetime and thus don't want them to be copied into
+  // storages.
+  int a1_val{42};
+  auto a1 = fmt::arg("a1_", a1_val);
+  store.push_back(std::cref(a1));
+
+  std::string result = fmt::vformat("{a1_}",  // and {} and {}",
+                                    store);
+
+  EXPECT_EQ("42", result);
+}