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Add support for 'std::variant' in C++17 (#2941)

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Add support for 'std::variant' in C++17.

For C++17, if all the alternatives of a variant are formattable
the variant is now also formattable. In addition 'std::monostate'
is now formattable.

Moves implementation into 'std.h', and tests into 'std-test.cc'.

Avoid fold-expression since MSVC was crashing.

Add section for 'fmt/std.h' in API-docs.
This commit is contained in:
jehelset
2022-06-26 16:28:01 +02:00
committed by GitHub
parent 51535866d0
commit 6a775e9560
4 changed files with 162 additions and 3 deletions
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33
test/std-test.cc
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@ -7,6 +7,8 @@
#include "fmt/std.h"
#include <string>
#include "gtest/gtest.h"
TEST(std_test, path) {
@ -32,3 +34,34 @@ TEST(std_test, path) {
TEST(std_test, thread_id) {
EXPECT_FALSE(fmt::format("{}", std::this_thread::get_id()).empty());
}
TEST(std_test, variant) {
#ifdef __cpp_lib_variant
EXPECT_EQ(fmt::format("{}", std::monostate{}), "monostate");
using V0 = std::variant<int, float, std::string, char>;
V0 v0(42);
V0 v1(1.5f);
V0 v2("hello");
V0 v3('i');
EXPECT_EQ(fmt::format("{}", v0), "variant(42)");
EXPECT_EQ(fmt::format("{}", v1), "variant(1.5)");
EXPECT_EQ(fmt::format("{}", v2), "variant(\"hello\")");
EXPECT_EQ(fmt::format("{}", v3), "variant('i')");
struct unformattable {};
EXPECT_FALSE((fmt::is_formattable<unformattable>::value));
EXPECT_FALSE((fmt::is_formattable<std::variant<unformattable>>::value));
EXPECT_FALSE((fmt::is_formattable<std::variant<unformattable, int>>::value));
EXPECT_FALSE((fmt::is_formattable<std::variant<int, unformattable>>::value));
EXPECT_FALSE(
(fmt::is_formattable<std::variant<unformattable, unformattable>>::value));
EXPECT_TRUE((fmt::is_formattable<std::variant<int, float>>::value));
using V1 = std::variant<std::monostate, std::string, std::string>;
V1 v4{};
V1 v5{std::in_place_index<1>, "yes, this is variant"};
EXPECT_EQ(fmt::format("{}", v4), "variant(monostate)");
EXPECT_EQ(fmt::format("{}", v5), "variant(\"yes, this is variant\")");
#endif
}