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check for property 'mutable iterator' #921

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Oct 28, 2018
Merged

check for property 'mutable iterator' #921

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63 changes: 58 additions & 5 deletions include/fmt/format.h
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Original file line number Diff line number Diff line change
Expand Up @@ -3395,6 +3395,51 @@ inline typename buffer_context<Char>::type::iterator format_to(
basic_format_args<context>(as));
}

namespace internal {

// Detect the iterator category of *any* given type in a SFINAE-friendly way.
// Unfortunately, older implementations of std::iterator_traits are not safe
// for use in a SFINAE-context.

// the gist of C++17's void_t magic
template<typename... Ts>
struct void_ { typedef void type; };

template <typename T, typename Enable = void>
struct it_category : std::false_type {};

template <typename T>
struct it_category<T*> { typedef std::random_access_iterator_tag type; };

template <typename T>
struct it_category<T, typename void_<typename T::iterator_category>::type> {
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So this is basically a SFINAE detection of presence of T::iterator_category? A comment would be nice as well.

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Yeah, I've added a comment to make this clear.

typedef typename T::iterator_category type;
};

// Detect if *any* given type models the OutputIterator concept
template <typename It>
class is_output_iterator {
// Check for mutability because all iterator categories derived from
// std::input_iterator_tag *may* also meet the requirements of an
// OutputIterator, thereby falling into the category of 'mutable iterators'
// [iterator.requirements.general] clause 4.
// The compiler reveals this property only at the point of *actually
// dereferencing* the iterator!
template <typename U>
static decltype(*(internal::declval<U>())) test(std::input_iterator_tag);
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Why not just return const char&?

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Because dereferencing the iterator is the whole gist of all of this kaboodle. This returns either a const or a non-const refererence. The iterator traits give no clue - in particular with regard to const_iterator type members, and everything else fails on VS2013 as it's extra-finicky with expression-SFINAE.

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Makes sense. I was confused by the use of input_iterator_tag in the write context, but since iterator tags form a class hierarchy it makes sense now.

template <typename U>
static char& test(std::output_iterator_tag);
template <typename U>
static const char& test(...);

typedef decltype(test<It>(typename it_category<It>::type{})) type;
typedef typename std::remove_reference<type>::type result;
public:
static const bool value = !std::is_const<result>::value;
};

} // internal

template <typename OutputIt, typename Char = char>
//using format_context_t = basic_format_context<OutputIt, Char>;
struct format_context_t { typedef basic_format_context<OutputIt, Char> type; };
Expand All @@ -3407,8 +3452,9 @@ struct format_args_t {
};

template <typename String, typename OutputIt, typename... Args>
inline OutputIt vformat_to(
OutputIt out, const String &format_str,
inline typename std::enable_if<internal::is_output_iterator<OutputIt>::value,
OutputIt>::type
vformat_to(OutputIt out, const String &format_str,
typename format_args_t<OutputIt, FMT_CHAR(String)>::type args) {
typedef output_range<OutputIt, FMT_CHAR(String)> range;
return vformat_to<arg_formatter<range>>(range(out),
Expand All @@ -3427,7 +3473,9 @@ inline OutputIt vformat_to(
\endrst
*/
template <typename OutputIt, typename S, typename... Args>
inline FMT_ENABLE_IF_STRING(S, OutputIt)
inline typename std::enable_if<
internal::is_string<S>::value &&
internal::is_output_iterator<OutputIt>::value, OutputIt>::type
format_to(OutputIt out, const S &format_str, const Args &... args) {
internal::check_format_string<Args...>(format_str);
typedef typename format_context_t<OutputIt, FMT_CHAR(S)>::type context;
Expand Down Expand Up @@ -3463,7 +3511,9 @@ inline format_arg_store<
}

template <typename OutputIt, typename Char, typename... Args>
inline format_to_n_result<OutputIt> vformat_to_n(
inline typename std::enable_if<
internal::is_output_iterator<OutputIt>::value,
format_to_n_result<OutputIt>>::type vformat_to_n(
OutputIt out, std::size_t n, basic_string_view<Char> format_str,
typename format_to_n_args<OutputIt, Char>::type args) {
typedef internal::truncating_iterator<OutputIt> It;
Expand All @@ -3479,7 +3529,10 @@ inline format_to_n_result<OutputIt> vformat_to_n(
\endrst
*/
template <typename OutputIt, typename S, typename... Args>
inline FMT_ENABLE_IF_STRING(S, format_to_n_result<OutputIt>)
inline typename std::enable_if<
internal::is_string<S>::value &&
internal::is_output_iterator<OutputIt>::value,
format_to_n_result<OutputIt>>::type
format_to_n(OutputIt out, std::size_t n, const S &format_str,
const Args &... args) {
internal::check_format_string<Args...>(format_str);
Expand Down
18 changes: 18 additions & 0 deletions test/format-test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -197,6 +197,24 @@ TEST(IteratorTest, TruncatingBackInserter) {
EXPECT_EQ(buffer, "42");
}

TEST(IteratorTest, IsOutputIterator) {
EXPECT_TRUE(fmt::internal::is_output_iterator<char*>::value);
EXPECT_FALSE(fmt::internal::is_output_iterator<const char*>::value);
EXPECT_FALSE(fmt::internal::is_output_iterator<std::string>::value);
EXPECT_TRUE(fmt::internal::is_output_iterator<
std::back_insert_iterator<std::string>>::value);
EXPECT_TRUE(fmt::internal::is_output_iterator<
std::string::iterator>::value);
EXPECT_FALSE(fmt::internal::is_output_iterator<
std::string::const_iterator>::value);
EXPECT_FALSE(fmt::internal::is_output_iterator<std::list<char>>::value);
EXPECT_TRUE(fmt::internal::is_output_iterator<
std::list<char>::iterator>::value);
EXPECT_FALSE(fmt::internal::is_output_iterator<
std::list<char>::const_iterator>::value);
EXPECT_FALSE(fmt::internal::is_output_iterator<uint32_pair>::value);
}

TEST(MemoryBufferTest, Ctor) {
basic_memory_buffer<char, 123> buffer;
EXPECT_EQ(static_cast<size_t>(0), buffer.size());
Expand Down