amorfo77/json
1
0
Fork 0
mirror of https://github.com/nlohmann/json.git synced 2026-08-29 12:27:32 +00:00
Code Issues Packages Projects Releases Wiki Activity

Reduce the string_t and array_t members the library requires

Browse Source 
Several members were required only because of how the library happened to
be written, not because the functionality needs them. Dropping them widens
the set of usable string and array types, and one of them was also a
performance problem.

string_t:

- c_str() is gone. Every call site already knew the length and passed it
  along, so data() is enough. The one place that did not, the diagnostics
  path in exceptions.hpp, now builds the token from data() and size(),
  which also stops it from truncating keys that contain a null byte.
- back() is gone; the serializer indexes the last character instead.
- find(str, pos), replace(), and substr() are gone. escape() and
  unescape() rebuilt the string with one replace() per escaped character,
  which moves the tail every time: escaping a string of n characters that
  all need escaping cost O(n^2). Both now scan with find_first_of() -- a
  member the pointer parser already required -- and append whole runs, so
  the common case is one search and one copy. Escaping 64000 tildes drops
  from 717 ms to 20 ms; a string with nothing to escape gets faster too
  (8.4 ms to 5.8 ms), because the scan is still a single memchr per pass.
  json_pointer::split() takes its reference tokens with the
  (const char*, size_type) constructor rather than substr().
- json_pointer::to_string() accumulates with concat<string_t> instead of
  letting concat default to std::string and converting afterwards, so
  streaming a json_pointer no longer requires string_t to be assignable
  from a std::string.

array_t:

- at(size_type) is gone. basic_json::at(size_type) checked the index by
  calling array_t::at() and translating std::out_of_range, which also
  required the array type to throw that exact exception. It now compares
  against size() and uses operator[]. The thrown exception, its message,
  and the behaviour under JSON_NOEXCEPTION are unchanged.

The BSON writer wrote the terminating null byte out of the string's own
buffer (size() + 1). It now writes the byte itself, so string_t::data()
need not be null-terminated for to_bson().

The tests pin the reduced API: alt_string loses the five dropped members
and gains coverage of the escaping paths, and a std::vector whose at() is
hidden is used as an ArrayType.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-08-28 17:38:58 +00:00
parent 26b10a7b18
commit ccb290facf
9 changed files with 246 additions and 166 deletions
Download Patch File Download Diff File Expand all files Collapse all files
include/nlohmann/detail/exceptions.hpp
+1 -1
View File
@@ -101,7 +101,7 @@ class exception : public std::exception
{
if (&element.second == current)
{
tokens.emplace_back(element.first.c_str());
tokens.emplace_back(element.first.data(), element.first.size());
break;
}
}
include/nlohmann/detail/json_pointer.hpp
+4 -3
View File
@@ -72,7 +72,7 @@ class json_pointer
string_t{},
[](const string_t& a, const string_t& b)
{
return detail::concat(a, '/', detail::escape(b));
return detail::concat<string_t>(a, '/', detail::escape(b));
});
}
@@ -266,7 +266,7 @@ class json_pointer
JSON_THROW(detail::parse_error::create(109, 0, detail::concat("array index '", s, "' is not a number"), nullptr));
}
const char* p = s.c_str();
const char* p = s.data();
char* p_end = nullptr; // NOLINT(misc-const-correctness)
errno = 0; // strtoull doesn't reset errno
const unsigned long long res = std::strtoull(p, &p_end, 10); // NOLINT(runtime/int)
@@ -843,7 +843,8 @@ class json_pointer
{
// use the text between the beginning of the reference token
// (start) and the last slash (slash).
auto reference_token = reference_string.substr(start, slash - start);
const auto count = (slash == string_t::npos ? reference_string.size() : slash) - start;
auto reference_token = string_t(reference_string.data() + start, count);
// check reference tokens are properly escaped
for (std::size_t pos = reference_token.find_first_of('~');
include/nlohmann/detail/output/binary_writer.hpp
+12 -8
View File
@@ -261,7 +261,7 @@ class binary_writer
// step 2: write the string
oa->write_characters(
reinterpret_cast<const CharType*>(j.m_data.m_value.string->c_str()),
reinterpret_cast<const CharType*>(j.m_data.m_value.string->data()),
j.m_data.m_value.string->size());
break;
}
@@ -581,7 +581,7 @@ class binary_writer
// step 2: write the string
oa->write_characters(
reinterpret_cast<const CharType*>(j.m_data.m_value.string->c_str()),
reinterpret_cast<const CharType*>(j.m_data.m_value.string->data()),
j.m_data.m_value.string->size());
break;
}
@@ -798,7 +798,7 @@ class binary_writer
}
write_number_with_ubjson_prefix(j.m_data.m_value.string->size(), true, use_bjdata);
oa->write_characters(
reinterpret_cast<const CharType*>(j.m_data.m_value.string->c_str()),
reinterpret_cast<const CharType*>(j.m_data.m_value.string->data()),
j.m_data.m_value.string->size());
break;
}
@@ -950,7 +950,7 @@ class binary_writer
{
write_number_with_ubjson_prefix(el.first.size(), true, use_bjdata);
oa->write_characters(
reinterpret_cast<const CharType*>(el.first.c_str()),
reinterpret_cast<const CharType*>(el.first.data()),
el.first.size());
write_ubjson(el.second, use_count, use_type, prefix_required, use_bjdata, bjdata_version);
}
@@ -1013,8 +1013,11 @@ class binary_writer
{
oa->write_character(to_char_type(element_type));
oa->write_characters(
reinterpret_cast<const CharType*>(name.c_str()),
name.size() + 1u);
reinterpret_cast<const CharType*>(name.data()),
name.size());
// the terminating null byte is written explicitly rather than taken
// from the buffer, so that string_t::data() need not be null-terminated
oa->write_character(to_char_type(0x00));
}
/*!
@@ -1055,8 +1058,9 @@ class binary_writer
write_number<std::int32_t>(to_bson_length(value.size() + 1ul), true);
oa->write_characters(
reinterpret_cast<const CharType*>(value.c_str()),
value.size() + 1);
reinterpret_cast<const CharType*>(value.data()),
value.size());
oa->write_character(to_char_type(0x00));
}
/*!
include/nlohmann/detail/output/serializer.hpp
+10 -10
View File
@@ -135,7 +135,7 @@ class serializer
auto i = val.m_data.m_value.object->cbegin();
for (std::size_t cnt = 0; cnt < val.m_data.m_value.object->size() - 1; ++cnt, ++i)
{
o->write_characters(indent_string.c_str(), new_indent);
o->write_characters(indent_string.data(), new_indent);
o->write_character('\"');
dump_escaped(i->first, ensure_ascii);
o->write_characters("\": ", 3);
@@ -146,14 +146,14 @@ class serializer
// last element
JSON_ASSERT(i != val.m_data.m_value.object->cend());
JSON_ASSERT(std::next(i) == val.m_data.m_value.object->cend());
o->write_characters(indent_string.c_str(), new_indent);
o->write_characters(indent_string.data(), new_indent);
o->write_character('\"');
dump_escaped(i->first, ensure_ascii);
o->write_characters("\": ", 3);
dump(i->second, true, ensure_ascii, indent_step, new_indent);
o->write_character('\n');
o->write_characters(indent_string.c_str(), current_indent);
o->write_characters(indent_string.data(), current_indent);
o->write_character('}');
}
else
@@ -208,18 +208,18 @@ class serializer
for (auto i = val.m_data.m_value.array->cbegin();
i != val.m_data.m_value.array->cend() - 1; ++i)
{
o->write_characters(indent_string.c_str(), new_indent);
o->write_characters(indent_string.data(), new_indent);
dump(*i, true, ensure_ascii, indent_step, new_indent);
o->write_characters(",\n", 2);
}
// last element
JSON_ASSERT(!val.m_data.m_value.array->empty());
o->write_characters(indent_string.c_str(), new_indent);
o->write_characters(indent_string.data(), new_indent);
dump(val.m_data.m_value.array->back(), true, ensure_ascii, indent_step, new_indent);
o->write_character('\n');
o->write_characters(indent_string.c_str(), current_indent);
o->write_characters(indent_string.data(), current_indent);
o->write_character(']');
}
else
@@ -265,7 +265,7 @@ class serializer
indent_string.resize(indent_string.size() * 2, ' ');
}
o->write_characters(indent_string.c_str(), new_indent);
o->write_characters(indent_string.data(), new_indent);
o->write_characters("\"bytes\": [", 10);
@@ -281,7 +281,7 @@ class serializer
}
o->write_characters("],\n", 3);
o->write_characters(indent_string.c_str(), new_indent);
o->write_characters(indent_string.data(), new_indent);
o->write_characters("\"subtype\": ", 11);
if (val.m_data.m_value.binary->has_subtype())
@@ -293,7 +293,7 @@ class serializer
o->write_characters("null", 4);
}
o->write_character('\n');
o->write_characters(indent_string.c_str(), current_indent);
o->write_characters(indent_string.data(), current_indent);
o->write_character('}');
}
else
@@ -596,7 +596,7 @@ class serializer
{
case error_handler_t::strict:
{
JSON_THROW(type_error::create(316, concat("incomplete UTF-8 string; last byte: 0x", hex_bytes(static_cast<std::uint8_t>(s.back() | 0))), nullptr));
JSON_THROW(type_error::create(316, concat("incomplete UTF-8 string; last byte: 0x", hex_bytes(static_cast<std::uint8_t>(s[s.size() - 1] | 0))), nullptr));
}
case error_handler_t::ignore:
include/nlohmann/detail/string_escape.hpp
+68 -31
View File
@@ -8,50 +8,56 @@
#pragma once
#include <cstddef> // size_t
#include <nlohmann/detail/abi_macros.hpp>
NLOHMANN_JSON_NAMESPACE_BEGIN
namespace detail
{
/*!
@brief replace all occurrences of a substring by another string
@param[in,out] s the string to manipulate; changed so that all
occurrences of @a f are replaced with @a t
@param[in] f the substring to replace with @a t
@param[in] t the string to replace @a f
@pre The search string @a f must not be empty. **This precondition is
enforced with an assertion.**
@since version 2.0.0
*/
template<typename StringType>
inline void replace_substring(StringType& s, const StringType& f,
const StringType& t)
{
JSON_ASSERT(!f.empty());
for (auto pos = s.find(f); // find the first occurrence of f
pos != StringType::npos; // make sure f was found
s.replace(pos, f.size(), t), // replace with t, and
pos = s.find(f, pos + t.size())) // find the next occurrence of f
{}
}
/*!
* @brief string escaping as described in RFC 6901 (Sect. 4)
* @param[in] s string to escape
* @return escaped string
*
* Note the order of escaping "~" to "~0" and "/" to "~1" is important.
*
* The string is rebuilt in a single pass, appending whole runs between the
* characters that need escaping. Scanning with find_first_of() keeps the
* common case -- nothing to escape -- as fast as a single search, while
* repeated replace() calls would move the tail of the string once per
* escaped character.
*/
template<typename StringType>
inline StringType escape(StringType s)
inline StringType escape(const StringType& s)
{
replace_substring(s, StringType{"~"}, StringType{"~0"});
replace_substring(s, StringType{"/"}, StringType{"~1"});
return s;
auto next_special = [&s](std::size_t from)
{
const auto tilde = s.find_first_of('~', from);
const auto slash = s.find_first_of('/', from);
return tilde < slash ? tilde : slash; // npos is the largest value
};
auto pos = next_special(0);
if (pos == StringType::npos)
{
return s;
}
StringType result;
result.reserve(s.size() + 2);
std::size_t run = 0;
while (pos != StringType::npos)
{
result.append(s.data() + run, pos - run);
result.append(s[pos] == '~' ? "~0" : "~1", 2);
run = pos + 1;
pos = next_special(run);
}
result.append(s.data() + run, s.size() - run);
return result;
}
/*!
@@ -60,12 +66,43 @@ inline StringType escape(StringType s)
* @return unescaped string
*
* Note the order of escaping "~1" to "/" and "~0" to "~" is important.
*
* Rebuilt in a single pass, see @ref escape. A "~" that is followed by
* neither "0" nor "1" is passed through unchanged; @ref json_pointer rejects
* such input before it gets here.
*/
template<typename StringType>
inline void unescape(StringType& s)
{
replace_substring(s, StringType{"~1"}, StringType{"/"});
replace_substring(s, StringType{"~0"}, StringType{"~"});
auto pos = s.find_first_of('~', 0);
if (pos == StringType::npos)
{
return;
}
StringType result;
result.reserve(s.size());
std::size_t run = 0;
while (pos != StringType::npos)
{
result.append(s.data() + run, pos - run);
const auto next = pos + 1;
if (next < s.size() && (s[next] == '0' || s[next] == '1'))
{
result.append(s[next] == '0' ? "~" : "/", 1);
run = pos + 2;
}
else
{
result.append("~", 1);
run = pos + 1;
}
pos = s.find_first_of('~', run);
}
result.append(s.data() + run, s.size() - run);
s = result;
}
} // namespace detail
include/nlohmann/json.hpp
+6 -14
View File
@@ -2065,15 +2065,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// at only works for arrays
if (JSON_HEDLEY_LIKELY(is_array()))
{
JSON_TRY
if (JSON_HEDLEY_UNLIKELY(idx >= m_data.m_value.array->size()))
{
return set_parent(m_data.m_value.array->at(idx));
}
JSON_CATCH (std::out_of_range&)
{
// create a better exception explanation
JSON_THROW(out_of_range::create(401, detail::concat("array index ", std::to_string(idx), " is out of range"), this));
} // cppcheck-suppress[missingReturn]
}
return set_parent((*m_data.m_value.array)[idx]);
}
else
{
@@ -2088,15 +2084,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// at only works for arrays
if (JSON_HEDLEY_LIKELY(is_array()))
{
JSON_TRY
if (JSON_HEDLEY_UNLIKELY(idx >= m_data.m_value.array->size()))
{
return m_data.m_value.array->at(idx);
}
JSON_CATCH (std::out_of_range&)
{
// create a better exception explanation
JSON_THROW(out_of_range::create(401, detail::concat("array index ", std::to_string(idx), " is out of range"), this));
} // cppcheck-suppress[missingReturn]
}
return (*m_data.m_value.array)[idx];
}
else
{