Niels Lohmann
02dd3e67f2
* Compare values without recursing, and without comparing them twice Comparing two values compared their containers, which compare their elements, which brought the comparison back once per nesting level. Two values nested deeply enough exhausted the call stack and terminated the process with a segmentation fault - the same bug as #5387, in the last operation that still had it. Worse, an ordered comparison took exponentially long in the nesting depth before C++20. std::vector's operator< is a lexicographical comparison, which asks whether an element is less than its counterpart and then whether the counterpart is less than it - two full comparisons of everything below that element, at every level. Comparing two equal values nested 30 levels deep, which is nothing unusual, took 3.8 seconds; 40 levels would have taken an hour, and nothing about the value has to be pathological to get there. C++20 is unaffected: std::lexicographical_compare_three_way asks once. Compare a value that is nested too deeply to descend into on an explicit stack instead, in a single pass that yields less, equal, greater or unordered at once. Equality and the three-way comparison descend as they always did for the first 128 levels, which nothing measurable costs them; an ordered comparison no longer descends at all, which is what takes the exponent out of it. Objects and arrays that are not nested deeply are otherwise compared exactly as before. The results are unchanged for every pair of values: 68121 comparisons of a corpus that covers NaN, discarded values, mixed number types, binary values, empty containers and both object types are identical to develop, in C++11, C++17 and C++20, with and without thread_local storage and legacy discarded comparison. Reproducing that meant reproducing two subtleties: a lexicographic comparison steps over a pair it cannot order, where a three-way comparison stops at it, and an object compares its keys with < where its entries are ordered but with == where they are only checked for equality - not with the object's own comparator, which for nlohmann::ordered_map tells equality. Equality needs no ordering, so it no longer asks for any: a key or string type that can only be compared for equality still works. Measured (medians of 7 interleaved runs, clang -O3, C++11): comparing two equal values nested 30 levels deep 3778 ms -> 0.002 ms; ordering flat objects -33.6%; ordering flat arrays of numbers +27.3%, the one shape that pays for the single pass; equality unchanged throughout. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Describe comparison in the no-thread-local docs and CI target Comparing two values now bounds its descent with a thread_local counter just as copying does, so the JSON_NO_THREAD_LOCAL page, the macro overview and the ci_test_no_thread_local target cover both rather than copying alone. Also record what switching the macro on costs a comparison: on the benchmark documents, comparing two equal values takes 10% to 90% longer. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Take the descent flag as an argument rather than testing it MSVC reports the test of a constant as C4127 ("conditional expression is constant"), which the Windows builds treat as an error: may_descend is false for operator<, so the operand short-circuits the whole condition. Passing it to compare_descent_exhausted() puts the test where the value is an ordinary parameter, and leaves the call sites with no condition of their own. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Note the comparison fallback in the no-thread-local documentation The macro page describes what the library defines JSON_NO_THREAD_LOCAL for by itself in terms of copying alone; comparing falls back the same way. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Parenthesise the reserve() computation in the comparison test clang-tidy reports the mixed * and + as readability-math-missing- parentheses, as it does for the identical line in the copy test. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Use the shared descent bookkeeping rather than a second set Comparing kept a thread_local count, a limit and a guard of its own beside the ones copying already had, all three the same thing under a different name. They are gone; the shared count, limit and guard do the work. The guard grows a second constructor here, because the comparison operators are written as a macro and a macro cannot use the preprocessor: it cannot look the count up behind an #ifdef the way copy_structured does, so the guard looks it up for it. nesting_depth_exhausted() arrives for the same reason - whether an operator descends at all is a constant at every call site, and testing it there is what MSVC reports as C4127. Also say in compare_leaves what happens to a pair that is an array on one side and an object on the other, since the answer is not obvious from the code: an operator only descends into two values of the same type, so such a pair is told apart by its types alone - unequal, and ordered the way the types are - exactly as it is above the bound. And record what the explicit stack costs: the comparison operators are noexcept and the container comparison this replaces allocated nothing, so running out of memory here ends the process instead of throwing. It takes a value nested past the bound and an exhausted heap to reach, and the same comparison used to exhaust the call stack, but it is a new way to fail. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Amalgamate Signed-off-by: Niels Lohmann <mail@nlohmann.me> --------- Signed-off-by: Niels Lohmann <mail@nlohmann.me>
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Supported Macros
Some aspects of the library can be configured by defining preprocessor macros before including the json.hpp header.
See also the API documentation for macros for examples and more information.
JSON_ASSERT(x)
This macro controls which code is executed for runtime assertions of the library.
See full documentation of JSON_ASSERT(x).
JSON_BRACE_INIT_COPY_SEMANTICS
When defined to 1, single-element brace initialization of a basic_json value (e.g., #!cpp json j{value};) is
treated as a copy/move of the element rather than wrapping it in a single-element array. The default value is 0, which
preserves the existing behavior.
See full documentation of JSON_BRACE_INIT_COPY_SEMANTICS.
JSON_CATCH_USER(exception)
This macro overrides #!cpp catch calls inside the library.
See full documentation of JSON_CATCH_USER(exception).
JSON_DIAGNOSTICS
This macro enables extended diagnostics for exception messages. Possible values are 1 to enable or 0 to disable
(default).
When enabled, exception messages contain a JSON Pointer to the JSON value that triggered the exception, see Extended diagnostic messages for an example. Note that enabling this macro increases the size of every JSON value by one pointer and adds some runtime overhead.
The diagnostics messages can also be controlled with the CMake option
JSON_Diagnostics (OFF by default) which sets JSON_DIAGNOSTICS
accordingly.
See full documentation of JSON_DIAGNOSTICS.
JSON_DIAGNOSTIC_POSITIONS
When enabled, two new member functions start_pos() and
end_pos() are added to basic_json values. If the value
was created by calling theparse function, then these functions allow querying the byte
positions of the value in the input it was parsed from. The byte positions are also used in exceptions to help locate
errors.
The diagnostics positions can also be controlled with the CMake option
JSON_Diagnostic_Positions (OFF by default) which sets
JSON_DIAGNOSTIC_POSITIONS accordingly.
See full documentation of JSON_DIAGNOSTIC_POSITIONS
JSON_HAS_CPP_11, JSON_HAS_CPP_14, JSON_HAS_CPP_17, JSON_HAS_CPP_20, JSON_HAS_CPP_23, JSON_HAS_CPP_26
The library targets C++11, but also supports some features introduced in later C++ versions (e.g., std::string_view
support for C++17). For these new features, the library implements some preprocessor checks to determine the C++
standard. By defining any of these symbols, the internal check is overridden and the provided C++ version is
unconditionally assumed. This can be helpful for compilers that only implement parts of the standard and would be
detected incorrectly.
JSON_HAS_FILESYSTEM, JSON_HAS_EXPERIMENTAL_FILESYSTEM
When compiling with C++17, the library provides conversions from and to std::filesystem::path. As compiler support
for filesystem is limited, the library tries to detect whether <filesystem>/std::filesystem (JSON_HAS_FILESYSTEM)
or <experimental/filesystem>/std::experimental::filesystem (JSON_HAS_EXPERIMENTAL_FILESYSTEM) should be used.
To override the built-in check, define JSON_HAS_FILESYSTEM or JSON_HAS_EXPERIMENTAL_FILESYSTEM to 1.
See full documentation of JSON_HAS_FILESYSTEM and JSON_HAS_EXPERIMENTAL_FILESYSTEM.
JSON_NOEXCEPTION
Exceptions can be switched off by defining the symbol JSON_NOEXCEPTION.
See full documentation of JSON_NOEXCEPTION.
JSON_DISABLE_ENUM_SERIALIZATION
When defined, default parse and serialize functions for enums are excluded and have to be provided by the user, for example, using NLOHMANN_JSON_SERIALIZE_ENUM.
See full documentation of JSON_DISABLE_ENUM_SERIALIZATION.
JSON_NO_IO
When defined, headers <cstdio>, <ios>, <iosfwd>, <istream>, and <ostream> are not included and parse functions
relying on these headers are excluded. This is relevant for environment where these I/O functions are disallowed for
security reasons (e.g., Intel Software Guard Extensions (SGX)).
See full documentation of JSON_NO_IO.
JSON_NO_THREAD_LOCAL
When defined, the library does not use #!cpp thread_local storage. Copying a value and comparing two values then
always avoid the call stack rather than descending into a bounded number of levels first, which is slower but yields the
same values and the same comparisons.
See full documentation of JSON_NO_THREAD_LOCAL.
JSON_PRECISE_STREAM_POSITION
When defined to 1, operator>> and non-strict
sax_parse leave an input stream positioned right after the parsed value, instead of
also consuming the character that terminates a number. The default value is 0, which preserves the existing behavior;
this is planned to become the default in version 4.0.0.
See full documentation of JSON_PRECISE_STREAM_POSITION.
JSON_SKIP_LIBRARY_VERSION_CHECK
When defined, the library will not create a compiler warning when a different version of the library was already included.
See full documentation of JSON_SKIP_LIBRARY_VERSION_CHECK.
JSON_SKIP_UNSUPPORTED_COMPILER_CHECK
When defined, the library will not create a compile error when a known unsupported compiler is detected. This allows using the library with compilers that do not fully support C++11 and may only work if unsupported features are not used.
See full documentation of JSON_SKIP_UNSUPPORTED_COMPILER_CHECK.
JSON_STRICT_NUL_HANDLING
When defined to 1, a '\0' (NUL) byte anywhere in the input is rejected with parse_error.101, like any other
unexpected byte, instead of being silently treated as end of input (see the
FAQ entry for background). The default value is 0, which preserves the
existing behavior; this is planned to become the default in version 4.0.0.
The strict handling can also be enabled with the CMake option
JSON_StrictNulHandling (OFF by default) which sets
JSON_STRICT_NUL_HANDLING accordingly.
See full documentation of JSON_STRICT_NUL_HANDLING.
JSON_THROW_USER(exception)
This macro overrides #!cpp throw calls inside the library. The argument is the exception to be thrown.
See full documentation of JSON_THROW_USER(exception).
JSON_TRY_USER
This macro overrides #!cpp try calls inside the library.
See full documentation of JSON_TRY_USER.
JSON_USE_IMPLICIT_CONVERSIONS
When defined to 0, implicit conversions are switched off. By default, implicit conversions are switched on.
See full documentation of JSON_USE_IMPLICIT_CONVERSIONS.
JSON_USE_GLOBAL_UDLS
When defined to 1 (default), the user-defined string literals operator""_json and operator""_json_pointer are
placed into the global namespace instead of nlohmann::literals::json_literals.
See full documentation of JSON_USE_GLOBAL_UDLS.
JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON
When defined to 1, the library restores the legacy behavior in which a discarded value compared equal to itself. This
behavior is deprecated and switched off (0) by default.
See full documentation of JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON.
JSON_USE_SIMDUTF
When defined, UTF-8 validation of JSON strings read from contiguous byte input is delegated to the simdutf library instead of the built-in scalar validator. This is an opt-in external dependency and is not defined by default.
See full documentation of JSON_USE_SIMDUTF.
NLOHMANN_DEFINE_TYPE_*(...), NLOHMANN_DEFINE_DERIVED_TYPE_*(...)
The library defines 12 macros to simplify the serialization/deserialization of types. See the page on arbitrary type conversion for a detailed discussion.
NLOHMANN_JSON_NAMESPACE, NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END, NLOHMANN_JSON_NAMESPACE_NO_VERSION
These macros relate to the versioned, inline nlohmann namespace:
NLOHMANN_JSON_NAMESPACEevaluates to the full name of thenlohmannnamespace (including the inline ABI namespace).NLOHMANN_JSON_NAMESPACE_BEGIN/NLOHMANN_JSON_NAMESPACE_ENDopen and close the namespace (for example, to add specializations).NLOHMANN_JSON_NAMESPACE_NO_VERSION, when defined to1, omits the version component from the inline namespace.
See the nlohmann Namespace page, and the full documentation of
NLOHMANN_JSON_NAMESPACE,
NLOHMANN_JSON_NAMESPACE_BEGIN / NLOHMANN_JSON_NAMESPACE_END, and
NLOHMANN_JSON_NAMESPACE_NO_VERSION.
NLOHMANN_JSON_SERIALIZE_ENUM(type, ...)
This macro simplifies the serialization/deserialization of enum types. See Specializing enum conversion for more information.
See full documentation of NLOHMANN_JSON_SERIALIZE_ENUM.
A strict variant NLOHMANN_JSON_SERIALIZE_ENUM_STRICT throws an
exception on undefined input instead of falling back to the first mapping.
NLOHMANN_JSON_VERSION_MAJOR, NLOHMANN_JSON_VERSION_MINOR, NLOHMANN_JSON_VERSION_PATCH
These macros are defined by the library and contain the version numbers according to Semantic Versioning 2.0.0.