Niels Lohmann
346a73873d
basic_json's copy constructor copied objects and arrays by handing the container to its own copy constructor, which copy-constructs every element and so reaches this constructor again, once per nesting level. A value nested deeply enough exhausted the call stack and terminated the process with a segmentation fault - no exception, nothing the caller could catch. Parsing such a value works, as the parser is iterative, and so does destroying one, as #1436 made destruction iterative. Bound how far the copy descends rather than take the call stack away from it. The first levels are copied exactly as they were - the containers copy their own elements, which is by far the fastest way to fill them - and only once the copy has descended 128 levels is the value below it finished without the call stack, through an explicit worklist. Copying can therefore no longer exhaust the stack, however deeply a value is nested, while a value nested less deeply than the bound - all but a vanishing minority - is copied by the very same code as before and pays only for one counter. That counter lives in thread_local storage, as one shared between threads would be raced. JSON_NO_THREAD_LOCAL switches it off for toolchains without thread_local; copying then goes through the worklist right away, which yields the same values but is measurably slower. The deferred values are completed before the copy they belong to returns, so a value copied while another copy is going on - by a custom base class, say - is unaffected by the copy it is nested in. operator= takes its argument by value, so copy assignment is fixed as well. Copying is as fast as it was, within measurement noise (medians of 9 interleaved runs, clang -O3): -1.3% for an array of strings, +0.0% for a flat object, +0.1% for a flat array of numbers, +0.3% for nested arrays, +0.6% for nested objects and +1.2% for a twitter-like document. Copying a three-key object costs about ten nanoseconds more, the counter. Deferring every level instead, rather than only those below the bound, measured between 3% and 9% slower depending on the shape of the value. This fixes #5387 for the copy constructor. dump() is still recursive. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
9.3 KiB
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 then always avoids the call stack
rather than descending into a bounded number of levels first, which is slower but yields the same values.
See full documentation of JSON_NO_THREAD_LOCAL.
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_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.
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.