Niels Lohmann
759dd2e1b1
Add json_document and json_view, a read-only, zero-copy index of a JSON text, as the first public slice of the zero-copy view (#5295). A parse produces a flat array of 16-byte nodes in document order, one per value and one per object key. Strings stay in the source text; escaped strings are decoded into an arena. Integers are converted while their digits are in the cache; floats keep only their digit layout and are converted on read. Containers store the size of their subtree, so a reader can step over one in constant time. A document makes a handful of allocations, however many values it has. The parser accepts exactly what json::parse accepts, with every combination of ignore_comments and ignore_trailing_commas, with and without a trailing NUL, and under JSON_STRICT_NUL_HANDLING. It is portable C++11 and does not depend on byte order. basic_json_document adds parse, parse_copy, accept, read (reuses a document's memory), root, is_discarded, source, owns_source, node_count, memory_usage, and shrink_to_fit. basic_json_view adds type, the is_* queries, operator bool, size, empty, materialize, and source_offset. A parse error throws the same exception basic_json::parse would throw for the same input, message and position included. detail::abi_config keeps JSON_STRICT_NUL_HANDLING and JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON readable after json.hpp undefines them, in the ABI namespace so they always match the basic_json in use. A NUL byte that ends a // comment is the end of the input, as in parse() since #5696. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2.6 KiB
Integration
There are several ways to add this header-only library to a C++ project. The following flowchart summarizes how to pick one:
flowchart TD
A[Add the library to a C++ project] --> B{Already using CMake?}
B -- no --> C{Using pkg-config or plain Makefiles?}
C -- yes --> D[pkg-config]
C -- no --> E[Copy the single header]
B -- yes --> F{Library installed system-wide?}
F -- yes --> G["find_package()"]
F -- no --> H{Use a package manager?}
H -- yes --> I[Package manager]
H -- no --> J["add_subdirectory() or FetchContent"]
- Copy the single header, as described below — no build-system integration required.
- CMake: use
find_package()if the library is already installed,add_subdirectory()to embed the source tree, orFetchContentto download it at configure time; see CMake. - Package managers: install the library with a package manager such as Homebrew, Conan, or vcpkg; see Package Managers.
- pkg-config: if you use bare Makefiles instead of CMake, pkg-config can supply the include flags for an already-installed library.
Once the library is integrated, see the Migration Guide for how to keep your code future-proof across releases.
Header only
json.hpp is the single required
file in single_include/nlohmann or released here. You need to add
#include <nlohmann/json.hpp>
// for convenience
using json = nlohmann::json;
to the files you want to process JSON and set the necessary switches to enable C++11 (e.g., -std=c++11 for GCC and
Clang).
You can further use file
single_include/nlohmann/json_fwd.hpp
for forward declarations (see Compile times), and file
single_include/nlohmann/json_literals.hpp
for the user-defined string literals if you define
JSON_NO_AUTOMATIC_UDLS.
For the read-only, non-owning basic_json_document/basic_json_view
types, additionally include
single_include/nlohmann/json_view.hpp;
see Zero-copy JSON views.