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* Add JSON_NO_UDLS to leave out the user-defined string literals The bodies of operator""_json and operator""_json_pointer call the parser, so every translation unit including the library instantiates it, even if it never parses anything. Defining JSON_NO_UDLS leaves the literals out entirely, which saves 15-35% compile time for such translation units (#5294). Nothing changes if the macro is not defined. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Mention JSON_NO_UDLS in the list of exported module symbols Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Move the user-defined string literals to <nlohmann/json_literals.hpp> Following the review in #5294, the literals now live in their own header instead of being removed entirely: <nlohmann/json.hpp> includes it at the end unless JSON_NO_AUTOMATIC_UDLS (renamed from JSON_NO_UDLS) is defined, so a project can opt out globally and include the header only where the literals are used. The header only uses public and standard macros, because the library's internal macros are undefined at the end of json.hpp and the amalgamation inlines macro_scope.hpp only once. For the same reason, the library no longer defines and undefines JSON_USE_GLOBAL_UDLS, so a user's definition is still visible to the header. The single-header copy is identical to the multi-header one, as it only includes <nlohmann/json.hpp>. The module always exports the literals. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Fix CI: include cycle, GCC 4.8 literal operator spacing, and global UDLs off in the JSON_NO_AUTOMATIC_UDLS test - Suppress clang-tidy misc-header-include-cycle on the intentional mutual include of json.hpp and json_literals.hpp. - Use operator"" _json with a space for GCC 4.8 in the test's detection aliases, as the header does. - Only test the global literal operators when JSON_USE_GLOBAL_UDLS is on. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Declare the literal operators through a local macro The GCC 4.8 spacing condition was repeated for both operator definitions and the global using-declarations. NLOHMANN_JSON_LITERAL_OPERATOR(suffix) now selects operator""##suffix or operator"" suffix in one place and is undefined at the end of json_literals.hpp. Suggested by gregmarr in review. Signed-off-by: Niels Lohmann <mail@nlohmann.me> --------- Signed-off-by: Niels Lohmann <mail@nlohmann.me>
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JSON_NO_AUTOMATIC_UDLS
#define JSON_NO_AUTOMATIC_UDLS
When defined, <nlohmann/json.hpp> does not include <nlohmann/json_literals.hpp>, so the user-defined string
literals operator""_json and
operator""_json_pointer are not declared. Include
<nlohmann/json_literals.hpp> in the files that use them.
The literals are ordinary inline functions whose bodies call the parser, so every translation unit that includes them
instantiates the parser — even if it never parses anything itself. Defining JSON_NO_AUTOMATIC_UDLS for a whole project
avoids this cost in translation units that do not parse (e.g., ones that only define types and conversions or pass
json values around) and reduces their compile time.
Default definition
By default, #!cpp JSON_NO_AUTOMATIC_UDLS is not defined, and <nlohmann/json.hpp> includes
<nlohmann/json_literals.hpp>.
#undef JSON_NO_AUTOMATIC_UDLS
Notes
!!! info "Header <nlohmann/json_literals.hpp>"
The header includes `<nlohmann/json.hpp>` itself and places the literals according to
[`JSON_USE_GLOBAL_UDLS`](json_use_global_udls.md). It is part of the multi-header sources (`include/nlohmann`)
and of the single-header sources (`single_include/nlohmann`), next to `json.hpp`.
!!! info "C++ modules"
The `nlohmann.json` [module](../../features/modules.md) always exports the literals, regardless of this macro.
Examples
??? example
The code below includes the library without the literals and adds them in a single translation unit.
```cpp
// compiled with -DJSON_NO_AUTOMATIC_UDLS for the whole project
#include <nlohmann/json.hpp>
// this file uses the literals, so it includes them explicitly
#include <nlohmann/json_literals.hpp>
int main()
{
auto j = R"({"foo": 42})"_json;
return j.at("/foo"_json_pointer) == 42 ? 0 : 1;
}
```
Without the include of `<nlohmann/json_literals.hpp>`, the code would fail to compile.
See also
operator""_jsonoperator""_json_pointerJSON_USE_GLOBAL_UDLS- place user-defined string literals (UDLs) into the global namespace
Version history
- Added in version 3.13.0.