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Niels Lohmann
330fbead59
* Document clang-tidy exclusions and drop unneeded ones Re-enable every excluded check in a run of clang-tidy 22 over all tests and remove the seven that never fire (cert-err58-cpp, clang-analyzer-webkit.NoUncountedMemberChecker, cppcoreguidelines-virtual-class-destructor, google-readability-function-size, hicpp-function-size, hicpp-no-assembler, readability-function-size). Document the reason for every remaining exclusion at the top of the file. Exclude in advance the checks that only exist in clang-tidy 23 and are noise here (bugprone-signed-bitwise, *-explicit-constructor, readability-trailing-comma, modernize-use-string-view), plus modernize-use-integer-sign-comparison, which needs C++20 std::cmp_less and flags deliberate casts. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Fix clang-tidy findings in documentation examples and replace redundant aliases Apply clang-tidy fixes to the documentation examples: - performance-avoid-endl: std::endl -> '\n' - readability-qualified-auto: auto -> auto* / const auto* - performance-for-range-copy: copy -> const reference - google-readability-casting: C-style cast -> static_cast Replace redundant aliases (readability-redundant-qualified-alias) such as "using json = nlohmann::json;" with "using nlohmann::json;" in the documentation, README, examples, tests, and benchmarks. Aliases at class scope and aliases of other types (with_base_class_t etc.) are kept. No changes to the library or its public API. All changed examples were compiled and run; their output matches the stored .output files (except max_size and meta, which depend on the platform and compiler). Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Format doc examples with the pinned astyle Signed-off-by: Niels Lohmann <mail@nlohmann.me> --------- Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2.4 KiB
2.4 KiB
nlohmann::basic_json::get_ptr
template<typename PointerType>
PointerType get_ptr() noexcept;
template<typename PointerType>
constexpr const PointerType get_ptr() const noexcept;
Implicit pointer access to the internally stored JSON value. No copies are made.
Template parameters
PointerType- pointer type; must be a pointer to
array_t,object_t,string_t,boolean_t,number_integer_t, ornumber_unsigned_t,number_float_t, orbinary_t. Other types will not compile.
Return value
pointer to the internally stored JSON value if the requested pointer type fits to the JSON value; #!cpp nullptr
otherwise
Exception safety
No-throw guarantee: this function never throws exceptions.
Complexity
Constant.
Notes
!!! danger "Undefined behavior"
The pointer becomes invalid if the underlying JSON object changes.
Consider the following example code where the pointer `ptr` changes after the array is resized. As a result,
reading or writing to `ptr` after the array change would be undefined behavior. The address of the first array
element changes, because the underlying `std::vector` is resized after adding a fifth element.
```cpp
#include <iostream>
#include <nlohmann/json.hpp>
using nlohmann::json;
int main()
{
json j = {1, 2, 3, 4};
auto* ptr = j[0].get_ptr<std::int64_t*>();
std::cout << "value at " << ptr << " is " << *ptr << std::endl;
j.push_back(5);
ptr = j[0].get_ptr<std::int64_t*>();
std::cout << "value at " << ptr << " is " << *ptr << std::endl;
}
```
Output:
```
value at 0x6000012fc1c8 is 1
value at 0x6000029fc088 is 1
```
Examples
??? example
The example below shows how pointers to internal values of a JSON value can be requested. Note that no type
conversions are made and a `#!cpp nullptr` is returned if the value and the requested pointer type does not match.
```cpp
--8<-- "examples/get_ptr.cpp"
```
Output:
```json
--8<-- "examples/get_ptr.output"
```
See also
- get_ref() get a reference value
Version history
- Added in version 1.0.0.
- Extended to binary types in version 3.8.0.