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nlohmann
95 lines
2.4 KiB
Markdown
95 lines
2.4 KiB
Markdown
# <small>nlohmann::basic_json::</small>get_ptr
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```cpp
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template<typename PointerType>
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PointerType get_ptr() noexcept;
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template<typename PointerType>
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constexpr const PointerType get_ptr() const noexcept;
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```
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Implicit pointer access to the internally stored JSON value. No copies are made.
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## Template parameters
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`PointerType`
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: pointer type; must be a pointer to [`array_t`](array_t.md), [`object_t`](object_t.md), [`string_t`](string_t.md),
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[`boolean_t`](boolean_t.md), [`number_integer_t`](number_integer_t.md), or
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[`number_unsigned_t`](number_unsigned_t.md), [`number_float_t`](number_float_t.md), or [`binary_t`](binary_t.md).
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Other types will not compile.
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## Return value
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pointer to the internally stored JSON value if the requested pointer type fits to the JSON value; `#!cpp nullptr`
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otherwise
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## Exception safety
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No-throw guarantee: this function never throws exceptions.
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## Complexity
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Constant.
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## Notes
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!!! danger "Undefined behavior"
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The pointer becomes invalid if the underlying JSON object changes.
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Consider the following example code where the pointer `ptr` changes after the array is resized. As a result,
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reading or writing to `ptr` after the array change would be undefined behavior. The address of the first array
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element changes, because the underlying `std::vector` is resized after adding a fifth element.
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```cpp
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#include <iostream>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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json j = {1, 2, 3, 4};
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auto* ptr = j[0].get_ptr<std::int64_t*>();
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std::cout << "value at " << ptr << " is " << *ptr << std::endl;
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j.push_back(5);
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ptr = j[0].get_ptr<std::int64_t*>();
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std::cout << "value at " << ptr << " is " << *ptr << std::endl;
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}
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```
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Output:
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```
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value at 0x6000012fc1c8 is 1
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value at 0x6000029fc088 is 1
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```
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## Examples
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??? example
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The example below shows how pointers to internal values of a JSON value can be requested. Note that no type
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conversions are made and a `#!cpp nullptr` is returned if the value and the requested pointer type does not match.
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```cpp
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--8<-- "examples/get_ptr.cpp"
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```
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Output:
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```json
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--8<-- "examples/get_ptr.output"
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```
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## See also
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- [get_ref()](get_ref.md) get a reference value
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## Version history
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- Added in version 1.0.0.
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- Extended to binary types in version 3.8.0.
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