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nlohmann
108 lines
3.2 KiB
Markdown
108 lines
3.2 KiB
Markdown
# nlohmann::basic_json::get_ptr
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```
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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` : pointer type; must be a pointer to [`array_t`](https://json.nlohmann.me/api/basic_json/array_t/index.md), [`object_t`](https://json.nlohmann.me/api/basic_json/object_t/index.md), [`string_t`](https://json.nlohmann.me/api/basic_json/string_t/index.md), [`boolean_t`](https://json.nlohmann.me/api/basic_json/boolean_t/index.md), [`number_integer_t`](https://json.nlohmann.me/api/basic_json/number_integer_t/index.md), or [`number_unsigned_t`](https://json.nlohmann.me/api/basic_json/number_unsigned_t/index.md), [`number_float_t`](https://json.nlohmann.me/api/basic_json/number_float_t/index.md), or [`binary_t`](https://json.nlohmann.me/api/basic_json/binary_t/index.md). 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; `nullptr` 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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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, 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.
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```
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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 conversions are made and a `nullptr` is returned if the value and the requested pointer type does not match.
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```
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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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// create a JSON number
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json value = 17;
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// explicitly getting pointers
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auto p1 = value.get_ptr<const json::number_integer_t*>();
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auto p2 = value.get_ptr<json::number_integer_t*>();
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auto p3 = value.get_ptr<json::number_integer_t* const>();
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auto p4 = value.get_ptr<const json::number_integer_t* const>();
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auto p5 = value.get_ptr<json::number_float_t*>();
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// print the pointees
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std::cout << *p1 << ' ' << *p2 << ' ' << *p3 << ' ' << *p4 << '\n';
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std::cout << std::boolalpha << (p5 == nullptr) << '\n';
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}
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```
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Output:
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```
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17 17 17 17
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true
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```
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## See also
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- [get_ref()](https://json.nlohmann.me/api/basic_json/get_ref/index.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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