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nlohmann
89 lines
2.1 KiB
Markdown
89 lines
2.1 KiB
Markdown
# nlohmann::basic_json::front
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```
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reference front();
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const_reference front() const;
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```
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Returns a reference to the first element in the container. For a JSON container `c`, the expression `c.front()` is equivalent to `*c.begin()`.
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## Return value
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In the case of a structured type (array or object), a reference to the first element is returned. In the case of number, string, boolean, or binary values, a reference to the value is returned.
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## Exception safety
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Strong guarantee: if an exception is thrown, there are no changes in the JSON value.
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## Exceptions
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If the JSON value is `null`, exception [`invalid_iterator.214`](https://json.nlohmann.me/home/exceptions/#jsonexceptioninvalid_iterator214) is thrown.
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## Complexity
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Constant.
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## Notes
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Precondition
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The array or object must not be empty. Calling `front` on an empty array or object yields undefined behavior.
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## Examples
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Example
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The following code shows an example for `front()`.
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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 JSON values
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json j_null;
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json j_boolean = true;
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json j_number_integer = 17;
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json j_number_float = 23.42;
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json j_object = {{"one", 1}, {"two", 2}};
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json j_object_empty(json::value_t::object);
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json j_array = {1, 2, 4, 8, 16};
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json j_array_empty(json::value_t::array);
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json j_string = "Hello, world";
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// call front()
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//std::cout << j_null.front() << '\n'; // would throw
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std::cout << j_boolean.front() << '\n';
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std::cout << j_number_integer.front() << '\n';
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std::cout << j_number_float.front() << '\n';
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std::cout << j_object.front() << '\n';
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//std::cout << j_object_empty.front() << '\n'; // undefined behavior
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std::cout << j_array.front() << '\n';
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//std::cout << j_array_empty.front() << '\n'; // undefined behavior
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std::cout << j_string.front() << '\n';
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}
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```
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Output:
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```
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true
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17
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23.42
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1
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1
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"Hello, world"
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```
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## See also
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- [back](https://json.nlohmann.me/api/basic_json/back/index.md) to access the last element
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## Version history
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- Added in version 1.0.0.
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- Adjusted code to return reference to binary values in version 3.8.0.
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