Created nlohmann::basicjson::at (markdown)

nlohmann::basicjson::at.md

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+```cpp
+reference at(size_type idx);
+const_reference at(size_type idx) const;                  // 1
+```
+```cpp
+reference at(const object_t::key_type& key)
+const_reference at(const object_t::key_type& key) const   // 2
+```
+
+## Description
+
+1. Returns a reference to the element at specified index `idx`, with bounds checking. If `idx` not within the range of the container, an exception of type `std::out_of_range` is thrown (only works for arrays).
+2. Returns a reference to the mapped value of the element with key equivalent to `key`. If no such element exists, an exception of type `std::out_of_range` is thrown (only works for objects).
+
+## Parameters
+
+- `idx`: index of the element to return
+- `key`: key of the element to return
+
+## Return value
+
+Reference to the specified value.
+
+## Complexity
+
+1. Constant.
+2. Logarithmic in the size of the container.
+
+## Exceptions
+
+1. `std::runtime_error` when called on a JSON container with other type than `array`; `std::out_of_range` if `!(idx < size())`.
+2. `std::runtime_error` when called on a JSON container with other type than `object`; `std::out_of_range` if the container does not have an element with the specified key.