Created nlohmann::basicjson::size (markdown)

nlohmann::basicjson::size.md

@@ -0,0 +1,54 @@
+```cpp
+size_type size() const;
+```
+
+## Description
+
+Returns the number of elements of a JSON value, i.e. `std::distance(begin(), end())`.
+
+## Return value
+
+The return value depends on the different value types and is defined as follows:
+
+| Value type | return value         |
+| ---------- | -------------------- |
+| null       | `0`                  |
+| boolean    | `1`                  |
+| string     | `1`                  |
+| number     | `1`                  |
+| object     | `ObjectType::size()` |
+| array      | `ArrayType::size()`  |
+
+## Parameter
+
+None.
+
+## Complexity
+
+Constant, as long as `ArrayType` and `ObjectType` satisfy the [Container concept](http://en.cppreference.com/w/cpp/concept/Container).
+
+## Exceptions
+
+None. The function's noexcept-specification is `noexcept`.
+
+## Example
+
+The following code uses empty to check if a `nlohmann::json` contains any elements:
+
+```cpp
+#include <iostream>
+#include <json.hpp>
+ 
+int main()
+{ 
+    nlohmann::json values {"string", true, 17, 23.42};
+ 
+    std::cout << "values contains " << values.size() << " elements.\n";
+}
+```
+
+Output:
+
+```
+values contains 4 elements.
+```