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### length
```
integer|null length(array|object|string value)
```
Returns the length of an array, object or string.
If array, returns the number of items in the array
If object, returns the number of key-value pairs in the object
If string, returns the number of codepoints in the string
Otherwise, returns null.
### Examples
```cpp
#include <iostream>
#include <jsoncons/json.hpp>
#include <jsoncons_ext/jsonpath/jsonpath.hpp>
// for brevity
using jsoncons::json;
namespace jsonpath = jsoncons::jsonpath;
int main()
{
std::string data = R"(
{
"books":
[
{
"title" : "A Wild Sheep Chase",
"author" : "Haruki Murakami",
"price" : 22.72
},
{
"title" : "The Night Watch",
"author" : "Sergei Lukyanenko",
"price" : 23.58
},
{
"title" : "The Comedians",
"author" : "Graham Greene",
"price" : 21.99
},
{
"title" : "The Night Watch",
"author" : "Phillips, David Atlee"
}
]
}
)";
json j = json::parse(data);
json result1 = jsonpath::json_query(j, "length($.books[*])");
std::cout << "(1) " << result1 << "\n\n";
json result2 = jsonpath::json_query(j, "length($.books[*].price)");
std::cout << "(2) " << result2 << "\n\n";
}
```
Output:
```
(1) [4]
(2) [3]
```
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