JSON for Modern C++  3.0
template<template< typename U, typename V, typename...Args > class ObjectType = std::map, template< typename U, typename...Args > class ArrayType = std::vector, class StringType = std::string, class BooleanType = bool, class NumberIntegerType = int64_t, class NumberFloatType = double, template< typename U > class AllocatorType = std::allocator>
using nlohmann::basic_json::array_t = ArrayType<basic_json, AllocatorType<basic_json>>

RFC 7159 describes JSON arrays as follows:

An array is an ordered sequence of zero or more values.

To store objects in C++, a type is defined by the template parameters ArrayType which chooses the container (e.g., std::vector or std::list) and AllocatorType which chooses the allocator to use.

Default type

With the default values for ArrayType (std::vector) and AllocatorType (std::allocator), the default value for array_t is:

std::vector<
basic_json, // value_type
std::allocator<basic_json> // allocator_type
>

Limits

RFC 7159 specifies:

An implementation may set limits on the maximum depth of nesting.

In this class, the array's limit of nesting is not constraint explicitly. However, a maximum depth of nesting may be introduced by the compiler or runtime environment. A theoretical limit can be queried by calling the max_size function of a JSON array.

Storage

Arrays are stored as pointers in a basic_json type. That is, for any access to array values, a pointer of type array_t* must be dereferenced.