a more forgiving array behavior

README.md

@@ -29,7 +29,7 @@ Other aspects were not so important to us:
 As of February 2015, the following updates were made to the library
 
 - *Changed:* In the generic class `basic_json`, all JSON value types (array, object, string, bool, integer number, and floating-point) are now **templated**. That is, you can choose whether you like a `std::list` for your arrays or an `std::unordered_map` for your objects. The specialization `json` sets some reasonable defaults.
-- *Changed:* The library now consists of a single header, called `json.hpp`. Consequently, build systems such as Automake or CMake are not any longer required.
+- *Changed:* The library now consists of a **single header**, called `json.hpp`. Consequently, build systems such as Automake or CMake are not any longer required.
 - *Changed:* The **deserialization** is now supported by a lexer generated with [re2c](http://re2c.org) from file [`src/json.hpp.re2c`](https://github.com/nlohmann/json/blob/master/src/json.hpp.re2c). As a result, we follow the JSON specification more strictly. Note neither the tool re2c nor its input are required to use the class.
 - *Added:* The library now satisfies the [**ReversibleContainer**](http://en.cppreference.com/w/cpp/concept/ReversibleContainer) requirement. It hence provides four different iterators (`iterator`, `const_iterator`, `reverse_iterator`, and `const_reverse_iterator`), comparison functions, `swap()`, `size()`, `max_size()`, and `empty()` member functions.
 - *Added*: The class uses **user-defined allocators** which default to `std::allocator`, but can be templated via parameter `Allocator`.
@@ -184,7 +184,7 @@ These operators work for any subclasses of `std::istream` or `std::ostream`.
 
 ### STL-like access
 
-We designed the JSON class to behave just like an STL container:
+We designed the JSON class to behave just like an STL container. In fact, it satisfies the [**ReversibleContainer**](http://en.cppreference.com/w/cpp/concept/ReversibleContainer) requirement.
 
 ```cpp
 // create an array using push_back

src/json.hpp

@@ -712,12 +712,26 @@ class basic_json
     /// access specified element
     inline reference operator[](size_type pos)
     {
-        // at only works for arrays
+        // implicitly convert null to object
+        if (m_type == value_t::null)
+        {
+            m_type = value_t::array;
+            Allocator<array_t> alloc;
+            m_value.array = alloc.allocate(1);
+            alloc.construct(m_value.array);
+        }
+
+        // [] only works for arrays
         if (m_type != value_t::array)
         {
             throw std::runtime_error("cannot use [] with " + type_name());
         }
 
+        for (size_t i = m_value.array->size(); i <= pos; ++i)
+        {
+            m_value.array->push_back(basic_json());
+        }
+
         return m_value.array->operator[](pos);
     }
 
@@ -769,7 +783,7 @@ class basic_json
             alloc.construct(m_value.object);
         }
 
-        // at only works for objects
+        // [] only works for objects
         if (m_type != value_t::object)
         {
             throw std::runtime_error("cannot use [] with " + type_name());

src/json.hpp.re2c

@@ -712,12 +712,26 @@ class basic_json
     /// access specified element
     inline reference operator[](size_type pos)
     {
-        // at only works for arrays
+        // implicitly convert null to object
+        if (m_type == value_t::null)
+        {
+            m_type = value_t::array;
+            Allocator<array_t> alloc;
+            m_value.array = alloc.allocate(1);
+            alloc.construct(m_value.array);
+        }
+
+        // [] only works for arrays
         if (m_type != value_t::array)
         {
             throw std::runtime_error("cannot use [] with " + type_name());
         }
 
+        for (size_t i = m_value.array->size(); i <= pos; ++i)
+        {
+            m_value.array->push_back(basic_json());
+        }
+
         return m_value.array->operator[](pos);
     }
 
@@ -769,7 +783,7 @@ class basic_json
             alloc.construct(m_value.object);
         }
 
-        // at only works for objects
+        // [] only works for objects
         if (m_type != value_t::object)
         {
             throw std::runtime_error("cannot use [] with " + type_name());

test/unit.cpp

@@ -2033,10 +2033,20 @@ TEST_CASE("element access")
             {
                 SECTION("null")
                 {
-                    json j_nonarray(json::value_t::null);
-                    const json j_nonarray_const(j_nonarray);
-                    CHECK_THROWS_AS(j_nonarray[0], std::runtime_error);
-                    CHECK_THROWS_AS(j_nonarray_const[0], std::runtime_error);
+                    SECTION("standard tests")
+                    {
+                        json j_nonarray(json::value_t::null);
+                        const json j_nonarray_const(j_nonarray);
+                        CHECK_NOTHROW(j_nonarray[0]);
+                        CHECK_THROWS_AS(j_nonarray_const[0], std::runtime_error);
+                    }
+
+                    SECTION("implicit transformation to properly filled array")
+                    {
+                        json j_nonarray;
+                        j_nonarray[3] = 42;
+                        CHECK(j_nonarray == json({nullptr, nullptr, nullptr, 42}));
+                    }
                 }
 
                 SECTION("boolean")