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Advanced-Use-Cases.md

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Advanced Use Cases

Custom Base Class for Values

By default, tao::json::basic_value<> uses tao::json::internal::empty_base, an empty dummy class, as public base class. This can be changed to a custom public base class, allowing applications to add arbitrary custom member variables or member functions to every single JSON Value instance, including, recursively, all nested sub-values contained in Arrays and Objects.

To set up a custom public base class it is necessary to use custom traits where the specialisation for void defines a different public_base.

template< typename T >
struct my_traits
   : tao::json::traits< T >
{};

struct my_base
{};

template<>
struct my_traits< void >
   : tao::json::traits< void >
{
   template< typename > using public_base = my_base;
};

Note that public_base is a template, and tao::json::value< my_traits > will use my_traits< void >::public_base< tao::json::value< my_traits > > as public base class. This design pattern is also known as CRTP, the curiously recurring template pattern.

In the example my_base is not a template and the template parameter to my_traits< void >::public_base<> is ignored, however it is possible and to let my_base use the tao::json::value< my_traits > template parameter when necessary.

The library preserves the contents of the public base class, and otherwise ignores it. More precisely:

  • The base is copied when the value is copy constructed or assigned.
  • The base is moved when the value is move constructed or assigned.
  • Otherwise the base is default initialised when constructing a value.
  • The base is ignored by the default comparison operators.

Access to the public base class instance of a value is given through two accessor functions named public_base().

template< template< typename... > class Traits >
Traits< void >::public_base< basic_value< Traits > >& basic_value< Traits >::public_base() noexcept;

template< template< typename... > class Traits, class Base >
const Traits< void >::public_base< basic_value< Traits > >& basic_value< Traits, Base >::public_base() const noexcept;

The application can use the public base for whatever it wants, for example to add filenames and line numbers to parsed values as shown below, or to add new public member functions to all values.

Annotate with filename and line number

A natural use case for custom base classes for Values is to add the filename, line number, and starting byte position to every (sub-)value during parsing.

The two functions tao::json::parse_file_with_position() and tao::json::basic_parse_file_with_position() in include/tao/json/contrib/position.hpp do just that and can be directly used, or customised as required.

The public base class of type tao::json::position, and a helper to add the base class definition to any traits class template, is defined in the same header. The following example shows how to read a filename, and, if the file contains a JSON array, prints the line numbers each array element starts on.

#include <iostream>

#include <tao/json.hpp>
#include <tao/json/contrib/position.hpp>

void example( const std::string& filename )
{
   const auto v = tao::json::parse_file_with_position( filename );
   if( v.is_array() ) {
      for( const auto& i : v.get_array() ) {
         std::cout << "element on line " << i.base().line << std::endl;
      }
   }
}

Writing a recursive function that takes any value with positions and prints the types and line numbers of all sub-values is left as an exercise to the reader.

Copyright (c) 2018-2019 Dr. Colin Hirsch and Daniel Frey