Printing attribute names

[luxe]

What would be the best way to iterate over a json object and print the attribute names?
My goal would be to have something like this:

example json:

{
"example": {
    "a": "hello",
    "b": "world",
    "c": "text"
  }
}

example printing code:

auto j = nlohmann::json::parse(Read_Entire_File_Into_String("example.json"));
auto element = j["example"];
for (auto const& it: element) {
    std::cout << it.attribute_name() << std::endl;
}
// I made up 'Read_Entire_File_Into_String', but it does what you'd expect.
// the attribute_name() method doesn't exist, but I wish that it had.

example results:

a
b
c

Not everything has an attribute name, in which case, it could return an empty string, throw an exception, or return std::optional.

I thought this might be easy to implement, but it appears that all of the attribute names are stored in the parent object, which means that when we iterate over an object, each iterator points to an object that has no way of finding out its own attribute name.

for example, if I were to edit json.hpp and give the basic_json class this public member function:

    inline string_t attribute_name() const noexcept
    {
        std::string str;
        if (m_type == value_t::object){
            for (auto i = m_value.object->cbegin(); i != m_value.object->cend(); ++i){
                str += escape_string(i->first) + " ";
            }
        }

        return str;
    }

and run client code like this:

#include "json.hpp"
#include <iostream>

void Recursive_Print(nlohmann::json const& element, unsigned int depth);

int main(){
  auto element = nlohmann::json::parse(Read_Entire_File_Into_String("changelog.json"));

  if (element.is_null()){ std::cout << "null " << element.attribute_name() << std::endl;}
  if (element.is_boolean()){std::cout << "bool " << element.attribute_name() << std::endl;}
  if (element.is_number()){std::cout << "number " << element.attribute_name() << std::endl;}
  if (element.is_object()){std::cout << "object " << element.attribute_name() << std::endl;}
  if (element.is_array()){std::cout << "array " << element.attribute_name() << std::endl;}
  if (element.is_string()){std::cout << "string " << element.attribute_name() << std::endl;}
}
void Recursive_Print(nlohmann::json const& element, unsigned int depth){
  for (auto const& it: element) {
    Print_Character_N_Times('-',depth); 
    if (it.is_null()){ std::cout << "null " << it.attribute_name() << std::endl;}
    if (it.is_boolean()){std::cout << "bool " << it.attribute_name() << std::endl;}
    if (it.is_number()){std::cout << "number " << it.attribute_name() << std::endl;}
    if (it.is_object()){std::cout << "object " << it.attribute_name() << std::endl; Recursive_Print(it,++depth);}
    if (it.is_array()){std::cout << "array " << it.attribute_name() << std::endl; Recursive_Print(it,++depth);}
    if (it.is_string()){std::cout << "string " << it.attribute_name() << std::endl;}
  }
}
//PRETTY MESSY.  BUT YOU GET THE GIST

it would result in the following:

object example 
-object a b c 
--string 
--string 
--string

I think that illustrates my point about attribute names being stored in the parents.

Anyway, do you have any thoughts on how I could accomplish this?

[Iced-Sun]

Yes I found this is very annoying because it FORCES the user know all the possible attributes.

I think it would be convenient to just follow the convention of std::map, i.e., returning a (key, value) pair instead of the value only, when enumerating the json object.

[nlohmann]

Dear @luxe,

thanks for reporting! I shall have a look once I'm back from holidays.

All the best
Niels

[amirmasoudabdol]

That would be really useful. Even if you know the keys beforehand, being able to iterate over them will save a lot of lines.

[nlohmann]

This is related to #46 and #49. I have an idea how to solve it. Stay tuned...

[luxe]

Yay!

[nlohmann]

All iterators have now member functions key() to retrieve the key for objects and value() to retrieve the value. This should fix the mentioned issue.

[amirmasoudabdol]

That's great. Thanks!