Custom aggregate initializer list constructor

Question

For example, nlohmann json has a way of converting an aggregate initializer list to a JSON object:

json j = {
  {"pi", 3.141},
  {"happy", true},
  {"name", "Niels"},
  {"nothing", nullptr},
  {"answer", {
    {"everything", 42}
  }},
  {"list", {1, 0, 2}},
  {"object", {
    {"currency", "USD"},
    {"value", 42.99}
  }}
};

and c++ std::map also has a aggeragte initalizer list

{
{"one": 1},
{"two": 2}
}

I'm curious about how you can write a custom (aggregate) initalizer list initalizer

There is no such thing as an "aggregate initializer list" constructor. This is just an application of list-initialization. Each of those braced-init-lists is just a means for calling a constructor. — Nicol Bolas
– Nicol Bolas, Commented Apr 29, 2020 at 23:03
yes, but how do can i write a custom initializer for it (if I can)? — FloweyTF
– FloweyTF, Commented Apr 29, 2020 at 23:07
You can look inside json.hpp to see how it writes its constructor — M.M
– M.M, Commented Apr 30, 2020 at 1:04

3CEZVQ · Accepted Answer · 2020-04-29 23:15:24Z

1

It is easy to learn howtos in the standard library.

Look at the std::map constructor:

map( std::initializer_list<value_type> init,
     const Compare& comp = Compare(),
     const Allocator& alloc = Allocator() );

value_type is

std::pair<const Key, T>, Key is std::string, T is int

So the constructor is

map( std::initializer_list<std::pair<std::string, int>> init,
     const Compare& comp = Compare(),
     const Allocator& alloc = Allocator() );

and can be used like with

{
  std::pair("one", 1),
  std::pair("two", 2),
}

Then look at std::pair constructor

pair( const T1& x, const T2& y );

It can be constructed like

std::pair<std::string, int> a{"one", 1};

or

std::pair<std::string, int> a = {"one", 1};

Taking into consideration all above, a map can be constructed like with

{
{"one", 1},
{"two", 2}
}

edited Apr 29, 2020 at 23:15

answered Apr 29, 2020 at 23:08

3CEZVQ

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5 Comments

FloweyTF Over a year ago

what about the more complex JSON example? if you can explain it, ill accept your answer!

Nicol Bolas Over a year ago

@FloweyTF: It's the same thing, just using a different type. It's nothing special. Just look at the json type's constructor.

3CEZVQ Over a year ago

More complex JSON can be done with std::map<std::string, std::any>

3CEZVQ Over a year ago

Or

class JSONItem : public std::variant<std::string, bool, int, std::map<std::string, JSONItem>, std::vector<JSONItem>> {};  using JSON = std::map<std::string, JSONItem>;

FloweyTF Over a year ago

I'd rather not use std::variant because its for c++17, but I guess?

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