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

Answer 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}
}