Thoughts on "Parsing JSON in C & C++: Singleton Tax"

[nlohmann]

Does anyone have thoughts on how to improve the issues discussed in https://ashvardanian.com/posts/parsing-json-with-allocators-cpp/?

• Reddit: https://www.reddit.com/r/cpp/comments/1hvug61/parsing_json_in_c_c_singleton_tax/

[gregmarr]

There are three functions in json.hpp where we do AllocatorType<*> alloc;. They are basic_json::create, json_value::destroy, and basic_json::erase.

The last two have 6 sections of the form

                    AllocatorType<Foo> alloc;
                    std::allocator_traits<decltype(alloc)>::destroy(alloc, foo);
                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, foo, 1);

In each case, foo has a get_allocator() function that I think should be used instead of AllocatorType<Foo> alloc;

    auto alloc = foo.get_allocator();

For the first, it's more complicated because basic_json::create is a static member function. I think that means that we need to pass in the allocator.

    template<typename T, typename... Args>
    static T* create(Args&& ... args)
    {
        AllocatorType<T> alloc;

becomes

    template<typename T, typename... Args>
    static T* create(AllocatorType<T>& alloc, Args&& ... args)
    {

and then every call to create() needs to add the allocator up front. basic_json also has a get_allocator() function, so that shouldn't be too difficult.

[nlohmann]

Indeed, that should be doable.