add `reserve` function for array to reserve memory before adding json values into it

[fnc12]

std::vector has a very useful reserve function that is used to preallocate memory for future push_backs. It reduces push_back time in case if a number of items is high. So it would be useful to add this function to basic_json for array reserve

[gregmarr]

You can currently do this manually using this function:

/// get a pointer to the value (array)
    array_t* get_impl_ptr(array_t* /*unused*/) noexcept
    {
        return is_array() ? m_value.array : nullptr;
    }

Once the array exists in the json object, this function will give you a pointer to it, which you can use to call reserve yourself. (That's not to say that this shouldn't be added.)

[fnc12]

no, get_impl_ptr is private

[gregmarr]

Oh, sorry, I thought these had been added as public. Maybe I was thinking of a request that never went through.

[nlohmann]

There is get_ptr.

[fnc12]

Thanks.