4 ms·

it's not only type-safe, it uses much less memory, takes less time to construct and does not duplicate the "null" state: - your JSONStruct takes 160 bytes (on

it's not only type-safe, it uses much less memory, takes less time to construct and does not duplicate the "null" state: - your JSONStruct takes 160 bytes (on 64-bit systems with libstdc++), the variant one takes 56, almost three times less. - it needs std::optional<T> for each member to avoid construction, while std::variant does that by default. But the compiler still have to initialize all these optionals to their default state. - you need on every access two branches: * one for the type switch * one for the optional the variant only has one level of indirection.

eatonphil 5y ago
That's fair. I assumed it would be clear to anyone who knows C++ better than I that the point of this post wasn't exactly performance. Actually I was most interested in just how simple and readable I could make it. But I didn't say that in the post anywhere. Will add.
- jcelerier 5y ago
  > I assumed it would be clear to anyone who knows It's just not possible to assume that at all on the internet. Every year I have students who learn from blog posts like this for instance.
- gravypod 5y ago
  As someone who does quite a bit of C++ recently: for most uses readability is king. If you did want squeeze out the extra performance you could also do something where you store `std::variant<>` inside a struct and provide an API like this: if (auto int_value = value.i()) { // *int_value == your int } and expose a `std::variant<> &variant();` for use cases where you want the more complex/faster stuff.
  - tcbawo 5y ago
    Using variant as the storage type while providing a nice public accessor API makes sense given the huge difference in semantics between a struct containing optional fields and a variant.
- bialpio 5y ago
  Performance aside, using variant has an added benefit of making illegal states unrepresentable. Optionals + an enum still allows users to either nullopt all the members, or have more than one member that is not nullopt.
DilutedCoke 5y ago
How do you get the size of the struct? Is there a compiler command to do that?
- anonymoushn 5y ago
  https://en.cppreference.com/w/cpp/language/sizeof https://en.cppreference.com/w/cpp/language/sizeof

eatonphil 5y ago

That's fair. I assumed it would be clear to anyone who knows C++ better than I that the point of this post wasn't exactly performance. Actually I was most interested in just how simple and readable I could make it. But I didn't say that in the post anywhere. Will add.

jcelerier 5y ago
> I assumed it would be clear to anyone who knows It's just not possible to assume that at all on the internet. Every year I have students who learn from blog posts like this for instance.
gravypod 5y ago
As someone who does quite a bit of C++ recently: for most uses readability is king. If you did want squeeze out the extra performance you could also do something where you store `std::variant<>` inside a struct and provide an API like this: if (auto int_value = value.i()) { // *int_value == your int } and expose a `std::variant<> &variant();` for use cases where you want the more complex/faster stuff.
- tcbawo 5y ago
  Using variant as the storage type while providing a nice public accessor API makes sense given the huge difference in semantics between a struct containing optional fields and a variant.
bialpio 5y ago
Performance aside, using variant has an added benefit of making illegal states unrepresentable. Optionals + an enum still allows users to either nullopt all the members, or have more than one member that is not nullopt.

DilutedCoke 5y ago

How do you get the size of the struct? Is there a compiler command to do that?

anonymoushn 5y ago
https://en.cppreference.com/w/cpp/language/sizeof https://en.cppreference.com/w/cpp/language/sizeof