4 ms·

How much stack space does that use, are there allocations and how many are there and how are they managed? These questions seem utterly opaque to me with moder

How much stack space does that use, are there allocations and how many are there and how are they managed? These questions seem utterly opaque to me with modern C++. Perhaps I just need to spend a weekend reading the docs again?

jcelerier 9y ago
> sizeof(std::variant<size_t, std::string>) == sizeof(size_t) + sizeof(std::string) + sizeof(discriminant) (at most 4 or 8 I'd guess?) std::variant will never allocate by itself.
- saghm 9y ago
  Shouldn't it be `max(sizeof(size_t), sizeof(std::string)) + sizeof(discriminant)`? At least in other languages, that's how sum types work
  - steveklabnik 9y ago
    I think both of you are also forgetting padding/alignment, though that obviously depends on the exact details.
    - jcelerier 9y ago
      from what I can see sizeof(std::variant<char, uint8_t>) == 2 so there is no padding between the values and the discriminant
      - steveklabnik 9y ago
        In this specific case, yes.
    - dleslie 9y ago
      And I wasn't clear, in that I was curious about more than the union.
  - jcelerier 9y ago
    yes, forgot the max.
- dleslie 9y ago
  That's the union, but what about the algorithm?
  - jcelerier 9y ago
    why would it allocate ? creating objects does not trigger allocations most of the time. The only stack space used is the internal state of the functor used, which is a lot of time empty (and won't even exist anymore by the time the compiler has optimized your code). eg. look here: https://godbolt.org/g/AdLAi1 https://godbolt.org/g/AdLAi1 there's not a single new / malloc / whatever
    - dleslie 9y ago
      Now I know that there's a stack allocation for the functor. Not all platforms have Sufficiently Smart Compilers; I've got code relying on SDCC and TCC, for instance. :) That's what I'm getting at, there's this whole new suite of functionality that is sort of opaque to me, whereas I'm used to having a grasp of what's going on at-a-glance when working with C and C++98. I'll just have to spend a weekend working with it.
      - jcelerier 9y ago
        > Not all platforms have Sufficiently Smart Compilers; I've got code relying on SDCC and TCC, for instance. :) I'm pretty sure that it's not a matter of compiler brightness; the C and C++ standards are both pretty explicit about when the automatic and dynamic storages are used. If anything, I guess that it would be harder for compilers to allocate such things on the heap. I checked and tcc doesn't allocate anything for instance for this code; neither does gcc 4.4 in c++98 mode : int main() { struct foo { int a, b, c; char x[3000]; } f; }
        dleslie 9y ago
        You're basically confirming my supposition that I'll need to spend time reviewing the new C++ standards. ;)

jcelerier 9y ago

> sizeof(std::variant<size_t, std::string>) == sizeof(size_t) + sizeof(std::string) + sizeof(discriminant) (at most 4 or 8 I'd guess?) std::variant will never allocate by itself.

saghm 9y ago
Shouldn't it be `max(sizeof(size_t), sizeof(std::string)) + sizeof(discriminant)`? At least in other languages, that's how sum types work
- steveklabnik 9y ago
  I think both of you are also forgetting padding/alignment, though that obviously depends on the exact details.
  - jcelerier 9y ago
    from what I can see sizeof(std::variant<char, uint8_t>) == 2 so there is no padding between the values and the discriminant
    - steveklabnik 9y ago
      In this specific case, yes.
  - dleslie 9y ago
    And I wasn't clear, in that I was curious about more than the union.
- jcelerier 9y ago
  yes, forgot the max.
dleslie 9y ago
That's the union, but what about the algorithm?
- jcelerier 9y ago
  why would it allocate ? creating objects does not trigger allocations most of the time. The only stack space used is the internal state of the functor used, which is a lot of time empty (and won't even exist anymore by the time the compiler has optimized your code). eg. look here: https://godbolt.org/g/AdLAi1 https://godbolt.org/g/AdLAi1 there's not a single new / malloc / whatever
  - dleslie 9y ago
    Now I know that there's a stack allocation for the functor. Not all platforms have Sufficiently Smart Compilers; I've got code relying on SDCC and TCC, for instance. :) That's what I'm getting at, there's this whole new suite of functionality that is sort of opaque to me, whereas I'm used to having a grasp of what's going on at-a-glance when working with C and C++98. I'll just have to spend a weekend working with it.
    - jcelerier 9y ago
      > Not all platforms have Sufficiently Smart Compilers; I've got code relying on SDCC and TCC, for instance. :) I'm pretty sure that it's not a matter of compiler brightness; the C and C++ standards are both pretty explicit about when the automatic and dynamic storages are used. If anything, I guess that it would be harder for compilers to allocate such things on the heap. I checked and tcc doesn't allocate anything for instance for this code; neither does gcc 4.4 in c++98 mode : int main() { struct foo { int a, b, c; char x[3000]; } f; }
      - dleslie 9y ago
        You're basically confirming my supposition that I'll need to spend time reviewing the new C++ standards. ;)