4 ms·

What's the equivalent pattern in C++ to do this then? Specifically a contiguously allocated but variable length struct. I was thinking you can use template:

by rawnlq 10y ago

What's the equivalent pattern in C++ to do this then? Specifically a contiguously allocated but variable length struct. I was thinking you can use template: template <int N> struct ion_system_heap { struct ion_heap heap; struct ion_page_pool *pools[N]; }; But that is not quite the same thing since ion_system_heap<10> and ion_system_heap<20> won't be the same type and makes it super hard to use. And a whole bunch of other problems like not knowing how many pages you want to allocate at compile time. Yes you can just use a vector for "pools" but that allocates it on the heap instead of in the struct.

CyberDildonics 10y ago
Two types must be the same size, if they weren't how would sizeof() work, or vector<> for that matter.
- rawnlq 10y ago
  This problem is also on stackoverflow: http://stackoverflow.com/a/1391001 http://stackoverflow.com/a/1391001. It solved it by wrapping with a superclass that you downcast from. You won't be able to work with them by value due to object slicing but you can still have a vector of pointers: vector<Panda_without_bamboo*> pandas; I am not sure how to do the downcasting though. Assuming the classes look like this: public Panda_without_bamboo { int bambooCount; } template<std::size_t N> class Panda_with_bamboo : public Panda_without_bamboo { int a; int b; Bamboo bamboo[N]; } You can't really do: static_cast<Panda<pandas[0].bambooCount>*>(pandas[0]) So it does seem like using flexible array members (first answer in that stackoverflow and the topic discussed in this submission) is a cleaner solution if you need contiguous allocation: http://www.codeatcpp.com/2007/10/dynamic-array-template.html http://www.codeatcpp.com/2007/10/dynamic-array-template.html
  - CyberDildonics 10y ago
    Of course you can create a vector of pointers and do whatever you want. If you need contiguous memory though this is likely a terrible way to go about it since you are not only having to dereference pointers that could go anywhere, but each one will mean a separate heap allocation which will be slow for small objects and not scale with concurrency.

by rawnlq 10y ago

CyberDildonics 10y ago

Two types must be the same size, if they weren't how would sizeof() work, or vector<> for that matter.

rawnlq 10y ago
This problem is also on stackoverflow: http://stackoverflow.com/a/1391001 http://stackoverflow.com/a/1391001. It solved it by wrapping with a superclass that you downcast from. You won't be able to work with them by value due to object slicing but you can still have a vector of pointers: vector<Panda_without_bamboo*> pandas; I am not sure how to do the downcasting though. Assuming the classes look like this: public Panda_without_bamboo { int bambooCount; } template<std::size_t N> class Panda_with_bamboo : public Panda_without_bamboo { int a; int b; Bamboo bamboo[N]; } You can't really do: static_cast<Panda<pandas[0].bambooCount>*>(pandas[0]) So it does seem like using flexible array members (first answer in that stackoverflow and the topic discussed in this submission) is a cleaner solution if you need contiguous allocation: http://www.codeatcpp.com/2007/10/dynamic-array-template.html http://www.codeatcpp.com/2007/10/dynamic-array-template.html
- CyberDildonics 10y ago
  Of course you can create a vector of pointers and do whatever you want. If you need contiguous memory though this is likely a terrible way to go about it since you are not only having to dereference pointers that could go anywhere, but each one will mean a separate heap allocation which will be slow for small objects and not scale with concurrency.