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  1. #include <iostream>
  2. #include <vector>
  3.  
  4. using namespace std;
  5.  
  6. void use(void *p)
  7. {
  8.     static auto volatile t=p;
  9. };
  10.  
  11. template<typename T,unsigned N,typename F>
  12. auto cps_alloca_static(F &&f) -> decltype(f(nullptr,nullptr))
  13. {
  14.     T data[N];
  15.     return f(&data[0],&data[0]+N);
  16. }
  17.  
  18. template<typename T,typename F>
  19. auto cps_alloca_dynamic(unsigned n,F &&f) -> decltype(f(nullptr,nullptr))
  20. {
  21.     vector<T> data(n);
  22.     return f(&data[0],&data[0]+n);
  23. }
  24.  
  25. template<typename T,typename F>
  26. auto cps_alloca(unsigned n,F &&f) -> decltype(f(nullptr,nullptr))
  27. {
  28.     switch(n)
  29.     {
  30.         case 1: return cps_alloca_static<T,1>(f);
  31.         case 2: return cps_alloca_static<T,2>(f);
  32.         case 3: return cps_alloca_static<T,3>(f);
  33.         case 4: return cps_alloca_static<T,4>(f);
  34.         case 0: return f(nullptr,nullptr);
  35.         default: return cps_alloca_dynamic<T>(n,f);
  36.     }; // mpl::for_each / array / index pack / recursive bsearch / etc variación
  37. }
  38.  
  39. struct Payload
  40. {
  41.     char d[1 << 14];
  42. };
  43.  
  44. void test(unsigned n)
  45. {
  46.     volatile char begin;
  47.     cps_alloca<Payload>(n,[&](Payload *first,Payload *last)
  48.     {
  49.         volatile char end;
  50.         const auto overhead = &begin-&end - int(n*sizeof(Payload));
  51.         cout << "n=" << n << "\toverhead=" << overhead << endl;
  52.         use(first); use(last);
  53.     });
  54. }
  55.  
  56. int main()
  57. {
  58.     for(auto i=1u;i!=6u;++i)
  59.         test(i);
  60. }
Success #stdin #stdout 0s 3032KB
comments (?)
stdin 
Standard input is empty
stdout 
n=1	overhead=38
n=2	overhead=38
n=3	overhead=38
n=4	overhead=38
n=5	overhead=-81922
https://ideone.com/HsrQqc
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
secret

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