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  1. #include <string>
  2. #include <stack>
  3.  
  4. namespace exprtemplates
  5. {
  6.     template <typename T> struct stack_indexer_expr
  7.     {
  8.         typedef std::stack<T> S;
  9.         S& s;
  10.         std::size_t n;
  11.         stack_indexer_expr(S& s, std::size_t n) : s(s), n(n) {}
  12.  
  13.         operator T() const {
  14.             auto i = s.size()-n;
  15.             for (auto clone = s; !clone.empty(); clone.pop())
  16.                 if (0 == --i) return clone.top();
  17.         }
  18.  
  19.         operator std::string() const {
  20.             return std::to_string(this->operator T());
  21.         }
  22.     };
  23.  
  24.     template <typename T> struct stack_expr
  25.     {
  26.         typedef std::stack<T> S;
  27.         S& s;
  28.         stack_expr(S& s) : s(s) {}
  29.  
  30.         stack_indexer_expr<T> operator[](std::size_t n) const {
  31.             return { s, n };
  32.         }
  33.     };
  34. }
  35.  
  36. template <typename T> 
  37. exprtemplates::stack_expr<T> extend(std::stack<T>& s) {
  38.     return { s };
  39. }
  40.  
  41. #include <iostream>
  42. int main()
  43. {
  44.     std::stack<double> s;
  45.     s.push(0.5);
  46.     s.push(0.6);
  47.     s.push(0.7);
  48.     s.push(0.8);
  49.  
  50.     std::string demo = extend(s)[3];
  51.     std::cout << demo << "\n";
  52.  
  53.     // some more usage:
  54.     auto magic_stack = extend(s);
  55.     std::cout << magic_stack[0] << "\n";
  56.     std::cout << magic_stack[1] << "\n";
  57.     std::cout << magic_stack[2] << "\n";
  58.     std::cout << magic_stack[3] << "\n";
  59.  
  60.     double      as_double = magic_stack[0];
  61.     std::string as_string = magic_stack[0];
  62. }
  63.  
Success #stdin #stdout 0s 3032KB
comments (?)
stdin 
Standard input is empty
stdout 
0.800000
0.5
0.6
0.7
0.8
https://ideone.com/Hacgcy
language:
C++14 (gcc 8.3)
created:
12 years ago
visibility:
public

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