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  1. #include <iostream>
  2.  
  3. template <typename T>
  4. class fast_arg
  5. {
  6. public:
  7.     fast_arg(const T & val)
  8.     : lval_(&val), rval_(nullptr) {}
  9.     fast_arg(T &&val)
  10.     : lval_(nullptr), rval_(&val) {}
  11.  
  12.     template <typename T1>
  13.     fast_arg(T1 &&val)
  14.     : lval_(nullptr), rval_(nullptr)
  15.     {
  16.         static T temp(std::forward<T1>(val));
  17.         rval_ = &temp;
  18.     }
  19.  
  20.     void copy_to(T &target)
  21.     {
  22.         if (lval_) target = *lval_;
  23.         else       target = std::move(*rval_);
  24.     }
  25.  
  26.     T create_as_copy()
  27.     {
  28.         if (lval_) return *lval_;
  29.         else       return std::move(*rval_);
  30.     }
  31.  
  32. private:
  33.     const T *lval_;
  34.     T *rval_;
  35. };
  36.  
  37. class logged_creation
  38. {
  39. public:
  40.     logged_creation() {std::cout << "def ctor\n";}
  41.     logged_creation(int) {std::cout << "int ctor\n";}
  42.     logged_creation(const logged_creation &) {std::cout << "copy ctor\n";}
  43.     logged_creation(logged_creation &&) {std::cout << "move ctor\n";}
  44.     logged_creation& operator = (const logged_creation &) {std::cout << "copy assignment\n";}
  45.     logged_creation& operator = (logged_creation &&) {std::cout << "move assignment\n";}
  46. };
  47.  
  48. template <typename T>
  49. class foo
  50. {
  51. public:
  52.     void func(fast_arg<T> val)
  53.     {
  54.         auto v = val.create_as_copy();
  55.     }
  56. };
  57.  
  58. int main()
  59. {
  60.     foo<logged_creation> f;
  61.     std::cout << "LValue:\n";
  62.     logged_creation i;
  63.     f.func(i);
  64.     std::cout << "RValue:\n";
  65.     f.func(logged_creation());
  66.     std::cout << "Converted Temporary:\n";
  67.     f.func(42);
  68. }
  69.  
Success #stdin #stdout 0s 3296KB
comments (?)
stdin 
Standard input is empty
stdout 
LValue:
def ctor
copy ctor
move ctor
RValue:
def ctor
move ctor
Converted Temporary:
int ctor
move ctor
https://ideone.com/uIIPMi
language:
C++14 (gcc 8.3)
created:
12 years ago
visibility:
public

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