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  1. #include <cstdio>
  2. #include <memory>
  3.  
  4. using namespace std;
  5.  
  6. #define KNOTE(fmt, args...) ::printf(fmt "\n", ##args)
  7.  
  8. class TClass {
  9. public:
  10.   TClass()
  11.     : IntValue(-1) {
  12.     KNOTE("%s@%p: %d", __func__, this, IntValue);
  13.   }
  14.   TClass(int i)
  15.     : IntValue(i) {
  16.     KNOTE("%s@%p: %d", __func__, this, IntValue);
  17.   }
  18.   ~TClass() {
  19.     KNOTE("%s@%p: %d", __func__, this, IntValue);
  20.   }
  21.   int IntValue;
  22. };
  23.  
  24. // A function can return a shared_ptr
  25. shared_ptr<TClass> GetATClass()
  26. {
  27.   auto c = std::make_shared<TClass>(0);
  28.   return c;
  29. }
  30.  
  31. // A function can take a shared_ptr parameter by value
  32. // but better by const reference
  33. void DoSomething(shared_ptr<TClass> c) /* Better const shared_ptr<TClass>& */
  34. {
  35.   c->IntValue = 4;
  36. }
  37.  
  38. void BasicUse()
  39. {
  40.   {
  41.     TClass c1; // TClass: -1
  42.     std::shared_ptr<TClass> c2(new TClass(2)); // TClass: 2
  43.     std::shared_ptr<TClass> c3 = std::make_shared<TClass>(3); // TClass: 3
  44.     {
  45.       std::shared_ptr<TClass> c4;
  46.       c4.reset(new TClass(4)); // TClass: 4
  47.       if (c4) {
  48.         c4->IntValue = 5;
  49.       }
  50.       c4.reset(); // ~TClass: 5
  51.       if (!c4) {
  52.         KNOTE("c4 becomes null");
  53.       }
  54.     }
  55.     c3 = GetATClass(); // c3's object is deleted and re-created
  56.                        // TClass: 0
  57.                        // ~TClass: 3
  58.     DoSomething(c3); // c3->IntValue becomes 4
  59.     c3.reset();  // ~TClass: 4
  60.     c3 = c2; // Now c3 and c2 share ownership of TClass
  61.     DoSomething(c3); // c2/c3 ->IntValue becomes 4
  62.   }
  63.   // c3, c2, c1 out of scope
  64.   // ~TClass: 4
  65.   // ~TClass: -1
  66. }
  67.  
  68. int main()
  69. {
  70.   BasicUse();
  71.   return 0;
  72. }
Success #stdin #stdout 0s 3432KB
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Standard input is empty
stdout
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TClass@0xbff0373c: -1
TClass@0x8d3e008: 2
TClass@0x8d3e040: 3
TClass@0x8d3e048: 4
~TClass@0x8d3e048: 5
c4 becomes null
TClass@0x8d3e068: 0
~TClass@0x8d3e040: 3
~TClass@0x8d3e068: 4
~TClass@0x8d3e008: 4
~TClass@0xbff0373c: -1
https://ideone.com/aEFxlk
Basic use of std::shared_ptr
language:
C++14 (gcc 8.3)
created:
9 years ago
visibility:
public

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