C++ Fundamentals for Professionals/

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- Examples

- Examples

The key question of the std::unique_ptr is when to delete the underlying resource. This occurs when the std::unique_ptr goes out of scope or receives a new resource. Let’s look at two use cases to better understand this concept.

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Example 1 #

C++
// uniquePtr.cpp
#include <iostream>
#include <memory>
#include <utility>
struct MyInt{
  MyInt(int i):i_(i){}
  ~MyInt(){
    std::cout << "Good bye from " << i_ << std::endl;
  }
  int i_;
};
int main(){
  std::cout << std::endl;
  std::unique_ptr<MyInt> uniquePtr1{ new MyInt(1998) };
  std::cout << "uniquePtr1.get(): " << uniquePtr1.get() << std::endl;
  std::unique_ptr<MyInt> uniquePtr2;
  uniquePtr2= std::move(uniquePtr1);
  std::cout << "uniquePtr1.get(): " << uniquePtr1.get() << std::endl;
  std::cout << "uniquePtr2.get(): " << uniquePtr2.get() << std::endl;
  std::cout << std::endl;
  {
    std::unique_ptr<MyInt> localPtr{ new MyInt(2003) };
  }
  std::cout << std::endl;
  uniquePtr2.reset(new MyInt(2011));
  MyInt* myInt= uniquePtr2.release();
  delete myInt;
  std::cout << std::endl;
  std::unique_ptr<MyInt> uniquePtr3{ new MyInt(2017) };
  std::unique_ptr<MyInt> uniquePtr4{ new MyInt(2022) };
  std::cout << "uniquePtr3.get(): " << uniquePtr3.get() << std::endl;
  std::cout << "uniquePtr4.get(): " << uniquePtr4.get() << std::endl;
  std::swap(uniquePtr3, uniquePtr4);
  std::cout << "uniquePtr3.get(): " << uniquePtr3.get() << std::endl;
  std::cout << "uniquePtr4.get(): " << uniquePtr4.get() << std::endl;
  std::cout << std::endl;
}

Explanation #

  • The class MyInt (lines 7 -17) is a simple wrapper for a number. We have adjusted the destructor in line 11 - 13 for observing the life cycle of MyInt.

  • We create, in line 24, an std::unique_ptr and return, in line 26, the address of its resource, new MyInt(1998). Afterward, we move the uniquePtr1 to uniquePtr2 (line 29). Therefore, uniquePtr2 is the owner of the resource. That is shown in the output of ...