Correct way of initializing a unique_ptr

Question

Working on learning how to use smart pointers and C++ in general... Assume that I have the following class:

template<typename T>
class MyClass {
public:
  MyClass(const T& def_val)
private:
  std::unique_ptr<T> default_val;
};

What is the idiomatic way of implementing the constructor if I would only like to store a pointer to an object of type T with the value given in the default_val class member? My understanding is also that I don't have to define a destructor at all, since the unique_ptr will automatically taking care of cleaning up itself?

unique_ptr<T> owns a pointer to a dynamically-allocated T ... so you need to dynamically-allocate a T and store it in default_val — Jonathan Wakely
– Jonathan Wakely, Commented Jan 15, 2015 at 12:05

Johann Gerell · Accepted Answer · 2015-01-15 11:44:16Z

3

The way you have written your code, MyClass can only store a unique pointer to a copy of the constructor parameter:

MyClass::MyClass(const T& def_val)
: default_val(new T(def_val))
{
}

This means that T must be copy constructible.

My understanding is also that I don't have to define a destructor at all, since the unique_ptr will automatically taking care of cleaning up itself?

Correct. That is 1 of 2 main purposes for unique_ptr, the 2nd being the guarantee that it has only one owner.

edited Jan 15, 2015 at 11:44

answered Jan 15, 2015 at 11:16

Johann Gerell

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Comments

Jonathan Wakely · Accepted Answer · 2015-01-15 12:01:15Z

3

If you're using C++11 you could add also a constructor that accepts an rvalue ref

template<typename T>
class MyClass {
public:
  MyClass(T&& def_val) : default_val(new T(std::move(def_val))) {}
  MyClass::MyClass(const T& def_val) : default_val(new T(def_val)) {}
private:
  std::unique_ptr<T> default_val;
};

now you accept both const ref, generating a copy, or temporaries

edited Jan 15, 2015 at 12:01

Jonathan Wakely

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answered Jan 15, 2015 at 11:21

dau_sama

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2 Comments

Sebastian Redl Over a year ago

You forgot to allocate an object in the T&& constructor.

Jonathan Wakely Over a year ago

(Edited to allocate the object in the T&& constructor). N.B. saying "If you're using C++11" is a bit redundant in a question about unique_ptr, which is also tagged c++11

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