CODEWITHSUNDEEP
c++c++11constructorshared-ptr
andrew231
andrew231

Reputation: 156

Conditionally make shared_ptr in initializer list null

I am in a situation where I need to make a shared_ptr either null or contain an instance of class Bar.

The approach below does not work though since, Bar and nullptr are not of the same type. How can achieve this? 

 class Bar {};

 class Foo {

    private:
       shared_ptr<Bar> b;

    public:
       Foo() : b(true ? Bar() : nullptr) {
       }

 };

Upvotes: 4

Views: 318

Answers (3)

Martin Bonner supports Monica
Martin Bonner supports Monica

Reputation: 28987

Your problem is that your initialization of b is incorrect. 

b(Bar())

won't compile either. You'd need 

b(new Bar())

and the equivalent with the ternary operator:

b(true?new Bar():nullptr)

is fine. However, I would recommend avoiding naked new wherever possible, and use

b(true?maked_shared<Bar>():nullptr)

Although make_shared returns a different type to nullptr, they can be coerced into the same type by constructing an empty shared_ptr from the nullptr

Upvotes: 0

R Sahu
R Sahu

Reputation: 206607

You can use 

Foo() : b(true ? std::make_shared<Bar>() : nullptr) {}

My suggestion will be to push that logic to a helper function.

class Foo {

   private:
      std::shared_ptr<Bar> b;

      static std::shared_ptr<Bar> getB(bool flag)
      {
         return (flag ? std::make_shared<Bar>() : nullptr);
      }

   public:
      Foo() : b(getB(true)) {}

};

Upvotes: 1

Vittorio Romeo
Vittorio Romeo

Reputation: 93284

b(true ? std::make_shared<Bar>() : nullptr)

Upvotes: 2

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