Lazy initialization of unique_ptr in C++11

Question

I'm developing c++ project in c++11 (gcc 4.8.2). Recently I found unique_ptr is useful for me. Unfortunately I can't use the std::make_unique function in my environment. So I tried lazy initialization of unique_ptr by using std::move.

Actually, the below code works, I don't have confidence in myself. Could you give any comments on better ways to initialize a unique_ptr? I think my initialization is a little redundant.

 class AppData {
     public:
         AppData(int id):_id(id){};

         int _id;
         void print() { std::cout << "id is " << _id << std::endl; };
 };

 class Test {
     public:
         Test(){};
         ~Test(){};
         void test();

         std::unique_ptr<AppData> p_data;
 };

 void Test::test() {
     // I am concerned with this part
     std::unique_ptr<AppData> p(new AppData(3));
     p_data = std::move(p);
     p_data->print();
 }

 int main() {
     Test t;
     t.test();

     return 0;
 }

You chose a very bad name for your function, as it is easily mistaken with the constructor. Do you plan to initialize the unique_ptr at construction time? Or later? — vsoftco, Jun 05 '15 at 05:09

score 6 · Accepted Answer · edited May 23 '17 at 12:05

6

You could more easily do

p_data.reset(new AppData(3));

rather than going through the extra step of creating a new variable just to move from it. reset will release ownership of the owned object (if any) and free it, then take ownership of the argument.

But make_unique is very easy to implement

To be clear, it would make your Test::test function:

void Test::test() {
    p_data.reset(new AppData(3));
    p_data->print();
}

edited May 23 '17 at 12:05

Community

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answered Jun 05 '15 at 05:23

Ryan Haining

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GREAT! Thanks for your elegant code. I'd love to use C++14. – jef Jun 06 '15 at 07:54

Lazy initialization of unique_ptr in C++11

1 Answers1