How to initialize std::unique_ptr in constructor?

c++ smart-pointers ifstream unique-ptr

user1529891 · Oct 8, 2013 · Viewed 38.9k times · Source

A.hpp:

class A {
  private:
   std::unique_ptr<std::ifstream> file;
  public:
   A(std::string filename);
};

A.cpp:

A::A(std::string filename) {
  this->file(new std::ifstream(filename.c_str()));
}

The error that I get is thrown:

A.cpp:7:43: error: no match for call to ‘(std::unique_ptr<std::basic_ifstream<char> >) (std::ifstream*)’

Does anyone have any insight as to why this is occurring? I've tried many different ways to get this to work but to no avail.

Answer

You need to initialize it through the member-initializer list:

A::A(std::string filename) :
    file(new std::ifstream(filename));
{ }

Your example was an attempt to call operator () on a unique_ptr which is not possible.

Update: BTW, C++14 has std::make_unique:

A::A(std::string filename) :
    file(std::make_unique<std::ifstream>(filename));
{ }