How to initialize std::unique_ptr in constructor?

Question

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.

[This](https://stackoverflow.com/questions/8114276/how-do-i-pass-a-unique-ptr-argument-to-a-constructor-or-a-function) is related — mrpandey, Dec 27 '22 at 10:19

David G · Accepted Answer · 2013-10-08T01:04:24.497

56

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));
{ }

edited Oct 08 '13 at 01:04

answered Oct 08 '13 at 00:41

David G

5

Alternatively call the `reset()` function on `file` to assign the `unique_ptr` if you need to do some checking in the constructor beforehand. – gigaplex Oct 08 '13 at 00:54
You don't necessarily have to use member-initializer list. It is more preferable though. – JohnJohn Jan 29 '15 at 10:30
1

@JohnJohn, if you're using Pimpl, you should use member-initializer list, since your std::unique_ptr is const. – csguth Sep 27 '16 at 17:48
1

Regarding the use of `file(new ...)` vs. `file(std::make_unique...)`: Is there any reason to prefer one over the other? – Dirk Herrmann Sep 25 '20 at 18:33

score 6 · Answer 2 · answered Oct 08 '13 at 00:41

6

You can do it like this:

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

answered Oct 08 '13 at 00:41

Jonathan Potter

2 Answers2