CODEWITHSUNDEEP
c++streamofstream
user14068460
user14068460

Reputation:

Moving Ofstream to class c'tor

In main I have started an ofstream like this:

std::ofstream to(argv[2]);

Now I want to move it (not a copy) to my class builder like this:

class MyClass{
    std::ofstream output;
    MyClass(const std::ofstream &output=std::cout) output(output): {}
};

But that pops up many errors, any idea why is this happening?

Upvotes: 0

Views: 38

Answers (2)

Remy Lebeau
Remy Lebeau

Reputation: 597941

Your code fails because you are trying to take a copy of the ofstream, but C++ streams are not copyable.

If you are using C++11 or later, you can use actual move semantics, eg

class MyClass{
    std::ofstream output;
public:
    MyClass(std::ofstream &&output) : output(std::move(output)) {}
};

...

std::ofstream to(argv[2]);
MyClass cls(std::move(to));

Otherwise, use a non-owning reference or pointer instead (just make sure the stream outlives your class):

class MyClass{
    std::ofstream &output;
public:
    MyClass(std::ofstream &output) : output(output) {}
};

...

std::ofstream to(argv[2]);
MyClass cls(to);

class MyClass{
    std::ofstream *output;
public:
    MyClass(std::ofstream *output) : output(output) {}
};

...

std::ofstream to(argv[2]);
MyClass cls(&to);

Or, you could simply change your constructor to take a filename instead of a pre-existing stream:

class MyClass{
    std::ofstream output;
public:
    MyClass(const char *filename) : output(filename) {}
};

...

MyClass cls(argv[2]);

Upvotes: 1

cigien
cigien

Reputation: 60278

Instead of moving the ostream you should take a reference to it. Also, your class should contain a ostream& otherwise a default constructed MyClass will not work. Also, the constructor should take an ostream&, instead of const ofstream&, like this:

class MyClass{
    std::ostream &output;
  public:
    MyClass(std::ostream &output = std::cout) : output(output) {}
};

Here's a demo.

Upvotes: 2

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