Insert instance of ofstream to container

问题

I created my a class file which is a wrapper for std::ofstream. I created a Container to contain all the instances of file.

class file
{
private:
      std::ofstream ofs;

public:
      void open(std::string filename);
      void write(std::string s);
      void close();
};

class Container
{
private:
  std::map<int, file> m;


public:
      void insert(file f,int i)
      {
            m.insert(std::pair<int,file> (i,f));
      }
      void get(int i)
      {
            m.at(i);
      }
};

However, there is an issue in this code. In the insert method I am attempting to copy a std::pair<int,file> which cannot be done as the copy constructor of std::ofstream is deleted (see compilation error below).

I would like to successively add instances of file in a container. How can I do that?

Here is the compilation error

In file included from src/test.cpp:1:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/iostream:38:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/ios:216:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/__locale:15:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/string:439:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/algorithm:627:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/utility:274:23: error: call to implicitly-deleted copy constructor of
      'file'
        : first(__x), second(__y) {}
                      ^      ~~~
src/test.cpp:35:22: note: in instantiation of member function 'std::__1::pair<int, file>::pair' requested here
            m.insert(std::pair<int,file> (i,f));
                     ^
src/test.cpp:9:21: note: copy constructor of 'file' is implicitly deleted because field 'ofs' has a deleted copy constructor
      std::ofstream ofs;
                    ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/fstream:1168:5: note: copy constructor is implicitly deleted because
      'basic_ofstream<char, std::__1::char_traits<char> >' has a user-declared move constructor
    basic_ofstream(basic_ofstream&& __rhs);
    ^
1 error generated.

回答1:

If you want to kind of initialize your Container with file without copying them, then you could rely on the move-constructor of std::ofstream:

void insert(file &&f, int i)
{
    m.insert(std::pair<int,file>(i, std::move(f)));
}

Then you would do the following:

cont.insert(file{}, 0); // Construct a file when inserting, you already have an rvalue

Or:

file myfile;
myfile.open(...); // Do whatever you want with myfile...
cont.insert(std::move(myfile), 0); // And then move it, as long as you are not using if after

If your file is not move-constructible, you could construct it in place:

template <typename... Args>
void insert(int i, Args&&... args) {
    m.emplace(std::piecewise_construct, std::tuple<int>(i),
              std::tuple<Args>(std::forward<Args>(args)...));
}

Also, if you want to insert default-initialized file (like in the first example), you could simply do:

void insert(int i)
{
    m[i];
}

来源：https://stackoverflow.com/questions/43429708/insert-instance-of-ofstream-to-container

标签

c++

design-patterns

copy-constructor

ofstream