将 ofstream 的实例插入到容器中

Question

我创建了 class file，它是 std::ofstream 的包装器。我创建了一个 Container 来包含 file.

的所有实例

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);
      }
};

但是，此代码中存在问题。在 insert 方法中，我试图复制一个 std::pair<int,file>，这不能完成，因为 std::ofstream 的复制构造函数被删除（见下面的编译错误）。

我想在容器中连续添加 file 的实例。我该怎么做？

这里是编译错误

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.

Answer 1

如果你想用 file 初始化你的 Container 而不是复制它们，那么你可以依赖 std::ofstream:

的移动构造函数

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

然后您将执行以下操作：

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

或：

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

如果您的 file 不可移动构造，您可以就地构造它：

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)...));
}

另外，如果你想插入默认初始化的文件（如第一个例子），你可以简单地做：

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

将 ofstream 的实例插入到容器中

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