将流的实例插入容器

Question

我创建了我的类file，它是std::ofstream的包装。我创建了一个Container来包含所有file的实例。

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，这个代码有问题。在insert方法中，我试图复制std::pair<int,file>，这是因为std::ofstream的复制构造函数被删除而无法完成（请参阅下面的编译错误）。

我想在容器中连续添加file的实例。我怎样才能做到这一点？

---

以下是编译错误

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

如果你想那种初始化Container与file无需将其复制，那么你可以依靠的std::ofstream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 

或者：

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file不动，构造的，你可以建立它在的地方：

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文件（就像第一个例子），你可以简单地做：

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