noncopyable

如果自己设计的一个类，不想支持复制操作，一般需要将copy constructor和copy assignment声明为private.

例子：

#include <cstdlib>
#include <iostream>
using namespace std;

class A {
public:
    A(int x = 0): x_(x) {
        
    }
    
    
    int X() const {
        return x_;
    }
private:
    A(A const&);
    A& operator=(A const&);
    
private:
    int x_;
};

/*
 * 
 */
int main(int argc, char** argv) {
    A a(6);
    cout << a.X() << endl;
    
    A a2(a);
    
    A a3;
    a3 = a;
    return 0;
}

编译器报错：

main.cpp:23:5: error: ‘A::A(const A&)’ is private
main.cpp:37:11: error: within this context
main.cpp:24:8: error: ‘A& A::operator=(const A&)’ is private
main.cpp:40:10: error: within this context

不过避免重复写这些代码是程序员的惯性。

看看boost提供的noncopyable类。现在很多C++产品都在用。改一下代码，这样使用：

#include <cstdlib>
#include <iostream>
using namespace std;

#include <boost/utility.hpp>
using namespace boost;

class A : noncopyable {
public:
    A(int x = 0): x_(x) {
        
    }
    
    int X() const {
        return x_;
    }
    
private:
    int x_;
};

/*
 * 
 */
int main(int argc, char** argv) {
    A a(6);
    cout << a.X() << endl;

    A a2(a);
    
    A a3;
    a3 = a;
    return 0;
}

编译器也报错，就是看起来累点：

/usr/include/boost/noncopyable.hpp: In copy constructor ‘A::A(const A&)’:
/usr/include/boost/noncopyable.hpp:27:7: error: ‘boost::noncopyable_::noncopyable::noncopyable(const boost::noncopyable_::noncopyable&)’ is private
main.cpp:15:7: error: within this context
main.cpp: In function ‘int main(int, char**)’:
main.cpp:36:11: note: synthesized method ‘A::A(const A&)’ first required here 
/usr/include/boost/noncopyable.hpp: In member function ‘A& A::operator=(const A&)’:
/usr/include/boost/noncopyable.hpp:28:26: error: ‘const boost::noncopyable_::noncopyable& boost::noncopyable_::noncopyable::operator=(const boost::noncopyable_::noncopyable&)’ is private
main.cpp:15:7: error: within this context
main.cpp: In function ‘int main(int, char**)’:
main.cpp:39:10: note: synthesized method ‘A& A::operator=(const A&)’ first required here 
make[2]: *** [build/Debug/GNU-Linux-x86/main.o] Error 1

现在看看实现代码：

namespace noncopyable_  // protection from unintended ADL
{
  class noncopyable
  {
   protected:
      noncopyable() {}
      ~noncopyable() {}
   private:  // emphasize the following members are private
      noncopyable( const noncopyable& );
      const noncopyable& operator=( const noncopyable& );
  };
}

typedef noncopyable_::noncopyable noncopyable;

} 

构造和析构函数设置为protocted权限，这样就不能直接创建noncopyable对象，只能由子类构造和析构函数调用它们。

然后声明了copy constructor和copy assignment为private.

非常简单实用。文档中说道总有些包含资源（比如文件和网络通信连接）的对象你不想被拷贝，那就用它吧。