发表新帖
发表新帖

Defining an object without calling its constructor in C++

后端 未结
关注
7 2154
执笔经年
执笔经年 2020-12-23 19:26

In C++, I want to define an object as a member of a class like this:

Object myObject;

However doing this will try to call it\'s parameterle

7条回答
  • 暖寄归人
    暖寄归人 (楼主)
    2020-12-23 19:51

    Others have posted solutions using raw pointers, but a smart pointer would be a better idea:

    class MyClass {
  std::unique_ptr pObj;
  // use boost::scoped_ptr for older compilers; std::unique_ptr is a C++0x feature
public:
  MyClass() {
    // ...
    pObj.reset(new Object(...));
    pObj->foo();
  }
  // Don't need a destructor
};



    This avoids the need to add a destructor, and implicitly forbids copying (unless you write your own operator= and MyClass(const MyClass &).
    


    If you want to avoid a separate heap allocation, this can be done with boost's aligned_storage and placement new. Untested:
    


    template
class DelayedAlloc : boost::noncopyable {
  boost::aligned_storage storage;
  bool valid;
public:
  T &get() { assert(valid); return *(T *)storage.address(); }
  const T &get() const { assert(valid); return *(const T *)storage.address(); }

  DelayedAlloc() { valid = false; }

  // Note: Variadic templates require C++0x support
  template
  void construct(Args&&... args)
  {
    assert(!valid);
    new(storage.address()) T(std::forward(args)...);
    valid = true;
  }

  void destruct() {
    assert(valid);
    valid = false;
    get().~T();
  }

  ~DelayedAlloc() { if (valid) destruct(); }
};

class MyClass {
  DelayedAlloc obj;
public:
  MyClass() {
    // ...
    obj.construct(...);
    obj.get().foo();
  }
}



    Or, if Object is copyable (or movable), you can use boost::optional:
    


    class MyClass {
  boost::optional obj;
public:
  MyClass() {
    // ...
    obj = Object(...);
    obj->foo();
  }
};

    
    
             
    
                                                        
    
            
            
    
              
                
                0
              
                   
                
               讨论(0)
              
                                                  
              
              
    
                          
             
       
          
              
    
                                       
    

    

                
    
                         
          
                              			
        
        
        
    
          
    
            
            
    
              
              
    
            
    


                                 
              
    
            
    
                          
    

        
         
    
                
                
    
                  
                
    
                
    
                   看不清?
                
    
              
    
                                  
    
                    
   
                 
             
              
    提交回复
    
            
    
          
    
        
    
      
       
      
    
    
    
          
 
     
 
    
        
    热议问题