问题
I'm a little confused as to the mechanics of the copy constructor. Correct me if I'm wrong:
If a method takes a reference to an object as a parameter, and the class defines a copy construtor, then the class uses the constructor to create a copy of itself and that gets passed to the function instead of a reference to the original object?
Furthermore, one can call
Object * obj = new Object(&anotherObject);
to create a copy of anotherObject?
回答1:
No, if a function take a reference:
void f1( Object & o ); // call by reference
then no copy is made. If a function takes a value:
void f2( Object o ); // call by value
then a copy is created by the compiler using the copy constructor.
And yes, when you say:
Object * obj = new Object(anotherObject); // not &anotherObject
the copy constructor is used explicitly (assuming anotherObject is of type Object.) There is nothing magic about the use of new here, however - in this case:
Object obj2(anotherObject);
the copy constructor is also used.
回答2:
If a method takes a reference to an object as a parameter, copy constructor will not be called. If that was the case, then a call to the copy constructor itself would have resulted in an infinite loop (since it takes a reference as an argument).
That line is not a valid way to call a copy constructor. It expects a reference as an argument, not a pointer.
回答3:
The fact that you are making a method call is of no importance here. Reference parameter initialization during a function call is no different from a standalone reference initialization and is governed by the same rules.
The rules of reference initialization are a bit complicated, but the bottom line is that if the initializer is an lvalue (the argument in the method call in your case) and the type of the reference is the same as the type of the initializer (i.e. type of the parameter is the same as the argument type), then the reference will be bound directly. I.e. no copy is created.
Object a; // class type
Object &r = a; // no copying
const Object &cr = a; // no copying
If these requirements are not met (like if the initializer is an rvalue, for example), then it all depends. In some cases the copying might and will take place. For example
const Object &tr = Object();
can be interpreted by the compiler as
const Object &tr = Object(Object(Object(Object())));
with implementation-dependent finite number of copyings. Of course, for efficiency reasons compilers normally are trying not to create unnecessary copies, even when they are allowed to copy.
A classic example that often stirs debate about the validity of the copying behavior of the compiler is the reference initialization in expressions like the following one
Object a;
const Object &r = <some condition> ? a : Object();
A person familiar with C++ reference semantics would understand that expressions like the above are likely the rationale behind the standard permission to perform superfluous copying during reference initialization.
回答4:
No in both the cases. In the first case, reference to that object itself is passed and copy is not created. In the second case you are passing a pointer to the constructor of object hence no copy is created. So object should have a constructor (not a copy constructor) which is something like object(anotherClass*)
回答5:
Copy constructor is called only when passing by value, not by reference. By reference no copying is needed (this is part of what references are for!) so no copy constructor called.
回答6:
yes using placement new like so:
Object dstObject;
new(&dstObject) Object(&anotherObject);
来源：https://stackoverflow.com/questions/2213894/can-i-call-a-copy-constructor-explicitly