C++ 对象创建和构造函数

Question

I'm learning ctors now and have a few question. At these lines:

Foo obj(args);

Foo obj2;
obj2 = Foo(args);

Foo obj3 = Foo(args);

First part : only 1 constructor called (Foo) and obj is initialized. So, 1 object creation.

Second part : creating of temporary object obj2 , calling default ctor for it. Next lines we create another copy of Foo and pass its copy into operator=() . Is that right? So, 3 local temporary objects, 2 constructor callings.

Third part : create 1 object Foo and pass its copy into operator=() . So, 2 temprorary objects and 1 ctor calling.

Do I understand this right? And if it's true, will compiler (last gcc, for example) optimize these in common cases?

Answer 1

I will comment on the third one first:

Foo obj3=Foo(args);

It doesn't use operator= which is called copy-assignment. Instead it invokes copy-constructor (theoretically). There is no assignment here. So theoretically, there is two objects creation, one is temporary and other is obj3 . The compiler might optimize the code, eliding the temporary object creation completely.

Now, the second one:

Foo obj2;         //one object creation
obj = Foo(args);  //a temporary object creation on the RHS

Here the first line creates an object, calling the default constructor. Then it calls operator= passing the temporary object created out of the expression Foo(args) . So there is two objects only the operator= takes the argument by const reference (which is what it should do).

And regarding the first one, you're right.

Answer 2

1. Yes, Foo obj(args) creates one Foo object and calls the ctor once.

2. obj2 is not considered a temporary object. But just like 1 Foo obj2 creates one object and calls the Foo ctor. Assuming that you meant obj2 = Foo(args) for the next line, this line creates one temporary Foo object and then calls obj2.operator=() . So for this second example there is only a single temporary object, a single non-temporary, Foo ctors are called twice (once for the non-temporary, once for the temporary) and the operator=() is called once.

3. No, this line does not call operator=() . When you initialize obj3 using the = syntax it is almost exactly as if you had used parentheses instead: Foo obj3(Foo(args)); So this line creates a temporary object, and then calls the Foo copy ctor to initialize obj3 using that temporary object.

Answer 3

Your terminology is a bit confusing.

The objects obj , obj2 obj3 are not called "temporary objects". Only the instance that is created in line 3 before being assign to obj is a temporary object.

Also, you don't create "a copy of Foo", you create either "an instance of Foo" or "an object of type Foo".