Passing temporary object as parameter by value - is copy constructor called?

问题

If a have a class with both standard and copy constructors

class Ex{
       //constructor definitions
}

and a function that takes it as an argument (by value)

void F(Ex _exin){...}

take the following piece of code:

Ex A;
F(A);   //F's parameter is copy constructed from A
F(Ex());  //F's parameter uses the default constructor

In the third line I'm passing to F a new (temporary) object of the Ex class using the default constructor. My question is: after this new object is created is it also copy constructed/assigned (like it happens in the second line) or is it directly created "inside" F?

回答1:

It was hard to find, but honestly it was bugging me. This is called copy constructor elision.

The standard illustrates this example:

class X{
public:
   X(int);
   X(const X&);
   ~X()
};

X f(X);

void g()
{
   X a(1);
   X b = f(X(2)); //identical to what you have:
   a = f(a);
}

And it states:

12.2/2 Temporary objects

Here, an implementation might use a temporary in which to construct X(2) before passing it to f() using X's copy-constructor; alternatively, X(2) might be constructed in the space used to hold the argument. /.../

After this the standard explains return value optimization, which is basically the same thing.

So it actually has nothing to do with observed behavior, it is up to the compiler.

回答2:

it should call the constructor and the copy-constructor

optimizers could delete unnecessary copying

来源：https://stackoverflow.com/questions/8451212/passing-temporary-object-as-parameter-by-value-is-copy-constructor-called

标签

c++

constructor