Re: assignment and coyp constructor
On Dec 14, 12:38 pm, samhas <sahaselho...@googlemail.com> wrote:
Hello,
I have the following class definition:
class test2{
public:
int y;
test2(int ui) : y(ui){}
test2(const test2& t) : y(5){}
test2& operator= (const test2& t){ y = 6; return *this; }
};
In an expression like
test2 t2(test2(20)); or
test2(t2) = test2(20);
neither the copy-constructor nor the assgnment operator get executed.
t2 end up with y=20.
But in the following case the copy constructor gets called:
test2 t = test2(20);
test2 t2(t); // or test2 t2 = t;
Can someone explain to me what's going on there?
Tried this out and got the same results. A friend referenced the
following from the standard:
12.2 Temporary objects [class.temporary]
1 Temporaries of class type are created in various contexts: binding
an rvalue to a reference (8.5.3), returning
an rvalue (6.6.3), a conversion that creates an rvalue (4.1, 5.2.9,
5.2.11, 5.4), throwing an exception (15.1),
entering a handler (15.3), and in some initializations (8.5). [Note:
the lifetime of exception objects is
described in 15.1. ] Even when the creation of the temporary object is
avoided (12.8), all the semantic
restrictions must be respected as if the temporary object was created.
[Example: even if the copy constructor
is not called, all the semantic restrictions, such as accessibility
(clause 11), shall be satisfied. ]
2 [Example:
class X {
// ...
public:
// ...
X(int);
X(const X&);
~X();
};
X f(X);
void g()
{
X a(1);
X b = f(X(2));
a = f(a);
}
Here, an implementation might use a temporary in which to construct X
(2) before passing it to f() using
X's copyconstructor;
alternatively, X(2) might be constructed in the space used to hold the
argument.
Also, a temporary might be used to hold the result of f(X(2)) before
copying it to b using X's copyconstructor;
alternatively, f()'s result might be constructed in b. On the other
hand, the expression
a=f(a) requires a temporary for either the argument a or the result of
f(a) to avoid undesired aliasing of
a. ]
Tom M.