C++Talk.NET

toton · Guest

Hi,
I have little confusion about static memory allocation & dynamic
allocation for a cluss member.

I have class like
class Bar{
public:
explicit Bar(){
cout<<"bar default"<<endl;
}
explicit Bar(int x){
cout<<"bar int "<<x<<endl;
}
~Bar(){
cout<<"bar dtor"<<endl;
}
};

class Foo{
private:
Bar bar;
public:
explicit Foo(){
cout<<"foo default"<<endl;
}
explicit Foo(int x) : bar(Bar(10)){
cout<<"foo int "<<x<<endl;
}
~Foo(){
//delete bar;
cout<<"foo dtor"<<endl;
}
};

alternative way to write class Foo as,

class Foo{
private:
Bar* bar;
public:
explicit Foo() : bar(new Bar()){
cout<<"foo default"<<endl;
}
explicit Foo(int x) : bar(new Bar(10)){
cout<<"foo int "<<x<<endl;
}
~Foo(){
delete bar;
cout<<"foo dtor"<<endl;
}
};

which Foo class is to be used when? Most of the GUI library (like
wxWindows and Qt) I find the second kind is used (.i.e dynamic
allocation).
Note, in this case the bar objects lifetime is same as Foo's, i.e its
created at Foo constructor & destroyed at Foo's destructor. I know that
if Bar to be initializd any other place that Foo's ctor & destroyed
other than Foo's dtor, the second method is the only way. But if that
is not the case, which one is preferred? any problem wth the virtual
function of Bar for the first implementation?
moreover,
if I initialize,
Foo foo = Foo(4);
for the first sizeof(foo) is 1, while for second it is 4. why is so?
Also how the memory layout is different between first and second case?

thanks in advance

abir

Guest

"if I initialize,
Foo foo = Foo(4);
for the first sizeof(foo) is 1, while for second it is 4. why is so? "

The first sizeof(foo) == 1, because for some reason, when c++ compile
meet a class with no member variable, means sizeof(myClass) == 0, it
will fake something into myClass, to make it larger than 0. That is,
You will never get a sizeof(myClass) == 0. I think I have read this
from C++ Primier.

The next sizeof(foo) is actually the size of Bar*.

toton wrote:

wei8010@gmail.com · Guest

toton 写道：

toton · Guest

peter koch wrote: