C ++ different behavior between reference and interface pointer
I was trying to write an interface for a variable value. The base class provides the only field enum
where the type of the variable is stored (eg int
, char
etc.)) and some virtual functions. Classes inheriting this interface must implement a variable type representation each.
#include <iostream>
enum Type
{
INT, CHAR
};
class Var
{
Type type;
public:
Var(Type t):
type(t)
{}
virtual void printValue()
{
std::cout << "-\n";
}
virtual void printType()
{
std::cout << type << std::endl;
}
};
class IntVar : public Var
{
int value;
public:
IntVar(int i):
Var(INT),
value(i)
{}
void printValue()
{
std::cout << value << std::endl;
}
};
class CharVar : public Var
{
char value;
public:
CharVar(char c):
Var(CHAR),
value(c)
{}
void printValue()
{
std::cout << value << std::endl;
}
};
Then I tried this:
Var* np = new IntVar(1);
np->printType();
np->printValue();
np = new CharVar('a');
np->printType();
np->printValue();
The way out was
0 (Type :: INT), 1, 1 (Type :: CHAR), a
so everything worked as expected, but when I tried the same thing with links, the result was a little strange.
Var& nr = *(new IntVar(1));
nr.printType();
nr.printValue();
nr = *(new CharVar('a'));
nr.printType();
nr.printValue();
Here the output was
0 (Type :: INT), 1, 1 (Type :: CHAR) and 1
Why was the code working when using pointers and not working with links? Or am I missing some obvious mistakes?
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The decision using pointers is done nr
first to IntVar
, and then to CharVar
.
The reference solution creates an object IntVar
, then creates a new name ( nr
) for that object , and then changes the values of this object based on the values CharVar
.
Links cannot be reinstalled as you did with pointers. The link refers to the same object throughout its life.
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Statement
nr = *(new CharVar('a'));
is an assignment with a completely different meaning than
np = new CharVar('a');
Section 12.8:
The implicitly defined copy / move assignment operator for a non-single class X performs incremental copy / move assignment of its subobjects. Direct base classes X are assigned first, in the order in which they are declared in the base-specifier list, and then the immediate non-static X data members are assigned in the order in which they were declared in the class definition ....
nr uses an implicit assignment operator Var::operator=()
that copies fields from Var. You are not changing the reference type, it is still a reference to the IntVar object.
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