Re: pure virtual functions and runtime id
cerenoc wrote:
I am fairly new to polymorphism with c++ and am having trouble
figuring out an error message. I narrowed it down to the following
simple example:
------------------------------------------------------------
#include <stdio.h>
#include <stdlib.h>
#include <iostream>
#include <string>
using namespace std;
class Base {
int temp1;
//string temp2;
public:
virtual void whoami() = 0;
};
class Derived1 : public Base {
public:
virtual void whoami() {
cout << "== Derived1 ==" << endl;
}
};
class Derived2 : public Base {
public:
virtual void whoami() {
cout << "== Derived2 ==" << endl;
}
};
int main(int argc, char *argv[]) {
int t = 0;
Base * var;
if (t == 1) {
Derived1 v;
var = &v;
var->whoami();
} else if (t == 0) {
Derived2 v;
var = &v;
var->whoami();
}
var->whoami();
return 0;
}
----------------------------------------------------------------------
This code, as shown, compiles, runs and produces the expected result.
However, if I uncomment the 'string temp2;' line, the code runs to
produce the following error message:
==============
== Derived2 ==
pure virtual method called
terminate called without an active exception
Abort
==============
Could someone please explain this behavior and clue me in on how to
correct it? Thanks a lot.
Look at this section of code:
} else if (t == 0) {
Derived2 v;
var = &v;
var->whoami();
}
var->whoami();
Derived2 v has a lifetime of the else block. After the block v goes out of
scope. Your 2nd call to var->whoami() is attempting to derefernce a pointer
to an instance that has gone out of scope. This is underfined behavior.
Undefined behavior means anything can happen, such as showing some output
when there is no variable defined ni the class or crashing when there is or
whatever, it is undefined.
How to correct it? Do not have your Base* point to a local instance of a
class then attempt to use it after the variable goes out of scope. This may
include using new or simply not attempting to access the variable after what
it points to goes out of scope, it depends on your program.
main() could be reduced farther to show this happening.
int main(int argc, char *argv[]) {
Base * var;
{
Derived2 v;
var = &v;
var->whoami();
}
var->whoami();
return 0;
}
It is only the block itself that causes the issue, doesn't matter if it's a
for block, if block, switch block or naked block as shown. Any variable
defined inside of a block has local scope to that block.
--
Jim Langston
tazmaster@rocketmail.com