Selecting derived class based on function parameter - c++

Question

I have these classes:
class Base
{
private:
string name;
public:
void setName(string n);
string getName();
void toString();
}
and two classes derived from this:
class DerivedA : public Base
{
private:
int width;
public:
void setWidth(int w);
int getWidth();
}
and
class DerivedB : public Base
{
private:
int height;
public:
void setHeight(int h);
int getHeight();
}
Now to my question. My main looks like this:
int main()
{
Base* b;
string line;
... file loading ...
while(...)
{
s = cin.getline(file,10);
if(s == "w")
{
b = new DerivedA();
}
else if(s == "h")
{
b = new DerivedB();
}
while(...)
{
b->toString();
}
}
return 0;
}
This always terminates my app. I found out that the b->toString(); part might be the source of the problem, because of different scopes. But anyway, is there a way how can I do this? (I left out boring and unrelated parts of code.)

Answer 1

Base should have a virtual destructor and every function you intend to override should be declared virtual. Additionally, your main function needs some modifications:
int main()
{
Base* b = nullptr; // initialize your pointer
string line;
// ... file loading ...
while(std::getline(file, line)) // this should be your while loop for your file parsing
{
//s = cin.getline(file,10); // why??? you appear to be trying to pass your ifstream object into cin's istream::getline method ... this won't even compile!
// I'm assuming s is a std::string, and you pull it out of the line variable at some point ...
if(s == "w")
{
if (b != nullptr) // properly free your memory
{
delete b;
b = nullptr;
}
b = new DerivedA();
}
else if(s == "h")
{
if (b != nullptr) // properly free your memory
{
delete b;
b = nullptr;
}
b = new DerivedB();
}
while(...)
{
if (b != nullptr) // make sure b is valid!
{
b->toString();
}
}
}
return 0;
}
This always terminates my app. I found out that the b->toString();
part might be the source of the problem, because of different scopes.
But anyway, is there a way how can I do this?
To start off with, what you have posted will (likely) not even compile. cin.getline will attempt to read from standard input. Your comment indicates you are loading a file, so (assuming that file is an std::ifstream instance, cin.getline(file, 10) is attempting to call a function std::istream::getline(std::istream&, int), which does not exist. std::getline does what it appears you want to do here. Additionally, even if you are attempting to read from standard input, it should be std::getline(std::cin, s), not cin.getline(file, 10).
Moving on, the next area is your memory leaks. Those are easy enough to fix by 1) initializing b when it is declared, and 2) properly deleteing it before you leak memory. The null checks are not totally necessary (with an initialized b), since delete will check for NULL anyway, but I wrote them in there to illustrate a point: you should be managing your memory properly!
Next up, your if-else if-condition has the potential to not do anything (that is, b would be uninitialized at worse, or NULL at best). If you don't want to do anything for non-"s"/"h" inputs, that is fine, but then you must do the following item (which you should do anyway).
Finally, the issue that is likely causing your crash is not checking if b is valid before attempting to use it: b->toString();. If b is invalid or null, you are invoking undefined behavior. Your program may crash, call your grandmother, or order a pizza for the President ... all would be valid options, and non of them are what you really intended to do.