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This code example is non-compliant because of the unintended data loss.
| Code Block | ||||
|---|---|---|---|---|
| ||||
class Employee {
public:
Employee(string theName) : name(theName) {};
string getName() const {return name;}
virtual void print() const {
cout << "Employee: " << getName() << endl;
}
private:
string name;
};
class Manager : public Employee {
public:
Manager(string theName, Employee theEmployee) :
Employee(theName), assistant(theEmployee) {};
Employee getAssistant() const {return assistant;}
virtual void print() const {
cout << "Manager: " << getName() << endl;
cout << "Assistant: " << assistant.getName() << endl;
}
private:
Employee assistant;
};
int main () {
Employee coder("Joe Smith");
Employee typist("Bill Jones");
Manager designer("Jane Doe", typist);
coder = designer; // slices Jane Doe!
coder.print();
}
|
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Assuming exactly the same class structure as above, if pointers to the objects are used so that objects are copied by reference, then slicing does not occur.
| Code Block | ||||
|---|---|---|---|---|
| ||||
int main () {
Employee *coder = new Employee("Joe Smith");
Employee *typist = new Employee("Bill Jones");
Manager *designer = new Manager("Jane Doe", *typist);
coder = designer;
coder->print();
}
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Alternatively, it is often safer to use a smart pointer, like std::auto_ptr, to hold the address of allocated memory. This is typically more robust than the use of raw pointers.
| Code Block | ||||
|---|---|---|---|---|
| ||||
int main () {
auto_ptr<Employee> coder( new Employee("Joe Smith") );
auto_ptr<Employee> typist( new Employee("Bill Jones") );
auto_ptr<Manager> designer( new Manager("Jane Doe", *typist) );
coder = designer; // Smith deleted, Doe xferred
coder->print();
// everyone deleted
}
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Alternatively, references may be used to refer to the various derived employee objects.
| Code Block | ||||
|---|---|---|---|---|
| ||||
int main () {
Employee coder("Joe Smith");
Employee typist("Bill Jones");
Manager designer("Jane Doe", typist);
Employee &toPrint = designer; // Jane remains entire
toPrint.print();
}
|
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The most effective way to avoid slicing of objects is to ensure, whenever possible, that polymorphic base classes are abstract.
| Code Block | ||||
|---|---|---|---|---|
| ||||
class Employee {
public:
Employee(string theName) : name(theName) {};
virtual ~Employee();
string getName() const {return name;}
virtual void print() const = 0;
private:
string name;
};
|
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