...
| 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();
}
|
In this code, class Manager is derived from class Employee and adds additional information, namely the data member assistant. In main, the object designer of class Manager, which contains an assistant data member typist, is copied by value to the object coder of class Employee. This results in the designer object being sliced, and only the Employee information is copied. Hence, the print() statement results in the output:
| Code Block |
|---|
Employee: Jane Doe
|
The information about Jane Doe's assistant is lost.
...
| 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();
}
|
Now, the object designer is not sliced, and the output is:
| Code Block |
|---|
Manager: Jane Doe
Assistant: Bill Jones
|
...
| 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
}
|
...
| 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();
}
|
...
| 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;
};
|
...
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
OOP33-CPP | low | probable | high | P2 | L3 |
Automated Detection
Tool | Version | Checker | Description | ||||||
| 3072,3073 |
Bibliography
[Dewhurst 02] Gotcha #38, "Slicing"
[ISO/IEC 14882-2003] Section 9, "Classes"
[Sutter 00] GotW #22: "Object Lifetimes - Part I"
...