Page History

...

When f() is called with the designer argument, the formal parameter in f() is sliced and information is lost. When the Manager object is object e is printed, Employee::print() is called instead of Manager::print(), resulting in the following output:

...

Using the same class definitions as the noncompliant code example, this compliant solution modifies the definition of f() to require raw pointers to the object, removing the slicing problem:.

// ...Remainder rest of code unchanged ...
 
void f(const Employee *e) {
  if (e) {
    std::cout << *e;
  }
}

int main() {
  Employee coder("Joe Smith");
  Employee typist("Bill Jones");
  Manager designer("Jane Doe", typist);
  
  f(&coder);
  f(&typist);
  f(&designer);
}

This compliant solution also complies with EXP34-C. Do not dereference null pointers in the implementation of f(). With this definition, the output becomesthe program correctly outputs the following.

Employee: Joe Smith
Employee: Bill Jones
Manager: Jane Doe
Assistant: 
	Employee: Bill Jones

...

An improved compliant solution, which does not require guarding against null pointers within f(), uses references instead of pointers:.

// ... Remainder of code unchanged ...
 
void f(const Employee &e) {
  std::cout << e;
}

int main() {
  Employee coder("Joe Smith");
  Employee typist("Bill Jones");
  Manager designer("Jane Doe", typist);
  
  f(coder);
  f(typist);
  f(designer);
}

Compliant Solution (Noncopyable)

Both of the previous compliant solutions depend on consumers of the Employee and Manager types to be declared in a compliant manner with the expected usage of the class hierarchy. This compliant solution ensures that consumers are unable to accidentally slice objects by removing the ability to copy-initialize an object that derives from Noncopyable. If copy-initialization is attempted, as in the original definition of f(), the program is ill-formed and a diagnostic will be emitted. However, such a solution also restricts the Manager object from attempting to copy-initialize its Employee object, which subtly changes the semantics of the class hierarchy.

...

#include <iostream>
#include <string>

class Noncopyable {
  Noncopyable(const Noncopyable &) = delete;
  void operator=(const Noncopyable &) = delete;
  
protected:
  Noncopyable() = default;
};

class Employee : Noncopyable {
  // Remainder of the definition is unchanged.
  std::string name;
  
protected:
  virtual void print(std::ostream &os) const {
    os << "Employee: " << get_name() << std::endl;      
  }
  
public:
  Employee(const std::string &name) : name(name) {}
  const std::string &get_name() const { return name; }
  friend std::ostream &operator<<(std::ostream &os, const Employee &e) {
    e.print(os);
    return os;
  }
};
 
class Manager : public Employee {
  const Employee &assistant; // Note: ThisThe definition of Employee has been modified.

  // Remainder of the definition is unchanged.
protected:
  void print(std::ostream &os) const override {
    os << "Manager: " << get_name() << std::endl;
    os << "Assistant: " << std::endl << "\t" << get_assistant() << std::endl;      
  }
  
public:
  Manager(const std::string &name, const Employee &assistant) : Employee(name), assistant(assistant) {}
  const Employee &get_assistant() const { return assistant; }
};
 
// If f() were declared as accepting an Employee, the program would be
// ill-formed because Employee cannot be copy-initialized.
void f(const Employee &e) {
  std::cout << e;
}

int main() {
  Employee coder("Joe Smith");
  Employee typist("Bill Jones");
  Manager designer("Jane Doe", typist);
  
  f(coder);
  f(typist);
  f(designer);
}

...

This noncompliant code example uses the same class definitions of Employee and Manager as in the previous examples noncompliant code example and attempts to store Employee objects in a std::vector. However, because std::vector requires a homogeneous list of elements, slicing occurs.

// In addition to the #includes from the previous example#include <iostream>
#include <string>
#include <vector>
 
void f(const std::vector<Employee> &v) {
  for (const auto &e : v) {
    std::cout << e;
  }
}

int main() {
  Employee typist("Joe Smith");
  std::vector<Employee> v{typist, Employee("Bill Jones"), Manager("Jane Doe", typist)};
  f(v);
}

Compliant Solution

This compliant solution stores uses a vector of std::unique_ptr smart pointers in the std::vector, which  objects, which eliminates the slicing problem:.

// In addition to the #includes from the previous example#include <iostream>
#include <memory>
#include <memory><string>
#include <vector>

void f(const std::vector<std::unique_ptr<Employee>> &v) {
  for (const auto &e : v) {
    std::cout << *e;
  }
}

int main() {
  std::vector<std::unique_ptr<Employee>> v;
  
  v.emplace_back(new Employee("Joe Smith"));
  v.emplace_back(new Employee("Bill Jones"));
  v.emplace_back(new Manager("Jane Doe", *v.front()));
  
  f(v);
}

...

Slicing results in information loss, which could lead to abnormal program execution or denial-of-service attacks.

Automated Detection

Related Vulnerabilities

Search for other vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography

...

...

Image Modified Image Modified Image Modified