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An object deriving from a base class typically contains additional member variables that extend the base class. When by-value assigning or copying an object of the derived type to an object of the base type, those additional member variables are not copied because there is insufficient space within the base class contains insufficient space in which to store them. This act action is commonly referred to as called slicing the object as because the additional members are "sliced off" the resulting object.

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In this noncompliant code example, an object of the derived Manager type is passed by value to a function accepting a base Employee type. This results in slicing Consequently, the Manager objects are sliced, resulting in information loss and unexpected behavior when the print() function is called.

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

Employee: Jane Doe

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

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This compliant solution also complies with EXP34-C. Do not dereference null pointers in the implementation of f(). With this definition, the output becomes:

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

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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. 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 requires restricts the Manager object to not attempt from attempting to copy-initialize its Employee object, which subtly changes the semantics of the class hierarchy.

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#include <iostream>
#include <string>

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

class Employee : Noncopyable {
  std::string Name;
  
protected:
  virtual void print(std::ostream &OS) const {
    OS << "Employee: " << getName() << std::endl;      
  }
  
public:
  Employee(const std::string &Name) : Name(Name) {}
  const std::string &getName() 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: thisThis definition has been modified
  
protected:
  void print(std::ostream &OS) const override {
    OS << "Manager: " << getName() << std::endl;
    OS << "Assistant: " << std::endl << "\t" << getAssistant() << std::endl;      
  }
  
public:
  Manager(const std::string &Name, const Employee &Assistant) : Employee(Name), Assistant(Assistant) {}
  const Employee &getAssistant() 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);
}

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This noncompliant code example uses the same class definitions of Employee and Manager from above, as in the previous examples 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 <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);
}

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// In addition to the #includes from the previous example.
#include <memory>
#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);
}

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

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

Related Guidelines

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