SEI CERT C++ Coding Standard

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Code Block
bgColor#FFCCCC
langcpp
#include <iostream>
#include <string>

class Employee {
  std::string Namename;
  
protected:
  virtual void print(std::ostream &OSos) const {
    OSos << "Employee: " << getNameget_name() << std::endl;      
  }
  
public:
  Employee(const std::string &Namename) : Namename(Namename) {}
  const std::string &getNameget_name() const { return Namename; }
  friend std::ostream &operator<<(std::ostream &OSos, const Employee &Ee) {
    Ee.print(OSos);
    return OSos;
  }
};
 
class Manager : public Employee {
  Employee Assistantassistant;
  
protected:
  void print(std::ostream &OSos) const override {
    OSos << "Manager: " << getNameget_name() << std::endl;
    OSos << "Assistant: " << std::endl << "\t" << getAssistantget_assistant() << std::endl;      
  }
  
public:
  Manager(const std::string &Namename, const Employee &Assistantassistant) : Employee(Namename), Assistantassistant(Assistantassistant) {}
  const Employee &getAssistantget_assistant() const { return Assistantassistant; }
};

void f(Employee Ee) {
  std::cout << Ee;    
}

int main() {
  Employee Codercoder("Joe Smith");
  Employee Typisttypist("Bill Jones");
  Manager Designerdesigner("Jane Doe", Typisttypist);
  
  f(Codercoder);
  f(Typisttypist);
  f(Designerdesigner);
}

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

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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:.

Code Block
bgColor#ccccff
langcpp
// Remainder of code unchanged...
 
void f(const Employee *Ee) {
  if (Ee) {
    std::cout << *Ee;
  }
}

int main() {
  Employee Codercoder("Joe Smith");
  Employee Typisttypist("Bill Jones");
  Manager Designerdesigner("Jane Doe", Typisttypist);
  
  f(&Codercoder);
  f(&Typisttypist);
  f(&Designerdesigner);
}

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

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

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

Code Block
bgColor#ccccff
langcpp
// ... Remainder of code unchanged ...
 
void f(const Employee &Ee) {
  std::cout << Ee;
}

int main() {
  Employee Codercoder("Joe Smith");
  Employee Typisttypist("Bill Jones");
  Manager Designerdesigner("Jane Doe", Typisttypist);
  
  f(Codercoder);
  f(Typisttypist);
  f(Designerdesigner);
}

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.

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Code Block
bgColor#ccccff
langcpp
#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 Namename;
  
protected:
  virtual void print(std::ostream &OSos) const {
    OSos << "Employee: " << getNameget_name() << std::endl;      
  }
  
public:
  Employee(const std::string &Namename) : Namename(Namename) {}
  const std::string &getNameget_name() const { return Namename; }
  friend std::ostream &operator<<(std::ostream &OSos, const Employee &Ee) {
    Ee.print(OSos);
    return OSos;
  }
};
 
class Manager : public Employee {
  const Employee &Assistantassistant; // Note: ThisThe definition of Employee has been modified.

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

int main() {
  Employee Codercoder("Joe Smith");
  Employee Typisttypist("Bill Jones");
  Manager Designerdesigner("Jane Doe", Typisttypist);
  
  f(Codercoder);
  f(Typisttypist);
  f(Designerdesigner);
}

Noncompliant Code Example

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.

Code Block
bgColor#FFCCCC
langcpp
// In addition to the #includes from the previous example#include <iostream>
#include <string>
#include <vector>
 
void f(const std::vector<Employee> &Vv) {
  for (const auto &Ee : Vv) {
    std::cout << Ee;
  }
}

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

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:.

Code Block
bgColor#ccccff
langcpp
// 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>> &Vv) {
  for (const auto &Ee : Vv) {
    std::cout << *Ee;
  }
}

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

Risk Assessment

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

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

OOP51-CPP

Low

Probable

Medium

P4

L3

Automated Detection

Tool

Version

Checker

Description

CodeSonar
Include Page
CodeSonar_V
CodeSonar_V

LANG.CAST.OBJSLICE

Object Slicing

Helix QAC

Include Page
Helix QAC_V
Helix QAC_V

C++3072
Parasoft C/C++test
9.5OOP-02, JSF-117_a 

 PRQA QA-C++

  Include PagePRQA QA-C++_VPRQA QA-C++_V

3072, 3073

 
Include Page
Parasoft_V
Parasoft_V

CERT_CPP-OOP51-a

Avoid slicing function arguments / return value

Polyspace Bug Finder

Include Page
Polyspace Bug Finder_V
Polyspace Bug Finder_V

CERT C++: OOP51-CPPChecks for object slicing (rule partially covered)
PVS-Studio

Include Page
PVS-Studio_V
PVS-Studio_V

V1054

Related Vulnerabilities

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

Related Guidelines

SEI CERT C++ Coding Standard

ERR61-CPP. Catch exceptions by lvalue reference
CTR56-CPP. Do not use pointer arithmetic on polymorphic objects 

SEI CERT C Coding Standard 

EXP34-C. Do not dereference null pointers

Bibliography

[Dewhurst
02
2002]Gotcha #38, "Slicing"
[ISO/IEC 14882-2014]Subclause 12.8, "Copying and Moving Class Objects"
[Sutter
00
2000]Item 40, "Object Lifetimes—Part I"

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