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| Code Block | ||||
|---|---|---|---|---|
| ||||
#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:
...
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 | ||||
|---|---|---|---|---|
| ||||
// 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 |
...
An improved compliant solution, which does not require guarding against null pointers within f(), uses references instead of pointers:.
| Code Block | ||||
|---|---|---|---|---|
| ||||
// ... 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.
...
| Code Block | ||||
|---|---|---|---|---|
| ||||
#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 | ||||
|---|---|---|---|---|
| ||||
// 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 | ||||
|---|---|---|---|---|
| ||||
// 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 |
| LANG.CAST.OBJSLICE | Object Slicing | ||||||
| Helix QAC |
|
| C++ |
| 3072 | ||
| Parasoft C/C++test |
|
| CERT_CPP-OOP51-a | Avoid slicing function arguments / return value | |||||||
| Polyspace Bug Finder |
| CERT C++: OOP51-CPP | Checks for object slicing (rule partially covered) | ||||||
| PVS-Studio |
| V1054 |
3072, 3073
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 |
| SEI CERT C Coding Standard |
Bibliography
| [Dewhurst 2002] | Gotcha #38, "Slicing" |
| [ISO/IEC 14882-2014] | Subclause 12.8, "Copying and Moving Class Objects" |
| [ |
| Sutter 2000] | Item 40, "Object |
| Lifetimes—Part I" |
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