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The definition of pointer arithmetic from The the C++ Standard, [expr.add], paragraph 7, states 7 [ISO/IEC 14882-2014], states the following:

For addition or subtraction, if the expressions P or Q have type “pointer to cv T”, where T is different from the cv-unqualified array element type, the behavior is undefined. [Note: In particular, a pointer to a base class cannot be used for pointer arithmetic when the array contains objects of a derived class type. —end note]

Pointer arithmetic does not account for polymorphic object sizes, and attempting to perform pointer arithmetic on a polymorphic object value results in undefined behavior.

The C++ StardardStandard, [expr.sub], paragraph 11 [ISO/IEC 14882-2014], defines array subscripting as being identical to pointer arithmetic. Specifically, it states the following:

The expression E1[E2] is identical (by definition) to *((E1)+(E2))...

Do not use pointer arithmetic, including array subscripting, on polymorphic objects.

Noncompliant Code Example

In this noncompliant code example, f() accepts an array of S objects as its first parameter. However, main() passes an array of T objects as the first argument to f(), which results in undefined behavior due to the pointer arithmetic used within the for loopThe following code examples assume the following static variables and class definitions.

#include <iostream>

int GlobIglobI;
double GlobDglobD;

struct S {
  int Ii;
  
  S() : Ii(GlobIglobI++) {}
};

struct T : S {
  double Dd;
  
  T() : S(), Dd(GlobDglobD++) {}
};

Noncompliant Code Example (Pointer Arithmetic)

In this noncompliant code example, f() accepts an array of S objects as its first parameter. However, main() passes an array of T objects as the first argument to f(), which results in undefined behavior due to the pointer arithmetic used within the for loop.

#include <iostream>
 
// ... definitions for S, T, globI, globD ...

void f(const S *SomeSessomeSes, std::size_t Countcount) { 
  for (const S *Endend = SomeSessomeSes + Countcount; SomeSessomeSes != Endend; ++SomeSessomeSes) {
    std::cout << SomeSessomeSes->I>i << std::endl;
  }
}

int main() {
  T Testtest[5];
  f(Testtest, 5);
}

This example would still be noncompliant if the for loop had instead been written to use array subscripting, like:

Noncompliant Code Example (Array Subscripting)

In this noncompliant code example, the for loop uses array subscripting. Since array subscripts are computed using pointer arithmetic, this code also results in undefined behavior.

#include <iostream>
 
// ... definitions for S, T, globI, globD ...

void f(const S *someSes, std::size_t count) { 
  

for (std::size_t i = 0; i < Countcount; ++i) {
    std::cout << SomeSessomeSes[i].Ii << std::endl;
  }
}

int main() {
  T test[5];
  f(test, 5);
}

Compliant Solution (Array)

Instead of having an array of objects, an array of pointers solves the problem of the objects being of different sizes, as in this compliant solution:.

#include <iostream>

int GlobI;
double GlobD;

struct S {
  int I;
  
  S() : I(GlobI++) {}
};

struct T : S {
  double D;
  
  T() : S(), D(GlobD++) {}
};

// ... definitions for S, T, globI, globD ...

void f(const S * const *SomeSessomeSes, std::size_t Countcount) { 
  for (const S * const *Endend = SomeSessomeSes + Countcount; SomeSessomeSes != Endend; ++SomeSessomeSes) {
    std::cout << (*SomeSessomeSes)->I>i << std::endl;
  }
}

int main() {
  S *Testtest[] = {new T, new T, new T, new T, new T};
  f(Testtest, 5);
  for (auto Vv : Testtest) {
    delete Vv;
  }
}

The elements in the arrays are no longer polymorphic objects (instead, they are pointers to polymorphic objects), and so there is no no undefined behavior with the pointer arithmetic.

Compliant Solution (std::vector)

Another approach is to use an a standard template library (STL) container instead of an array and have f() accept iterators as parameters, as in this compliant solution. However, since because STL containers require homogeneous elements, pointers are still required within the container.

#include <iostream>
#include <vector>

int GlobI;
double GlobD;

struct S {
  int I;
  
  S() : I(GlobI++) {}
};

struct T : S {
  double D;
  
  T() : S(), D(GlobD++) {}
};
// ... definitions for S, T, globI, globD ...
template <typename Iter>
void f(Iter Ii, Iter Ee) {
  for (; Ii != Ee; ++Ii) {
    std::cout << (*Ii)->I>i << std::endl;
  }
}

int main() {
  std::vector<S *> Testtest{new T, new T, new T, new T, new T};
  f(Testtest.cbegin(), Testtest.cend());
  for (auto Vv : Testtest) {
    delete Vv;
  }
}

Risk Assessment

Using arrays polymorphically can result in memory corruption, which could lead to an attacker being able to execute arbitrary code.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography

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