rspec/rules/S5405/cfamily/rule.adoc

A base class and its derived class often differ in size.

 Accessing an array of a derived class through a pointer to the base class leads to wrong pointer arithmetic and can then corrupt memory.


== Noncompliant Code Example

----
struct Base { /*...*/};
struct Derived : public Base { /*...*/};
void f(Base const &b);

int main() {
  const size_t size = 4;
  Derived derivedArray[size];
  Base* basePointer = derivedArray; // Noncompliant
  f(basePointer[2]); // The arithmetic will use the size of Base, not the size of Derived, and point to a random byte in the array
}
----
If you need to do a cast from derived to base in an array, do it element by element.


== Compliant Solution

----
#include<iostream>

using namespace std;

struct Base {
  int iBase = 0;
};

struct Derived : public Base {
  int iDerived = 0;
};

int main() {
  const size_t size = 4;
  Derived derivedArray[size];
  for(int i=0; i<size; ++i) {
    derivedArray[i].iBase = i;                                           // store : 0 1 2 3
    derivedArray[i].iDerived = i*1000;
  }
  for(int i=0; i<size; ++i) {
    cout<<"derivedArray["<<i<<"].iBase="<<derivedArray[i].iBase<<endl;   // display : 0 1 2 3
    cout<<"base of derivedArray["<<i<<"].iBase="<<static_cast<Base*>(derivedArray+i)->iBase<<endl;     // display : 0 1 2 3
  }
  return 0;
}
----


== See

* https://github.com/isocpp/CppCoreGuidelines/blob/036324/CppCoreGuidelines.md#c152-never-assign-a-pointer-to-an-array-of-derived-class-objects-to-a-pointer-to-its-base[{cpp} Core Guidelines C.152] - Never assign a pointer to an array of derived class objects to a pointer to its base


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