22b4470f2a
Update all links to C++ Core Guidelines to `e49158a`.
Refresh done using the following script and some manual edits:
db76e34e74/personal/fred-tingaud/rspec/refresh-cppcoreguidelines.py
When re-using this script, be mindful that:
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Co-authored-by: Marco Borgeaud <marco.borgeaud@sonarsource.com>
77 lines
2.2 KiB
Plaintext
77 lines
2.2 KiB
Plaintext
== Why is this an issue?
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A base class and its derived class often differ in size.
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Accessing an array of a derived class through a pointer to the base class leads to wrong pointer arithmetic and can then corrupt memory.
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=== Noncompliant code example
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[source,cpp]
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----
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struct Base { /*...*/};
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struct Derived : public Base { /*...*/};
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void f(Base const &b);
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int main() {
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const size_t size = 4;
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Derived derivedArray[size];
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Base* basePointer = derivedArray; // Noncompliant
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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
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}
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----
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If you need to do a cast from derived to base in an array, do it element by element.
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=== Compliant solution
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[source,cpp]
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----
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#include<iostream>
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using namespace std;
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struct Base {
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int iBase = 0;
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};
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struct Derived : public Base {
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int iDerived = 0;
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};
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int main() {
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const size_t size = 4;
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Derived derivedArray[size];
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for(int i=0; i<size; ++i) {
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derivedArray[i].iBase = i; // store : 0 1 2 3
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derivedArray[i].iDerived = i*1000;
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}
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for(int i=0; i<size; ++i) {
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cout<<"derivedArray["<<i<<"].iBase="<<derivedArray[i].iBase<<endl; // display : 0 1 2 3
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cout<<"base of derivedArray["<<i<<"].iBase="<<static_cast<Base*>(derivedArray+i)->iBase<<endl; // display : 0 1 2 3
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}
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return 0;
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}
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----
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== Resources
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* {cpp} Core Guidelines - https://github.com/isocpp/CppCoreGuidelines/blob/e49158a/CppCoreGuidelines.md#c152-never-assign-a-pointer-to-an-array-of-derived-class-objects-to-a-pointer-to-its-base[C.152: Never assign a pointer to an array of derived class objects to a pointer to its base]
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ifdef::env-github,rspecator-view[]
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'''
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== Comments And Links
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(visible only on this page)
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=== relates to: S5410
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=== on 25 Oct 2019, 16:27:29 Geoffray Adde wrote:
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First, we try a much stronger, constraining and simpler version of the rule: no array (C-style, std::array and std::vector) of publicly derived class.
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=== on 18 Dec 2019, 17:55:05 Loïc Joly wrote:
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If you want to try the stronger version, why not directly document it? I think it might have value on its own, as a code smell... And I would also prevent arrays of base classes.
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endif::env-github,rspecator-view[]
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