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For example, in C++03, std::auto_ptr had the following copy operation signatures [ISO/IEC 14882-2003]:

Both copy construction and copy assignment would mutate the source argument, A, by effectively calling this->reset(A.release()). However, this invalidated assumptions made by standard library algorithms such as std::sort(), which may need to make a copy of an object for later comparisons [Hinnant 05]. Consider an implementation the following implementation of std::sort() that implements the quick sort algorithm:.

// ...
value_type pivot_element = *mid_point;
// ...

At this point, the sorting algorithm assumes that pivot_element and *mid_point have equivalent value representations and will compare equal. However, for std::auto_ptr, this is not the case because *mid_point has been mutated, and mutated and results in unexpected behavior.

In C++11, the std::unique_ptr smart pointer class was introduced as a replacement for std::auto_ptr to better specify the ownership semantics of pointer objects. Rather than mutate the source argument in a copy operation, std::unique_ptr explicitly deletes the copy constructor and copy assignment operator, and instead uses a move constructor and move assignment operator. Subsequently, std::auto_ptr was deprecated in C++11.

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In this compliant solution, the copy operations for A no longer mutate the source operand, ensuring that the vector contains equivalent copies of obj. Instead, A has been given move operations that perform the mutation when it is safe to do so.

#include <algorithm>
#include <vector>

class A {
  int m;
  
public:
  A() : m(0) {}
  explicit A(int m) : m(m) {}
  
  A(const A &other) : m(other.m) {}
  A(A &&other) : m(other.m) { other.m = 0; }
  
  A& operator=(const A &other) {
    if (&other != this) {
      m = other.m;
    }
    return *this;
  }
 
  A& operator=(A &&other) {
    m = other.m;
    other.m = 0;
    return *this;
  }
  
  int get_m() const { return m; }
};

void f() {
  std::vector<A> v{10};
  A obj(12);
  std::fill(v.begin(), v.end(), obj);
}

Exceptions

OOP58-CPP-EX0: Reference counting, and implementations such as std::shared_ptr<> constitute an exception to this rule. Any copy or assignment operation of a reference-counted object requires the reference count to be incremented. The semantics of reference counting are well-understood, and it can be argued that the reference count is not a salient part of the shared_pointer object.

Risk Assessment

Copy operations that mutate the source operand or global state can lead to unexpected program behavior. In the case of using Using such a type in a Standard Template Library container or algorithm , this can also lead to undefined behavior.

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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