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Self-copy assignment can occur in situations of varying complexity, but essentially, all self-copy assignments entail some variation of the following:.

#include <utility>
 
struct S { /* ... */ }
 
void f() {
  S s;
  s = s; // Self-copy assignment
}

...

The C++ Standard, [copyassignable], specifies that types must ensure that self-copy assignment leave the object in a consistent state when passed to Standard Template Library (STL) functions. Since objects of STL types are used in contexts where CopyAssignable is required, STL types are required to gracefully handle self-copy assignment.

...

#include <new>
 
struct S { /* ... */ }; // Has nonthrowing copy constructor
 
class T {
  int n;
  S *s1;
 
public:
  T(const T &rhs) : n(rhs.n), s1(rhs.s1 ? new S(*rhs.s1) : nullptr) {}
  ~T() { delete s1; }

  // ...
 
  T& operator=(const T &rhs) {
    if (this != &rhs) {
      n = rhs.n;
      delete s1;
      try {
        s1 = new S(*rhs.s1);
      } catch (std::bad_alloc &) {
        s1 = nullptr; // For basic exception guarantees
        throw;
      }
    }
    return *this;
  }
};

Note that this This solution does not provide a strong exception guarantee for the copy assignment. Specifically, if an exception is called when evaluating the new expression, this has already been modified. However, this solution does provide a basic exception guarantee because no resources are leaked and all data members contain valid values. Consequently, this code complies with ERR56-CPP. Guarantee exception safety.

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