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Virtual functions allow for static dispatch the choice of member function calls to be determined at runtime run time based on the dynamic type of the object that the member function is being called on. This convention supports object-oriented programming practices commonly associated with object inheritance and function overriding. When calling a nonvirtual member function or when using a class member access expression to denote a call, the specified function is called. Otherwise, a virtual function call is made to the final overrider in the dynamic type of the object expression.

However, during the construction and destruction of an object, the rules for virtual method dispatch on that object are restricted. The C++ Standard, [class.cdtor], paragraph 4 [ISO/IEC 14882-2014], states the following:

Member functions, including virtual functions, can be called during construction or destruction. When a virtual function is called directly or indirectly from a constructor or from a destructor, including during the construction or destruction of the class’s non-static data members, and the object to which the call applies is the object (call it x) under construction or destruction, the function called is the final overrider in the constructor’s or destructor’s class and not one overriding it in a more-derived class. If the virtual function call uses an explicit class member access and the object expression refers to the complete object of x or one of that object’s base class subobjects but not x or one of its base class subobjects, the behavior is undefined.

Do not directly or indirectly invoke a virtual function from a constructor or destructor that attempts to call into the object under construction or destruction. Because the order of construction starts with base classes and moves to more derived classes, attempting to call a derived class function from a base class under construction is dangerous: the . The derived class has not had the opportunity to initialize its resources, which is why calling a virtual function from a constructor does not result in a call to a function in a more derived class. Similarly, an object is destroyed in reverse order from construction, so attempting to call a function in a more derived class from a destructor may access resources that have already been released.

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In this noncompliant code example, the base class attempts to seize and release an object's resources through calls to virtual functions from the constructor and destructor. However, it does not result in calling the B::B() constructor calls B::seize() rather than D::seize(). Likewise, the B::~B() destructor calls B::release() or  rather than D::release().

struct B {
  B() { seize(); }
  virtual ~B() { release(); }
 
protected:
  virtual void seize();
  virtual void release();
};

struct D : B {
  virtual ~D() = default;
 
protected:
  void seize() override {
    B::seize();
    // Get derived resources...
  }
 
  void release() override {
    // Release derived resources...
    B::release();
  }
};

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classstruct A {
  A() {
    // f();   // WRONG!
    A::f();   // Okay
  }
  virtual void f();
};

OOP50-CPP-EX2: It is permissible to call a virtual function that has the final virt-specifier from a constructor or destructor, as in this example:.

classstruct A {
  A();
  virtual void f();
};
 
classstruct B : public A {
  B() : A() {
    f();  // Okay
  }
  void f() override final;
};

Similarly, it is permissible to call a virtual function from a constructor or destructor of a class that has the final class-virt-specifier, as in this example:.

classstruct A {
  A();
  virtual void f();
};
 
classstruct B final : public A {
  B() : A() {
    f();  // Okay
  }
  void f() override;
};

In either case, f() must be the final overrider, guaranteeing consistent behavior of the function being called.

Risk Assessment

Automated Detection

Related Vulnerabilities

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

Bibliography

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