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Tip of the Day
Language: C++
Expertise: Advanced
Jun 15, 1999

The Exception Specification of an Implicitly-Declared Default Constructor

An implicitly-declared default constructor has an exception specification. The exception specification contains all the exceptions of every other special member functions (for example, the constructors of base class and embedded objects) that the constructor invokes directly. To demonstrate that, consider this class hierarchy:

 
  struct A 
  {
    A(); //can throw any type of exception
  };

  struct B 
  {
    B() throw(); //empty exception specification; not allowed to throw any exceptions
  };

Here, the classes, C and D, have no user-declared constructors and consequently, the implementation implicitly declares constructors for them:

 
  struct C : public B
  {
    //implicitly-declared constructor
    // public: inline C::C() throw;
  }

  struct D: public A, public B 
  {
    //implicitly-declared constructor
    // public inline D::D(); 
  };

The implicitly-declared constructor in class C is not allowed to throw any exception because it directly invokes the constructor of class B, which is not allowed to throw any exception either. On the other hand, the implicitly-declared constructor in class D is allowed to throw any type of exception because it directly invokes the constructors of the classes A and B. Since the constructor of class A is allowed to throw any type of exception, D's implicitly-declared constructor has a matching exception specification. In other words, an implicitly-declared constructor allows all exceptions if any function it directly invokes allows all exceptions; it allows no exceptions if every function it directly invokes allows no exceptions.

Danny Kalev
 
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