Assuming they’re protected, they probably Normally they do things that public users don’t need/want to do.
Note that if you do stash them in the base class, you should normally make them protected, since
Of the base class, but that’s not necessary and it might not even be best. A common place to stash the little pieces is in the protected part
Base constructor code#
Ultimately defined in the derived classes, and the little pieces of common code can be written once (to avoid codeĭuplication) and stashed somewhere (anywhere!). In this case you’d put the overall algorithm in a public virtual that’s Member function whose overall structure is different in each derived class, yet it has little pieces that are the same in Suppose you have the exact opposite situation from the previous FAQ, where you have a.The answer is to make it virtual if you think that some derived class might need The most critical question in this situation is whether or not the public member function containing the overallĪlgorithm should be virtual. They’re often pure virtual, and they’re certainly virtual), and they’d ultimately be defined in each derivedĬlass. The little pieces would be declared in the base class (they’re often protected, In this case you’d write the overallĪlgorithm in the base class as a public member function (that’s sometimes non- virtual), and you’d write the little So the algorithm is the same, but the primitives are different. Overall structure is the same for each derived class, but has little pieces that are different in each derivedĬlass. Suppose you have the situation described in the previous FAQ: you have a member function whose.Yes, there really are two different basic ways to use virtual functions: Is it a different strategy from the other ways to use virtual functions? What’s going on? #include "Shape.h"įloat a = this->area() // area() is pure virtual Printing, be a non- virtual defined in the base class Shape. Shape::print() could, if we were guaranteed no derived class wanted a different algorithm for In this case Shape’s area() member function would necessarily have to be virtual (probably pure virtual) but Printing, but this algorithm depends on their area and they all have a potentially different way to compute their area. For example, suppose all Shape objects have a common algorithm for It’s sometimes ( not always!) a great idea. Is it okay for a non- virtual function of the base class to call a virtual function? See the reasons under given for final classes. How can I set up my member function so it won’t be overridden in a derived class?īut again, ask yourself why you want to. Such cases may warrant a design review for overuse of virtual.
Note, however, that frequent such use may indicate other problems with the design - virtual functions work only in tandem with polymorphism and indirect use (pointers and references). one allocated on the stack of the caller), the compiler inserts code for a regular call. When calling a function directly for a named object (e.g.
The virtual function call mechanism is typically used only when calling through a pointer or a reference. an accessor or a forward), the cost of a virtual call can be measurable and sometimes significant. If, however, the function body is simple (i.e. If that work is significant, the cost of the call itself is negligible by comparison and often cannot be measured. Although a virtual call seems to be a lot more work, the right way to judge costs is in comparison to the work actually carried by the function. A regular call is most often a direct call to a literal address. the pointer to the virtual table) from the object, indexing into it via a constant, and calling the function indirectly via the pointer to function found at that location. In today’s usual implementations, calling a virtual function entails fetching the “vptr” (i.e.
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