From the standard, [temp. A specialization of a member function template does not override a virtual function from a base class. The compliation error disppears but it still doesn't work as you expected; the function template in the derived class doesn't override the virtual function of the base class. You can check it with dynamic dispatch :. If a derived class is handled using pointer or reference to the base class, a call to an overridden virtual function would invoke the behavior defined in the derived class.
You can also apply override specifier to help you confirm overridding. The problem is that virtual functions and template functions doesn't mix very well. So you can't make a template function that directly override a virtual function: if you define func as a null virtual function. When you call b. It's because the template func "hide" the func inherited virtual version. You can "un-hide" the virtual func version in B adding, in the body of B definition. This way, calling b.
This might be considered besides the point but there's a very nice mnemonic that you can use when designing with templates:. In your case you are attempting to provide a body to a virtual method solve a runtime polymorphism through template specialization. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. If not, it throws an error. Using the example above to explain this, the compiler sees line 8, and will make a note to look in any Implementer for a method that returns the type specified as the first template parameter called load that takes an integer as its argument.
For example, you can have multiple extensions of the interface provided by the base class. Another example is in defining the specific implementation to use when you compile.
With this, I can have configurations for different builds for, say different systems, all by substituting out different versions of LoaderConfig. In contrast, virtual member functions are resolved dynamically at run-time. This is called "dynamic binding. The compiler creates a v-table for each class that has at least one virtual function.
For example, if class Circle has virtual functions for draw and move and resize , there would be exactly one v-table associated with class Circle, even if there were a gazillion Circle objects, and the v-pointer of each of those Circle objects would point to the Circle v-table. The v-table itself has pointers to each of the virtual functions in the class. For example, the Circle v-table would have three pointers: a pointer to Circle::draw , a pointer to Circle::move , and a pointer to Circle::resize.
During a dispatch of a virtual function, the run-time system follows the object's v-pointer to the class's v-table, then follows the appropriate slot in the v-table to the method code. The space-cost overhead of the above technique is nominal: an extra pointer per object but only for objects that will need to do dynamic binding , plus an extra pointer per method but only for virtual methods.
The time-cost overhead is also fairly nominal: compared to a normal function call, a virtual function call requires two extra fetches one to get the value of the v-pointer, a second to get the address of the method.
None of this runtime activity happens with non-virtual functions, since the compiler resolves non-virtual functions exclusively at compile-time based on the type of the pointer. I'm attempting to use something like this now for a cubefile base class with templated optimized load functions which will be implemented differently for different types of cubes some stored by pixel, some by image, etc.
Contributing to Arch is a measure of your competence. Griemak-Bleeding edge, not bleeding flat. Edge denotes falls will occur from time to time. Bring your own parachute. If the run function is always the same, why is it pure? What about done by memory, unchecked :. I have not actually tried the code above been away from the office , but from a quick look at it it seems to me that runTemplate cannot be different for different derived classes?
Or could it be possible to use the pointer passed to runTemplate to call a templated function in class derived, sort of like Man, just typing that down is making me excited.
I'll try it out first thing tomorrow in the office and check back here. But does this work? The following declaration:. Generally you can get around this by templating the class instead of the function, and reworking the logic flow a bit.
However, this means that the run method you've defined above, with a switch on some type, has to be re-thought, because you'll have to instantiate your class based on the type. My use-case is actually quite well-specified in the above. Basically, I have a base class which must have a function called run. Derived classes will each implement their own run function.
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