How can I better learn to “not pay for what you don't use”?

Question

I've just gotten answers to this question which, at the bottom line, tell me: "Doing X doesn't make sense since it would make you pay for things you might not use."

I find this maxim difficult to follow; my instincts lean more towards seeing what I consider clear semantics, with things defined "in their place". More generally, it's not immediate for me to realize what the hidden costs and secret tariffs would be for a particular design choice?.

Is this covered by (non-reference) books on C++? Is there someplace relevant online to better enlighten myself on following this principle?

Answer 1

In the case you are presenting it is not as general a statement as it seems.

Doing X doesn't make sense since it would make you pay for things you might not use.

This is merely a statement that if you can, avoid using virtual functions. They add overhead to the function call.

Virtual functions can often be redesigned by using templates and regular function calls. One std:: example is std::vector. In Java for instance a Vector implements interfaces to be usable in algorithms. Accomplished by virtual function calls. std::vector uses iterators.

Answer 2

Despite the question being overly broad and asking for off site material I think it is interesting enough to deserve an answer. Remember that C++ was originally just "C with classes" and it is still possible today to write what is basically C code without using any of the nice abstractions that C++ gives you. For example if you don't want the cost of using exceptions then don't use them, if you don't want the cost of RTTI (virtual functions) then don't use them, if you don't want the overhead of using templates... etc.

As for resources, I'm going to break the rules and recommend Game Programming Patterns which despite the name is a good general purpose guide to writing performant C++.

Answer 3

How can I better learn to “not pay for what you don't use”?

The key to "not paying for what you don't use" is abstractions. When you clearly understand the purpose of a class or a function, you add the data and arguments that are absolutely necessary for the class and the function to work correctly, with as little overhead as possible.

You have to be very vigilant about adding member variables, member functions (virtual as well as non-virtual) to a class. Every member variable adds to the memory requirements of the class. Every member function requires maintenance. Presence of virtual member functions add to the memory requirements of the class as well as a small penalty at run time.

You have to be very vigilant about the arguments to a function. You don't want the user to be burdened with supplying arguments that don't make sense. You also don't want to leave out any arguments by making hidden assumptions.