Virtual functions & Inline constructors

Posted: December 2, 2010 in C++
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When an object containing virtual functions is created, its VPTR must be initialized to point to the proper VTABLE. This must be done before there’s any possibility of calling a virtual function. A constructor cannot be virtual because at the time when the constructor is invoked the virtual table would not be available in the memory. Hence we cannot have a virtual constructor. As you might guess, because the constructor has the job of bringing an object into existence, it is also the constructor’s job to set up the VPTR. The compiler secretly inserts code into the beginning of the constructor that initializes the VPTR. In fact, even if you don’t explicitly create a constructor for a class, the compiler will create one for you with the proper VPTR initialization code (if you have virtual functions). This has several implications.

The first concerns efficiency. The reason for inline functions is to reduce the calling overhead for small functions. If C++ didn’t provide inline functions, the preprocessor might be used to create these “macros.” However, the preprocessor has no concept of access or classes, and therefore couldn’t be used to create member function macros. In addition, with constructors that must have hidden code inserted by the compiler, a preprocessor macro wouldn’t work at all.

You must be aware when hunting for efficiency holes that the compiler is inserting hidden code into your constructor function. Not only must it initialize the VPTR, it must also check the value of this (in case the operator new returns zero) and call base-class constructors. Taken together, this code can impact what you thought was a tiny inline function call. In particular, the size of the constructor can overwhelm the savings you get from reduced function-call overhead. If you make a lot of inline constructor calls, your code size can grow without any benefits in speed.

Of course, you probably won’t make all tiny constructors non-inline right away, because they’re much easier to write as inlines. But when you’re tuning your code, remember to remove inline constructors.

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