Question

Pure virtual functions are those member functions that are virtual and have the pure-specifier ( = 0; )

Clause 10.4 paragraph 2 of C++03 tells us what an abstract class is and, as a side note, the following:

[Note: a function declaration cannot provide both a pure-specifier and a definition
—end note] [Example:

struct C {
virtual void f() = 0 { }; // ill-formed
};

—end example]

For those who are not very familiar with the issue, please note that pure virtual functions can have definitions but the above-mentioned clause forbids such definitions to appear inline (lexically in-class). (For uses of defining pure virtual functions you may see, for example, this GotW)

Now for all other kinds and types of functions it is allowed to provide an in-class definition, and this restriction seems at first glance absolutely artificial and inexplicable. Come to think of it, it seems such on second and subsequent glances 🙂 But I believe the restriction wouldn’t be there if there weren’t a specific reason for that.

My question is: does anybody know those specific reasons? Good guesses are also welcome.

Notes:

• MSVC does allow PVF’s to have inline definitions. So don’t get surprised 🙂
• the word inline in this question does not refer to the inline keyword. It is supposed to mean lexically in-class

Answer 1

In the SO thread "Why is a pure virtual function initialized by 0?" Jerry Coffin provided this quote from Bjarne Stroustrup’s The Design & Evolution of C++, section §13.2.3, where I’ve added some emphasis of the part I think is relevant:

The curious =0 syntax was chosen over the obvious alternative of introducing a new keyword pure or abstract because at the time I saw no chance of getting a new keyword accepted. Had I suggested pure, Release 2.0 would have shipped without abstract classes. Given a choice between a nicer syntax and abstract classes, I chose abstract classes. Rather than risking delay and incurring the certain fights over pure, I used the tradition C and C++ convention of using 0 to represent "not there." The =0 syntax fits with my view that a function body is the initializer for a function and also with the (simplistic, but usually adequate) view of the set of virtual functions being implemented as a vector of function pointers. [ … ]

So, when choosing the syntax Bjarne was thinking of a function body as a kind of initializer part of the declarator, and =0 as an alternate form of initializer, one that indicated “no body” (or in his words, “not there”).

It stands to reason that one cannot both indicate “not there” and have a body – in that conceptual picture.

Or, still in that conceptual picture, having two initializers.

Now, that’s as far as my telepathic powers, google-foo and soft-reasoning goes. I surmise that nobody’s been Interested Enough™ to formulate a proposal to the committee about having this purely syntactical restriction lifted, and following up with all the work that that entails. Thus it’s still that way.