Question

Consider:

struct device{
    virtual void switchon() = 0 {}
};

int main()
{

}

I wrote code similar to following and it gave an error:

pure-specifier on function-definition
compilation terminated due to
-Wfatal-errors.

When I asked him, he showed me the following quote from the standard:

A virtual function declared in a class
shall be defined, or declared pure
(10.4) in that class, or both; but no
diagnostic is required (3.2).

I can’t seem to understand what it means and I think this somehow is not relevant.

PS: If this is not the relevant quote, please guide me to the proper one so that I can have a better counterargument.

Answer 1

A pure virtual function may have a definition (out of class definition). That is completely optional. But what you are trying to do is plain wrong because

C++03 [Section 10.4/2] says:

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

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

However you are free to write

struct device{
    virtual void switchon() = 0;
};

void device::switchon() { } // Definition {optional}

int main()
{

}