Question

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Pure virtual functions may not have an inline definition. Why?

Just as the title says, why? And how about (not pure) virtual function, can it be inline format? And I’d like to know the real reason. Thanks!

Answer 1

A virtual function 99% of the time cannot be inlined because the function pointer is resolved at runtime using vtable.

The purpose of a virtual function is to be overloaded by subclasses.
An example :

class A
{
    virtual int foo() { return 1; }
}

class B : public A
{
    virtual int foo() { return 2; }
}

int main(void)
{
    B b;
    A *a = &b;

    return a->foo();
}

The main function will return 2 and not 1.

If the compiler inline the function it will return 1 because the compiler can only assume that the type of *a is A and not B. So the compiler will not do it if he cannot assume safely the type of *a.

In this example the compiler may successfully and safely assume that virtualization is not needed : This is really depending of the compiler and of optimization level.

In some case the compiler can safely assume that virtualization is not needed and only in these cases the inline keyword makes sense.

Even if you declare a function with the keywords inline, the function may not be inlined.

Anyway adding manually the inline keyword is most of the time not a good idea, compiler today are good and automatically inline function when necessary.
By adding inline you may in some case decrease performance, so it a good practice to not abuse of it