Question

Is there any way to eliminate the explicit constructor call in Foo and somehow have Bar::len assigned to the size of any of Bar’s sub-classes?

class Bar
{
    size_t len_;

    Bar(size_t len) : len_(len) { }
};

class Foo : public Bar
{    
    Foo() : Bar(sizeof(Foo)) { }
};

Answer 1

You could use a “curiously recursive template” to inform the base class of the derived class’s type:

template <typename Derived>
class Bar
{
    size_t len_;
protected:
    Bar() : len_(sizeof(Derived)) {}
};

class Foo : public Bar<Foo>
{
};