default constructor

Hi,

I couldn't figure out the answer myself about the default constructor --
does C++ create it by default? If yes, under what circumstances?

Actually, I've found out the partial answer myself via trying the
following code:

-----------------------------------------
#include <string>
#include <vector>

class F {
private:
    int f;
    };

class Fo {
public:
    Fo(int i): foo(i) {};
private:
    int foo;
    };

class Foo {
public:
    Foo(int i=0): foo(i) {};
private:
    int foo;
    };

class Bar: public Foo {
public:
    Bar(int i,int j=0): Foo(i), bar(j) {};
private:
    int bar;
    };

class Bar2: public Foo {
public:
    Bar2(int i=0,int j=0): Foo(i), bar(j) {};
private:
    int bar;
    };

class Monitor {
public:
    // interface member functions
    std::string::size_type height, width;
private:
    std::string vendor;
    };

using namespace std;

int main(){

    F f;
    //Fo f;
    Foo foo;
    //Bar bar;
    Bar2 bar2;

    vector<Fo> emptyf; // ok: no need for element default constructor
    //vector<Fo> bad(10); // error: no default constructor for Fo
    vector<Fo> okf(10, 1); // ok: each element initialized to 1
    
    vector<F> bad(10);

    Monitor m;
};
-----------------------------------------

(What's commented out are wrong). But still I don't know exactly under what
circumstances will C++ create default constructor implicitly.

Haven't found the answer from "C++ Primer, 4th Edition" and "The C++
Programming Language, 3rd Edition" yet, and I think it has nothing to do
with the concept of POD (Plain Old Data) object or Trivial Classes defined
in "C++ In A Nutshell".

All in all, the "default constructor" is still a mystery to me, and this
is the only question that I can ask now. Please explain.

Thanks a lot.

tong