Default parameters with C++ constructors

// Use this...
class foo
{
private:
    std::string name_;
    unsigned int age_;
public:
    foo(const std::string& name = "", const unsigned int age = 0) :
        name_(name),
        age_(age)
    {
        ...
    }
};

// Or this?
class foo
{
private:
    std::string name_;
    unsigned int age_;
public:
    foo() :
    name_(""),
    age_(0)
{
}

foo(const std::string& name, const unsigned int age) :
        name_(name),
        age_(age)
    {
        ...
    }
};

foo f1;
foo f2("Name", 30);

class Vehicle {
public:
  Vehicle(int wheels, std::string name = "Mini");
};

Vehicle x = 5;  // this compiles just fine... did you really want it to?

class Thingy2
{
public:
    enum Color{red,gree,blue};
    Thingy2();

    Thingy2 & color(Color);
    Color color()const;

    Thingy2 & length(double);
    double length()const;
    Thingy2 & width(double);
    double width()const;
    Thingy2 & height(double);
    double height()const;

    Thingy2 & rotationX(double);
    double rotationX()const;
    Thingy2 & rotatationY(double);
    double rotatationY()const;
    Thingy2 & rotationZ(double);
    double rotationZ()const;
}

main()
{
    // gets default rotations
    Thingy2 * foo=new Thingy2().color(ret)
    	.length(1).width(4).height(9)
    // gets default color and sizes
    Thingy2 * bar=new Thingy2()
    	.rotationX(0.0).rotationY(PI),rotationZ(0.5*PI);
    // everything specified.
    Thingy2 * thing=new Thingy2().color(ret)
    	.length(1).width(4).height(9)
    	.rotationX(0.0).rotationY(PI),rotationZ(0.5*PI);
}

// Header
void doSomething(int i = 25) ;

// Source
void doSomething(int i)
{
   // Do something with i
}

// Header
void doSomething() ;
void doSomething(int i) ;

// Source

void doSomething()
{
   doSomething(25) ;
}

void doSomething(int i)
{
   // Do something with i
}

foo bar[400];

foo bar("david", 34)[400]; // NOPE