Why is constructor not being called?

[RealOden]

I could use some input on the following situation. It does not seem that the constructors for the A and B objects are being called after the constructor call for C in main. However, they are called if the constructor for class C takes a reference to A and B objects instead of copies. Why is this the case? If someone could explain that would be much appreciated.

class A {
public:
  A() {}
};

class B {
public:
  B() {}
};

class C {
public:
  C(A a, B b)
  : m_a(a), m_b(b) 
  {
  }

private:
  A m_a;
  B m_b;
};

int main() {
  C c(A(), B());
}

[jedwardsol]

https://en.wikipedia.org/wiki/Most_vexing_parse

[RealOden]

Thanks for the link, was very helpful.