title: call base-class constructor in initializer list
body: <p>i write the following code: </p>
  
  <pre><code>class A {
  protected:
  
      int age;
  public:
      A(int a);
  };
  
  A::A(int a){
      cout &lt;&lt; "constructer A" &lt;&lt; a &lt;&lt; endl;
      age =a;
  }
  
  class B : public A{
      int id;
  public:
  B(int i,int a);
  };
  B::B(int i, int a):A::A(a)
  {
      cout &lt;&lt; "constructer B" &lt;&lt; endl;
      id = i;
  }
  </code></pre>
  
  <p>are there different between </p>
  
  <pre><code>B::B(int i, int a):A::A(a)
  </code></pre>
  
  <p>and </p>
  
  <pre><code>B::B(int i, int a):A(a)
  </code></pre>
  
  <p>?,
  i know both of theme work well, but my question is what is the different</p>