title: What is a value known at compile time?
body: <p>I'm studying the C++ programming language and in a chapter my book introduces me the concept of constant : </p>
  
  <blockquote>
    <p>A constexpr symbolic constant must be given a value that is known at compile time</p>
  </blockquote>
  
  <p>What is a value known at compile time ? Why we need of them ? </p>