title: How Can I use Null Lambda in C++?
body: <p>I want to declare a function like this:</p>
  
  <pre><code>template &lt;typename Lambda&gt;
  int foo(Lambda bar) {
      if(/* check bar is null lambda */)
          return -1;
      else
          return bar(3);
  }
  
  int main() {
      std::cout &lt;&lt; foo([](int a)-&gt;int{return a + 3;}) &lt;&lt; std::endl;
      std::cout &lt;&lt; foo(NULL_LAMBDA) &lt;&lt; std::endl;
  }
  </code></pre>
  
  <p>Then, how can I declare the <code>NULL_LAMBDA</code> and the condition checking passed lambda function whether is null?</p>