title: Need help in understanding meaning of ' string ltrim(const string &) '
body: <p>I have little C++ experience. My question is more about confirming if I'm thinking correctly with some queries. I was trying to understand a code and it has the following line:</p>
  
  <pre><code>string ltrim(const string &amp;);
  </code></pre>
  
  <p>I saw the following answers:
  1. <a href="https://stackoverflow.com/questions/35913098/what-is-the-difference-between-stdstring-const-s-and-const-stdstring-s">What is the difference between &quot;std::string const &amp;s&quot; and &quot;const std::string &amp;s&quot;?</a>
  2. <a href="https://www.geeksforgeeks.org/stdstring-class-in-c/" rel="nofollow noreferrer">https://www.geeksforgeeks.org/stdstring-class-in-c/</a>
  3. <a href="https://codereview.stackexchange.com/questions/32842/passing-parameters-by-reference">https://codereview.stackexchange.com/questions/32842/passing-parameters-by-reference</a></p>
  
  <p>I know the concept of call by value and call by reference. </p>
  
  <ol>
  <li>What is 'ltrim'?</li>
  </ol>
  
  <p>It seems to me that it is a function name declared here because I know C++ doesn't have a predefined function with this name. Am i correct to assume that it means that it is a function named 'ltrim' of type 'string' and this function has a parameter 'a constant string type'. </p>
  
  <p>However, usually we have functions like:</p>
  
  <pre><code>void abc (int &amp;a)
  </code></pre>
  
  <p>where a is a variable. What is the variable in:</p>
  
  <pre><code>string ltrim(const string &amp;);
  </code></pre>
  
  <p>because 'const' has a predefined meaning and 'string' is a datatype. What is the variable passed in the parameter?</p>
  
  <ol start="2">
  <li>Also, if there is no variable, what does '&amp;' do here. I understand call by reference some what w.r.t C++, but I don't see a variable name, I don't know what s being used to pass.</li>
  </ol>
  
  <p>I tried looking on internet for some information but i couldn't narro down to a place which explains this in terms of string.</p>