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<section id="writing-rules-3">
  <title>Part 3 - Introduction to writing rules with C++</title>

  <section>
    <title>Introduction</title>

    <para>The goal for this article is to introduce how
    Cppcheck rules are written with C++. With C++ it is
    possible to write more complex rules than is possible with regular
    expressions.</para>
  </section>

  <section>
    <title>Basics</title>

    <para>A C++ rule is written in a C++ function.</para>

    <para>Rules are organized into Check classes. For instance there is a
    class with the name <literal>CheckStl</literal> that contains various stl
    rules. The <literal>CheckOther</literal> can always be used if no other
    class suits you.</para>

    <para>When you have added your rule you must recompile Cppcheck before you
    can test it.</para>
  </section>

  <section>
    <title>Division by zero</title>

    <para>This simple regular expression will check for division by
    zero:</para>

    <programlisting>cppcheck --rule="/ 0"</programlisting>

    <para>Here is the corresponding C++ check:</para>

    <programlisting>// Detect division by zero
void CheckOther::divisionByZero()
{
    // Loop through all tokens
    for (const Token *tok = _tokenizer-&gt;tokens(); tok; tok = tok-&gt;next())
    {
        // check if there is a division by zero
        if (Token::Match(tok, "/ 0"))
        {
            // report error
            divisionByZeroError(tok);
        }
    }
}

// Report error
void CheckOther::divisionByZeroError()
{
    reportError(tok,                  // location
                Severity::error,      // severity
                "divisionByZero",     // id
                "Division by zero");  // message
}</programlisting>

    <para>The <literal>Token::Match</literal> matches tokens against
    expressions. A few rules about Token::Match expressions are:</para>

    <itemizedlist>
      <listitem>
        <para>tokens are either completely matched or not matched at all. The
        token "abc" is not matched by "ab".</para>
      </listitem>

      <listitem>
        <para>Spaces are used as separators.</para>
      </listitem>

      <listitem>
        <para>With normal regular expressions there are special meanings for +
        * ? ( ). These are just normal characters in
        <literal>Token::Match</literal> patterns.</para>
      </listitem>
    </itemizedlist>
  </section>

  <section>
    <title>Condition before deallocation</title>

    <para>In the first <link linkend="writing-rules-1">Writing rules</link> part
    I described a rule that looks for redundant conditions. Here is the regular
    expression that was shown:</para>

    <programlisting>if \( p \) { free \( p \) ; }</programlisting>

    <para>The corresponding <literal>Token::Match</literal> expression
    is:</para>

    <programlisting>if ( %var% ) { free ( %var% ) ; }</programlisting>

    <para>The <literal>%var%</literal> pattern match any variable name. Here
    is a C++ function:</para>

    <programlisting>// Find redundant condition before deallocation
void CheckOther::dealloc()
{
    // Loop through all tokens
    for (const Token *tok = _tokenizer-&gt;tokens(); tok; tok = tok-&gt;next())
    {
        // Is there a condition and a deallocation?
        if (Token::Match(tok, "if ( %var% ) { free ( %var% ) ; }"))
        {
            // Get variable name used in condition:
            const std::string varname1 = tok-&gt;strAt(2);

            // Get variable name used in deallocation:
            const std::string varname2 = tok-&gt;strAt(7);

            // Is the same variable used?
            if (varname1 == varname2)
            {
                // report warning
                deallocWarning(tok);
            }
        }
    }
}

// Report warning
void CheckOther::deallocWarning()
{
    reportError(tok,                      // location
                Severity::warning,        // severity
                "dealloc",                // id
                "Redundant condition");   // message
}</programlisting>

    <para>The strAt function is used to fetch strings from the token list. The
    parameter specifies the token offset. The result for "tok-&gt;tokAt(1)" is
    the same as for "tok-&gt;next()".</para>
  </section>

  <section>
    <title>Validate function parameters</title>

    <para>Sometimes it is known that a function can't handle certain
    parameters. Here is an example rule that checks that the parameters for
    strtol or strtoul are valid:</para>

    <programlisting>//---------------------------------------------------------------------------
// strtol(str, 0, radix)  &lt;- radix must be 0 or 2-36
//---------------------------------------------------------------------------

void CheckOther::invalidFunctionUsage()
{
    // Loop through all tokens
    for (const Token *tok = _tokenizer-&gt;tokens(); tok; tok = tok-&gt;next())
    {
        // Is there a function call for strtol or strtoul?
        if (!Token::Match(tok, "strtol|strtoul ("))
            continue;

        // Locate the third parameter of the function call..

        // Counter that counts the parameters.
        int param = 1;

        // Scan the function call tokens. The "tok-&gt;tokAt(2)" returns
        // the token after the "("
        for (const Token *tok2 = tok-&gt;tokAt(2); tok2; tok2 = tok2-&gt;next())
        {
            // If a "(" is found then jump to the corresponding ")"
            if (tok2-&gt;str() == "(")
                tok2 = tok2-&gt;link();

            // End of function call.
            else if (tok2-&gt;str() == ")")
                break;

            // Found a ",". increment param counter
            else if (tok2-&gt;str() == ",")
            {
                ++param;

                // If the param is 3 then check if the parameter is valid
                if (param == 3)
                {
                    if (Token::Match(tok2, ", %num% )"))
                    {
                        // convert next token into a number
                        MathLib::bigint radix;
                        radix = MathLib::toBigNumber(tok2-&gt;strAt(1));

                        // invalid radix?
                        if (!(radix == 0 || (radix &gt;= 2 &amp;&amp; radix &lt;= 36)))
                        {
                            dangerousUsageStrtolError(tok2);
                        }
                    }
                    break;
                }
            }
        }
    }
}

void CheckOther::dangerousUsageStrtolError(const Token *tok)
{
    reportError(tok,                          // location
                Severity::error,              // severity
                "dangerousUsageStrtol",       // id
                "Invalid radix");             // message
}</programlisting>

    <para>The link() member function is used to find the corresponding ( ) [ ]
    or { } token.</para>

    <para>The inner loop is not necessary if you just want to get the last
    parameter. This code will check if the last parameter is
    numerical..</para>

    <programlisting>..
        // Is there a function call?
        if (!Token::Match(tok, "do_something ("))
            continue;

        if (Token::Match(tok-&gt;next()-&gt;link()-&gt;tokAt(-2), "(|, %num% )"))
            ...</programlisting>

    <para>The pattern <literal>(|,</literal> can also be written as
    <literal>[(,]</literal>.</para>
  </section>
</section>