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| <H2>29.2 Checking the Stream State</H2> |
| <A NAME="idx733"><!></A> |
| <A NAME="idx734"><!></A> |
| <P>Let's first examine how you can check for errors using the stream state. Each steram class template inherits several member functions from the <SAMP>basic_ios</SAMP> template for this purpose, which are summarized with their effects in <A HREF="29-2.html#Table 31">Table 31</A>:</P> |
| <A NAME="idx735"><!></A> |
| <H4><A NAME="Table 31">Table 31: Stream member functions for error checking </A></H4> |
| <TABLE BORDER="1" CELLPADDING="3" CELLSPACING="3"> |
| <tr><td valign=top><B><B><I><A HREF="../stdlibref/ios-base.html">ios_base</A></I></B> member function</B> |
| </td><td valign=top><B><B>Effect</B></B> |
| </td></tr> |
| <tr><td valign=top><P CLASS="TABLE"><SAMP>bool good()</SAMP></P> |
| </td><td valign=top><P CLASS="TABLE"><SAMP>true</SAMP> if no error flag is set, <SAMP>false</SAMP> otherwise</P> |
| </td></tr> |
| <tr><td valign=top><P CLASS="TABLE"><SAMP>bool eof()</SAMP></P> |
| </td><td valign=top><P CLASS="TABLE"><SAMP>true</SAMP> if <SAMP>eofbit</SAMP> is set, <SAMP>false</SAMP> otherwise</P> |
| </td></tr> |
| <tr><td valign=top><P CLASS="TABLE"><SAMP>bool fail()</SAMP></P> |
| </td><td valign=top><P CLASS="TABLE"><SAMP>true</SAMP> if <SAMP>failbit</SAMP> or <SAMP>badbit</SAMP> is set, <SAMP>false</SAMP> otherwise</P> |
| </td></tr> |
| <tr><td valign=top><P CLASS="TABLE"><SAMP>bool bad()</SAMP></P> |
| </td><td valign=top><P CLASS="TABLE"><SAMP>true</SAMP> if <SAMP>badbit</SAMP> is set, <SAMP>false</SAMP> otherwise</P> |
| </td></tr> |
| <tr><td valign=top><P CLASS="TABLE"><SAMP>bool operator!()</SAMP></P> |
| </td><td valign=top><P CLASS="TABLE"><SAMP>fail()</SAMP></P> |
| </td></tr> |
| <tr><td valign=top><P CLASS="TABLE"><SAMP>operator void*()</SAMP></P> |
| </td><td valign=top><P CLASS="TABLE">Null pointer if <SAMP>fail()</SAMP> and non-null value otherwise</P> |
| </td></tr> |
| <tr><td valign=top><P CLASS="TABLE"><SAMP>iostate rdstate()</SAMP></P> |
| </td><td valign=top><P CLASS="TABLE">Value of stream state</P> |
| </td></tr> |
| </TABLE> |
| <P>It is a good idea to check the stream state in some central place, for example:</P> |
| |
| <UL><PRE> |
| if (!std::cout) error(); |
| </PRE></UL> |
| <P>The state of <SAMP>cout</SAMP> is examined with <SAMP>operator!()</SAMP>, which returns <SAMP>true</SAMP> if the stream state indicates that an error occurred.</P> |
| <P>An ostream can also appear in a boolean position to be tested as follows:</P> |
| |
| <UL><PRE> |
| if (std::cout << x) // okay! |
| </PRE></UL> |
| <P>The magic here is the <SAMP>operator void*()</SAMP>, which returns a nonzero value when the stream state is nonzero.</P> |
| <P>Finally, the explicit member functions can also be used:</P> |
| |
| <UL><PRE> |
| if ((std::cout << x).good()) // okay!; |
| </PRE></UL> |
| <P>Note that there is a difference between <SAMP>good()</SAMP> and <SAMP>operator!()</SAMP>. The function <SAMP>good()</SAMP> takes all flags into account; <SAMP>operator!()</SAMP> and <SAMP>fail()</SAMP> ignore <SAMP>eofbit</SAMP>.</P> |
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