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4 | <TITLE>Frequently Asked Questions</TITLE> |
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11 | |
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12 | <H1 CLASS="title">Frequently Asked Questions</H1> |
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13 | <HR CLASS="banner"> |
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16 | |
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17 | <DL class="page-index" style="margin-top:1em"> |
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18 | <DT><A href="#flush">Why is data I've written to a <CODE>filtering_stream</CODE> not reaching the Sink at the end of the chain?</A></DT> |
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19 | <DT><A href="#tee">How do I write to several ostreams at once?</A></DT> |
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20 | <DT><A href="#component_access">How do I access a Filter or Device after I've added it to a chain or attached it to a <CODE>stream</CODE> or <CODE>stream_buffer?</CODE></A></DT> |
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21 | <DT><A href="#offsets">How do perform file positioning operations with large (64-bit) offsets?</A></DT> |
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22 | <DT><A href="#stl">How do I read from or write to an STL sequence?</A></DT> |
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23 | <DT><A href="#multibyte">How do I write a stream which can read or write multibyte character encodings?</A></DT> |
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24 | <DT><A href="#auto_close">Can I swap Filters or Devices in the middle of a sequence of i/o operations?</CODE></A></DT> |
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25 | <DT><A href="#lifetime">Why does my filter chain work with <CODE>std::cout</CODE> but not with another <CODE>ostream</CODE>?</A></DT> |
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26 | <DT><A href="#pipe">Why do I get errors stating that <CODE>operator|</CODE> is ambiguous?</A></DT> |
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27 | <DT><A href="#finite_state">Why do I get errors when compiling the <CODE>finite_state_filter</CODE> examples?</A></DT> |
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28 | </DL> |
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29 | |
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30 | <HR style="margin-top:1em"> |
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31 | |
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32 | <A NAME="flush"></A> |
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33 | <H4>Why is data I've written to a <CODE>filtering_stream</CODE> not reaching the Sink at the end of the chain?</H4> |
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34 | |
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35 | <P> |
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36 | You may need to flush the stream. Note, however, that there is no guarantee that all data written to a <A HREF="classes/filtering_stream.html"><CODE>filtering_stream</CODE></A> will be forwarded to the final Sink until the stream is closed, unless all the Filters in the underlying <A HREF="classes/chain.html"><CODE>chain</CODE></A> are <A HREF="concepts/flushable.html">Flushable</A>. |
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37 | </P> |
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38 | <P> |
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39 | It's also possible that a buggy Filter is modifying data in a way you don't expect, <I>e.g.</I>, by silently discarding data. |
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40 | </P> |
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41 | |
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42 | <A NAME="tee"></A> |
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43 | <H4>How do I write to several ostreams at once?</H4> |
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44 | |
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45 | <P> |
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46 | Use a <A HREF="functions/tee.html#tee_filter"><CODE>tee_filter</CODE></A> or <A HREF="functions/tee.html#tee_device"><CODE>tee_device</CODE></A>. <I>See</I> <A HREF="functions/tee.html"><CODE>tee</CODE></A>. |
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47 | </P> |
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48 | |
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49 | <A NAME="component_access"></A> |
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50 | <H4>How do I access a Filter or Device after I've added it to a chain or attached it to a <CODE>stream</CODE> or <CODE>stream_buffer?</CODE></H4> |
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51 | |
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52 | <P> |
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53 | If you're using a <A HREF="guide/generic_streams.html#stream"><CODE>stream</CODE></A> or <A HREF="guide/generic_streams.html#stream_buffer"><CODE>stream_buffer</CODE></A>, use <CODE>operator*</CODE> or <CODE>operator-></CODE>. |
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54 | </P> |
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55 | |
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56 | <P> |
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57 | If you're using a <A HREF="classes/filtering_stream.html"><CODE>filtering_stream</CODE></A>, <A HREF="classes/filtering_streambuf.html"><CODE>filtering_streambuf</CODE></A> or <A HREF="classes/chain.html"><CODE>chain</CODE></A>, use the member function templates <A HREF="classes/chain.html#component_type"><CODE>component_type</CODE></A> and <A HREF="classes/chain.html#component"><CODE>component</CODE></A>. Alternatively, add your Filter or Device to the chain by reference, using a <A HREF="../../../doc/html/ref.html" TARGET="_top">reference wrapper</A>. |
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58 | </P> |
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59 | |
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60 | <A NAME="offsets"></A> |
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61 | <H4>How do perform file positioning operations with large (64-bit) offsets?</H4> |
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62 | |
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63 | <P> |
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64 | If you're using a raw Device and your compiler supports a 64-bit integral type, you can pass a large offset directly to <CODE>seek</CODE>. To convert the return value of seek to an integral type, use <A HREF="functions/positioning.html#position_to_offset"><CODE>position_to_offset</CODE></A>. |
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65 | </P> |
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66 | |
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67 | <P> |
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68 | If you're using a <A HREF="guide/generic_streams.html#stream_buffer"><CODE>stream_buffer</CODE></A> or <A HREF="classes/filtering_streambuf.html"><CODE>filtering_streambuf</CODE></A>, convert the offset to a <CODE>std::streampos</CODE> using <A HREF="functions/positioning.html#offset_to_position"><CODE>offset_to_position</CODE></A>, then pass it to <CODE>pubseekpos</CODE>. To convert the return value of seek to an integral type, use <A HREF="functions/positioning.html#position_to_offset"><CODE>position_to_offset</CODE></A>. |
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69 | </P> |
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70 | |
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71 | <P> |
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72 | If you're using a <A HREF="guide/generic_streams.html#stream"><CODE>stream</CODE></A> or <A HREF="classes/filtering_stream.html"><CODE>filtering_stream</CODE></A>, convert the offset to a <CODE>std::streampos</CODE> using <A HREF="functions/positioning.html#offset_to_position"><CODE>offset_to_position</CODE></A>, then pass it to the overload of <CODE>seekg</CODE> or <CODE>seekp</CODE> which takes a single <CODE>std::streampos</CODE> argument. To convert the return value of seek to an integral type, use <A HREF="functions/positioning.html#position_to_offset"><CODE>position_to_offset</CODE></A>. |
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73 | </P> |
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74 | |
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75 | <P><I>See</I> <A HREF="functions/positioning.html">Stream Offsets</A>.</P> |
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76 | |
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77 | <A NAME="stl"></A> |
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78 | <H4>How do I read from or write to an STL sequence?</H4> |
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79 | |
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80 | <P> |
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81 | You can append to an STL sequence using a <A HREF="classes/back_inserter.html"><CODE>back_insert_device</CODE></A>, or the function <A HREF="classes/back_inserter.html#back_inserter"><CODE>boost::iostreams::back_inserter</CODE></A>. You can read from an STL sequence |
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82 | by adding an instance of <A HREF="../../range/doc/utility_class.html#iter_range" TARGET="_top"><CODE>boost::itertator_range</CODE></A> to a <A HREF="classes/filtering_stream.html"><CODE>filtering_stream</CODE></A> or <A HREF="classes/filtering_streambuf.html"><CODE>filtering_streambuf</CODE></A>. |
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83 | </P> |
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84 | |
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85 | <P><I>See</I> <A HREF="tutorial/container_source.html">Writing a <CODE>container_source</CODE></A> and <A HREF="tutorial/container_sink.html">Writing a <CODE>container_sink</CODE></A>.</P> |
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86 | |
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87 | <A NAME="multibyte"></A> |
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88 | <H4>How do I write a stream which can read or write multibyte character encodings?</H4> |
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89 | |
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90 | <P> |
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91 | Use a <A HREF="classes/code_converter.html"><CODE>code_converter</CODE></A>. <I>See</I> <A HREF="guide/code_conversion.html"><CODE>Code Conversion</CODE></A>. |
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92 | </P> |
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93 | |
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94 | <A NAME="auto_close"></A> |
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95 | <H4>Can I swap Filters or Devices in the middle of a sequence of i/o operations?</H4> |
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96 | |
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97 | <P> |
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98 | If you're performing output, and if all the Filters in you chain are <A HREF="concepts/flushable.html">Flushable</A>, then yes. First call <A HREF="classes/chain.html#strict_sync"><CODE>strict_sync</CODE></A>. If it returns <CODE>true</CODE>, you can safely call <CODE><A HREF="classes/chain.html#set_auto_close">set_auto_close</A>(false)</CODE> and <A HREF="classes/chain.html#pop"><CODE>pop</CODE></A> one or more components without closing the stream. This applies to instances of <A HREF="classes/filtering_stream.html"><CODE>filtering_stream</CODE></A>, <A HREF="classes/filtering_streambuf.html"><CODE>filtering_streambuf</CODE></A> and <A HREF="classes/chain.html"><CODE>chain</CODE></A>. |
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99 | </P> |
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100 | |
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101 | <A NAME="lifetime"></A> |
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102 | <H4>Why does my filter chain work with <CODE>std::cout</CODE> but not with another <CODE>ostream</CODE>?</H4> |
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103 | |
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104 | <P> |
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105 | The Iostreams library stores streams and stream buffers by reference; consequently, streams and stream buffers must outlive any filter chain to which they are added. This is not a problem for <CODE>std::cout</CODE>, since it is guaranteed to live until the end of the program. |
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106 | </P> |
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107 | |
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108 | <P> |
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109 | Check to make sure that the <CODE>ostream</CODE> is not being destroyed before the <CODE>filtering_stream</CODE>. If both objects are constructed on the stack within the same block, make sure that the <CODE>ostream</CODE> is constructed <I>first</I>. |
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110 | </P> |
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111 | |
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112 | <A NAME="pipe"></A> |
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113 | <H4>Why do I get errors stating that <CODE>operator|</CODE> is ambiguous?</H4> |
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114 | |
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115 | <P> |
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116 | During overload resolution for an expression involving <CODE>operator|</CODE>, your compiler could be considering an implicit conversion from an intergral type to a <A HREF="concepts/pipable.html">Pipable</A> Filter. Make sure that all your Pipable Filters have <CODE>explicit</CODE> constructors. <I>See</I> <A HREF="guide/pipelines.html">Pipelines</A>. |
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117 | </P> |
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118 | |
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119 | <A NAME="finite_state"></A> |
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120 | <H4>Why do I get errors when compiling the <CODE>finite_state_filter</CODE> examples?</H4> |
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121 | |
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122 | <P> |
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123 | The template <CODE>finite_state_filter</CODE> requires a highly standard-conforming compiler. See <A HREF="portability.html">Portability</A> and the <A HREF="http://www.boost.org/status/compiler_status.html" TARGET="_top">Compiler Status Tables</A> for details. |
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124 | </P> |
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125 | |
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126 | <!-- Begin Footer --> |
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127 | |
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128 | <HR> |
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129 | <P CLASS="copyright">Revised |
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130 | <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan --> |
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131 | 20 May, 2004 |
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132 | <!--webbot bot="Timestamp" endspan i-checksum="38504" --> |
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133 | </P> |
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134 | |
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135 | <P CLASS="copyright">© Copyright <A HREF="http://www.kangaroologic.com" TARGET="_top">Jonathan Turkanis</A>, 2004</P> |
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136 | <P CLASS="copyright"> |
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137 | Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at <A HREF="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</A>) |
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138 | </P> |
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