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| 35 | <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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| 36 | <a name="bbv2.tutorial.conditions"></a>Conditions and alternatives</h2></div></div></div> |
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| 37 | <p>Sometimes, particular relationships need to be maintained |
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| 38 | among a target's build properties. This can be achieved with |
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| 39 | <em class="firstterm">conditional |
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| 40 | requirement</em>. For example, you might want to set |
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| 41 | specific <code class="computeroutput">#defines</code> when a library is built as shared, |
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| 42 | or when a target's <code class="computeroutput">release</code> variant is built in |
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| 43 | release mode. |
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| 44 | </p> |
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| 45 | <pre class="programlisting"> |
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| 46 | lib network : network.cpp |
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| 47 | : <span class="bold"><strong><link>shared:<define>NEWORK_LIB_SHARED</strong></span> |
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| 48 | <variant>release:<define>EXTRA_FAST |
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| 49 | ; |
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| 50 | </pre> |
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| 51 | <p> |
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| 52 | |
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| 53 | In the example above, whenever <code class="filename">network</code> is |
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| 54 | built with <code class="computeroutput"><link>shared</code>, |
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| 55 | <code class="computeroutput"><define>NEWORK_LIB_SHARED</code> will be in its |
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| 56 | properties, too. |
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| 57 | </p> |
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| 58 | <p> |
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| 59 | Sometimes the ways a target is built are so different that |
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| 60 | describing them using conditional requirements would be |
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| 61 | hard. For example, imagine that a library actually uses |
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| 62 | different source files depending on the toolset used to build |
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| 63 | it. We can express this situation using <em class="firstterm">target |
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| 64 | alternatives</em>: |
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| 65 | </p> |
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| 66 | <pre class="programlisting"> |
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| 67 | lib demangler : dummy_demangler.cpp ; # alternative 1 |
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| 68 | lib demangler : demangler_gcc.cpp : <toolset>gcc ; # alternative 2 |
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| 69 | lib demangler : demangler_msvc.cpp : <toolset>msvc ; # alternative 3 |
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| 70 | </pre> |
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| 71 | <p> |
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| 72 | In the example above, when built with <code class="literal">gcc</code> |
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| 73 | or <code class="literal">msvc</code>, <code class="filename">demangler</code> |
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| 74 | will use a source file specific to the toolset. Otherwise, it |
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| 75 | will use a generic source file, |
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| 76 | <code class="filename">dummy_demangler.cpp</code>. |
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| 77 | </p> |
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