Home Explore Help
Register Sign In
devn00b
/
EQ2EMu
9
3
Fork 0
Files Issues 59
Tree: ff02b05cfd
Branches Tags
EQ2EMu
/
EQ2
/
source
/
depends
/
boost_1_72_0
/
libs
/
outcome
/
doc
/
html
/
tutorial
/
essential
/
result
/
try.html

try.html 7.5 KB
History Raw

123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475 
  1. <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
    2. 

  2. <html><meta http-equiv="Content-Type" content="text/html; charset=utf-8">
    3. 

  3. <title>TRY operations - Boost.Outcome documentation</title>
    4. 

  4. <link rel="stylesheet" href="../../../css/boost.css" type="text/css">
    5. 

  5. <meta name="generator" content="Hugo 0.52 with Boostdoc theme">
    6. 

  6. <meta name="viewport" content="width=device-width,initial-scale=1.0"/>
    7. 

  7. 

  8. <link rel="icon" href="../../../images/favicon.ico" type="image/ico"/>
    9. 

  9. <body><div class="spirit-nav">
    10. 

  10. <a accesskey="p" href="../../../tutorial/essential/result/inspecting.html"><img src="../../../images/prev.png" alt="Prev"></a>
    11. 

  11.     <a accesskey="u" href="../../../tutorial/essential/result.html"><img src="../../../images/up.png" alt="Up"></a>
    12. 

  12.     <a accesskey="h" href="../../../index.html"><img src="../../../images/home.png" alt="Home"></a><a accesskey="n" href="../../../tutorial/essential/outcome.html"><img src="../../../images/next.png" alt="Next"></a></div><div id="content">
    13. 

  13.   <div class="titlepage"><div><div><h1 style="clear: both">TRY operations</h1></div></div></div>
    14. 

  14.   
    15. 

  15. 

  16. <p>In the implementation of function <code>print_half</code> we have seen the usage of the macro <a href="../../../reference/macros/tryv.html" class="api-reference"><code>BOOST_OUTCOME_TRYV(expr)/BOOST_OUTCOME_TRY(expr)</code></a>
    17. 

  17. :</p>
    18. 

  18. <div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="n">BOOST_OUTCOME_TRY</span> <span class="p">(</span><span class="n">i</span><span class="p">,</span> <span class="n">BigInt</span><span class="o">::</span><span class="n">fromString</span><span class="p">(</span><span class="n">text</span><span class="p">));</span>
    19. 

  19. </code></pre></div>
    20. 

  20. <p>The <code>BOOST_OUTCOME_TRY</code> macro uses C macro overloading to select between two implementations based on the number of
    21. 

  21. input parameters. If there is exactly one input parameter i.e. without the <code>i</code>, the control statement is
    22. 

  22. roughly equivalent to:</p>
    23. 

  23. <div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">auto</span><span class="o">&amp;&amp;</span> <span class="n">__result</span> <span class="o">=</span> <span class="n">BigInt</span><span class="o">::</span><span class="n">fromString</span><span class="p">(</span><span class="n">text</span><span class="p">);</span>
    24. 

  24. <span class="k">if</span> <span class="p">(</span><span class="o">!</span><span class="n">__result</span><span class="p">)</span>
    25. 

  25.   <span class="k">return</span> <span class="n">__result</span><span class="p">.</span><span class="n">as_failure</span><span class="p">();</span>
    26. 

  26. </code></pre></div>
    27. 

  27. <p>Where <code>__result</code> is a compile time generated unique name. This single argument form is equivalent to
    28. 

  28. <code>BOOST_OUTCOME_TRYV(expr)</code>, incidentally.</p>
    29. 

  29. 

  30. <p>If there are between two and eight parameters, this control statement is roughly equivalent to:</p>
    31. 

  31. <div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">auto</span><span class="o">&amp;&amp;</span> <span class="n">__result</span> <span class="o">=</span> <span class="n">BigInt</span><span class="o">::</span><span class="n">fromString</span><span class="p">(</span><span class="n">text</span><span class="p">);</span>
    32. 

  32. <span class="k">if</span> <span class="p">(</span><span class="o">!</span><span class="n">__result</span><span class="p">)</span>
    33. 

  33.   <span class="k">return</span> <span class="n">__result</span><span class="p">.</span><span class="n">as_failure</span><span class="p">();</span>
    34. 

  34. <span class="k">auto</span><span class="o">&amp;&amp;</span> <span class="n">i</span> <span class="o">=</span> <span class="n">__result</span><span class="p">.</span><span class="n">value</span><span class="p">();</span>
    35. 

  35. </code></pre></div>
    36. 

  36. <p>So here <code>i</code> as the first C macro parameter is set to the value of any successful result.</p>
    37. 

  37. 

  38. <p>C macro overloads are provided for up to eight arguments. To prevent the
    39. 

  39. confounding of the C preprocessor by commas in template specifications causing more than
    40. 

  40. eight arguments appearing to the C preprocessor, you should consider wrapping the
    41. 

  41. second argument in brackets.</p>
    42. 

  42. 

  43. <hr>
    44. 

  44. 

  45. <h3 id="compiler-specific-extension-boost-outcome-tryx">Compiler-specific extension: <code>BOOST_OUTCOME_TRYX</code></h3>
    46. 

  46. 

  47. <div class="notices note" style="background: url('../../../images/note.png') top left no-repeat padding-box padding-box;">
    48. 

  48. <div class="notices heading">note</div>
    49. 

  49. <div class="notices message"><p>This macro makes use of a proprietary extension in GCC and clang, and is not
    50. 

  50. portable. The macro is not made available on unsupported compilers,
    51. 

  51. so you can test for its presence using <code>#ifdef BOOST_OUTCOME_TRYX</code>.</p>
    52. 

  52. </div>
    53. 

  53. </div>
    54. 

  54. 

  55. 

  56. <p>GCC and Clang provide an extension to C++ known as
    57. 

  57. <a href="https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html" title="GCC docs on statement expressions">statement expressions</a>.
    58. 

  58. These make it possible to use a more convenient macro: <code>BOOST_OUTCOME_TRYX</code>, which is an expression. With the above macro, the above declaration of variable <code>i</code> can be rewritten to:</p>
    59. 

  59. <div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="n">BOOST_OUTCOME_TRYX</span> <span class="p">(</span><span class="n">BigInt</span><span class="o">::</span><span class="n">fromString</span><span class="p">(</span><span class="n">text</span><span class="p">));</span>
    60. 

  60. </code></pre></div>
    61. 

  61. <p>This has an advantage that you can use it any place where you can put an expression, e.g., in &ldquo;simple initialization&rdquo;:</p>
    62. 

  62. <div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">if</span> <span class="p">(</span><span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="n">BOOST_OUTCOME_TRYX</span><span class="p">(</span><span class="n">BigInt</span><span class="o">::</span><span class="n">fromString</span><span class="p">(</span><span class="n">text</span><span class="p">)))</span>
    63. 

  63.   <span class="n">use_a_non_zero_int</span><span class="p">(</span><span class="n">i</span><span class="p">);</span>
    64. 

  64. </code></pre></div>
    65. 

  65. <p>or in as a subexpression of a bigger full expression:</p>
    66. 

  66. <div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="kt">int</span> <span class="n">ans</span> <span class="o">=</span> <span class="n">BOOST_OUTCOME_TRYX</span><span class="p">(</span><span class="n">BigInt</span><span class="o">::</span><span class="n">fromString</span><span class="p">(</span><span class="s">&#34;1&#34;</span><span class="p">))</span> <span class="o">+</span> <span class="n">BOOST_OUTCOME_TRYX</span><span class="p">(</span><span class="n">BigInt</span><span class="o">::</span><span class="n">fromString</span><span class="p">(</span><span class="s">&#34;2&#34;</span><span class="p">));</span>
    67. 

  67. </code></pre></div>
    68. 

  68. 

  69.         </div><p><small>Last revised: February 08, 2019 at 22:18:08 UTC</small></p>
    70. 

  70. <hr>
    71. 

  71. <div class="spirit-nav">
    72. 

  72. <a accesskey="p" href="../../../tutorial/essential/result/inspecting.html"><img src="../../../images/prev.png" alt="Prev"></a>
    73. 

  73.     <a accesskey="u" href="../../../tutorial/essential/result.html"><img src="../../../images/up.png" alt="Up"></a>
    74. 

  74.     <a accesskey="h" href="../../../index.html"><img src="../../../images/home.png" alt="Home"></a><a accesskey="n" href="../../../tutorial/essential/outcome.html"><img src="../../../images/next.png" alt="Next"></a></div></body>
    75. 

  75. </html>
    76.