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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="math_toolkit.new_const"></a><a class="link" href="new_const.html" title="Defining New Constants">Defining New Constants</a>
</h2></div></div></div>
<p>
The library provides some helper code to assist in defining new constants;
the process for defining a constant called <code class="computeroutput"><span class="identifier">my_constant</span></code>
goes like this:
</p>
<p>
1. <span class="bold"><strong>Define a function that calculates the value of the
constant</strong></span>. This should be a template function, and be placed in
<code class="computeroutput"><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">constants</span><span class="special">/</span><span class="identifier">calculate_constants</span><span class="special">.</span><span class="identifier">hpp</span></code> if the
constant is to be added to this library, or else defined at the top of your
source file if not.
</p>
<p>
The function should look like this:
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">math</span><span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">constants</span><span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">detail</span><span class="special">{</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">Real</span><span class="special">></span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">int</span> <span class="identifier">N</span><span class="special">></span>
<span class="identifier">Real</span> <span class="identifier">constant_my_constant</span><span class="special"><</span><span class="identifier">Real</span><span class="special">>::</span><span class="identifier">compute</span><span class="special">(</span><span class="identifier">BOOST_MATH_EXPLICIT_TEMPLATE_TYPE_SPEC</span><span class="special">(</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="identifier">N</span><span class="special">>))</span>
<span class="special">{</span>
<span class="keyword">int</span> <span class="identifier">required_precision</span> <span class="special">=</span> <span class="identifier">N</span> <span class="special">?</span> <span class="identifier">N</span> <span class="special">:</span> <span class="identifier">tools</span><span class="special">::</span><span class="identifier">digits</span><span class="special"><</span><span class="identifier">Real</span><span class="special">>();</span>
<span class="identifier">Real</span> <span class="identifier">result</span> <span class="special">=</span> <span class="comment">/* value computed to required_precision bits */</span> <span class="special">;</span>
<span class="keyword">return</span> <span class="identifier">result</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">}}}}</span> <span class="comment">// namespaces</span>
</pre>
<p>
Then define a placeholder for the constant itself:
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">math</span><span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">constants</span><span class="special">{</span>
<span class="identifier">BOOST_DEFINE_MATH_CONSTANT</span><span class="special">(</span><span class="identifier">my_constant</span><span class="special">,</span> <span class="number">0.0</span><span class="special">,</span> <span class="string">"0"</span><span class="special">);</span>
<span class="special">}}}</span>
</pre>
<p>
For example, to calculate π/2, add to <code class="computeroutput"><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">constants</span><span class="special">/</span><span class="identifier">calculate_constants</span><span class="special">.</span><span class="identifier">hpp</span></code>
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">N</span><span class="special">></span>
<span class="keyword">inline</span> <span class="identifier">T</span> <span class="identifier">constant_half_pi</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">compute</span><span class="special">(</span><span class="identifier">BOOST_MATH_EXPLICIT_TEMPLATE_TYPE_SPEC</span><span class="special">(</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="identifier">N</span><span class="special">>))</span>
<span class="special">{</span>
<span class="identifier">BOOST_MATH_STD_USING</span>
<span class="keyword">return</span> <span class="identifier">pi</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">policies</span><span class="special">::</span><span class="identifier">policy</span><span class="special"><</span><span class="identifier">policies</span><span class="special">::</span><span class="identifier">digits2</span><span class="special"><</span><span class="identifier">N</span><span class="special">></span> <span class="special">></span> <span class="special">>()</span> <span class="special">/</span> <span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="number">2</span><span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
Then to <code class="computeroutput"><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">constants</span><span class="special">/</span><span class="identifier">constants</span><span class="special">.</span><span class="identifier">hpp</span></code> add:
</p>
<pre class="programlisting"><span class="identifier">BOOST_DEFINE_MATH_CONSTANT</span><span class="special">(</span><span class="identifier">half_pi</span><span class="special">,</span> <span class="number">0.0</span><span class="special">,</span> <span class="string">"0"</span><span class="special">);</span> <span class="comment">// Actual values are temporary, we'll replace them later.</span>
</pre>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
Previously defined constants like pi and e can be used, but by <span class="bold"><strong>not simply calling</strong></span> <code class="computeroutput"><span class="identifier">pi</span><span class="special"><</span><span class="identifier">T</span><span class="special">>()</span></code>;
specifying the precision via the policy <code class="computeroutput"><span class="identifier">pi</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span>
<span class="identifier">policies</span><span class="special">::</span><span class="identifier">policy</span><span class="special"><</span><span class="identifier">policies</span><span class="special">::</span><span class="identifier">digits2</span><span class="special"><</span><span class="identifier">N</span><span class="special">></span> <span class="special">></span> <span class="special">>()</span></code>
is essential to ensure full accuracy.
</p></td></tr>
</table></div>
<div class="warning"><table border="0" summary="Warning">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
<th align="left">Warning</th>
</tr>
<tr><td align="left" valign="top">
<p>
Newly defined constants can only be used once they are included in <code class="computeroutput"><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">constants</span><span class="special">/</span><span class="identifier">constants</span><span class="special">.</span><span class="identifier">hpp</span></code>. So
if you add <code class="computeroutput"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">N</span><span class="special">></span> <span class="identifier">T</span> <span class="identifier">constant_my_constant</span><span class="special">{...}</span></code>,
then you cannot define <code class="computeroutput"><span class="identifier">constant_my_constant</span></code>
until you add the temporary <code class="computeroutput"><span class="identifier">BOOST_DEFINE_MATH_CONSTANT</span><span class="special">(</span><span class="identifier">my_constant</span><span class="special">,</span> <span class="number">0.0</span><span class="special">,</span>
<span class="string">"0"</span><span class="special">)</span></code>.
Failing to do this will result in surprising compile errors:
</p>
<pre class="programlisting"><span class="identifier">error</span> <span class="identifier">C2143</span><span class="special">:</span> <span class="identifier">syntax</span> <span class="identifier">error</span> <span class="special">:</span> <span class="identifier">missing</span> <span class="char">';'</span> <span class="identifier">before</span> <span class="char">'<'</span>
<span class="identifier">error</span> <span class="identifier">C2433</span><span class="special">:</span> <span class="char">'constant_root_two_div_pi'</span> <span class="special">:</span> <span class="char">'inline'</span> <span class="keyword">not</span> <span class="identifier">permitted</span> <span class="identifier">on</span> <span class="identifier">data</span> <span class="identifier">declarations</span>
<span class="identifier">error</span> <span class="identifier">C2888</span><span class="special">:</span> <span class="char">'T constant_root_two_div_pi'</span> <span class="special">:</span> <span class="identifier">symbol</span> <span class="identifier">cannot</span> <span class="identifier">be</span> <span class="identifier">defined</span> <span class="identifier">within</span> <span class="keyword">namespace</span> <span class="char">'detail'</span>
<span class="identifier">error</span> <span class="identifier">C2988</span><span class="special">:</span> <span class="identifier">unrecognizable</span> <span class="keyword">template</span> <span class="identifier">declaration</span><span class="special">/</span><span class="identifier">definition</span>
</pre>
</td></tr>
</table></div>
<p>
2. <span class="bold"><strong>You will need an arbitrary precision type to use to
calculate the value</strong></span>. This library currently supports either <code class="computeroutput"><span class="identifier">cpp_float</span></code>, <code class="computeroutput"><span class="identifier">NTL</span><span class="special">::</span><span class="identifier">RR</span></code> or
<code class="computeroutput"><span class="identifier">mpfr_class</span></code> used via the bindings
in <code class="computeroutput"><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">bindings</span></code>.
The default is to use <code class="computeroutput"><span class="identifier">NTL</span><span class="special">::</span><span class="identifier">RR</span></code> unless you define an alternate macro,
for example, <code class="computeroutput"><span class="identifier">USE_MPFR</span></code> or <code class="computeroutput"><span class="identifier">USE_CPP_FLOAT</span></code> at the start of your program.
</p>
<p>
3. It is necessary to link to the Boost.Regex library, and probably to your
chosen arbitrary precision type library.
</p>
<p>
4. You need to add <code class="computeroutput"><span class="identifier">libs</span><span class="special">\</span><span class="identifier">math</span><span class="special">\</span><span class="identifier">include_private</span></code>
to your compiler's include path as the needed header is not installed in the
usual places by default (this avoids a cyclic dependency between the Math and
Multiprecision library's headers).
</p>
<p>
5. The complete program to generate the constant <code class="computeroutput"><span class="identifier">half_pi</span></code>
using function <code class="computeroutput"><span class="identifier">calculate_half_pi</span></code>
is then:
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">USE_CPP_FLOAT</span> <span class="comment">// If required.</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">constants</span><span class="special">/</span><span class="identifier">generate</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<span class="identifier">BOOST_CONSTANTS_GENERATE</span><span class="special">(</span><span class="identifier">half_pi</span><span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
The output from the program is a snippet of C++ code (actually a macro call)
that can be cut and pasted into <code class="computeroutput"><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">constants</span><span class="special">/</span><span class="identifier">constants</span><span class="special">.</span><span class="identifier">hpp</span></code> or else into your own code, for example:
</p>
<pre class="programlisting"> BOOST_DEFINE_MATH_CONSTANT(half_pi, 1.570796326794896619231321691639751442e+00, "1.57079632679489661923132169163975144209858469968755291048747229615390820314310449931401741267105853399107404326e+00");
</pre>
<p>
This macro BOOST_DEFINE_MATH_CONSTANT inserts a C++ struct code snippet that
declares the <code class="computeroutput"><span class="keyword">float</span></code>, <code class="computeroutput"><span class="keyword">double</span></code> and <code class="computeroutput"><span class="keyword">long</span>
<span class="keyword">double</span></code> versions of the constant, plus
a decimal digit string representation correct to 100 decimal digits, and all
the meta-programming machinery needed to select between them.
</p>
<p>
The result of an expanded macro for Pi is shown below.
</p>
<pre class="programlisting"><span class="comment">// Preprocessed pi constant, annotated.</span>
<span class="keyword">namespace</span> <span class="identifier">boost</span>
<span class="special">{</span>
<span class="keyword">namespace</span> <span class="identifier">math</span>
<span class="special">{</span>
<span class="keyword">namespace</span> <span class="identifier">constants</span>
<span class="special">{</span>
<span class="keyword">namespace</span> <span class="identifier">detail</span>
<span class="special">{</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">constant_pi</span>
<span class="special">{</span>
<span class="keyword">private</span><span class="special">:</span>
<span class="comment">// Default implementations from string of decimal digits:</span>
<span class="keyword">static</span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="identifier">get_from_string</span><span class="special">()</span>
<span class="special">{</span>
<span class="keyword">static</span> <span class="keyword">const</span> <span class="identifier">T</span> <span class="identifier">result</span>
<span class="special">=</span> <span class="identifier">detail</span><span class="special">::</span><span class="identifier">convert_from_string</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="string">"3.14159265358979323846264338327950288419716939937510582097494459230781640628620899862803482534211706798214808651e+00"</span><span class="special">,</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_convertible</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">char</span><span class="special">*,</span> <span class="identifier">T</span><span class="special">>());</span>
<span class="keyword">return</span> <span class="identifier">result</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">int</span> <span class="identifier">N</span><span class="special">></span> <span class="keyword">static</span> <span class="identifier">T</span> <span class="identifier">compute</span><span class="special">();</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="comment">// Default implementations from string of decimal digits:</span>
<span class="keyword">static</span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="identifier">get</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="identifier">construct_from_string</span><span class="special">>&)</span>
<span class="special">{</span>
<span class="identifier">constant_initializer</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="special">&</span> <span class="identifier">constant_pi</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">get_from_string</span> <span class="special">>::</span><span class="identifier">do_nothing</span><span class="special">();</span>
<span class="keyword">return</span> <span class="identifier">get_from_string</span><span class="special">();</span>
<span class="special">}</span>
<span class="comment">// Float, double and long double versions:</span>
<span class="keyword">static</span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="identifier">get</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="identifier">construct_from_float</span><span class="special">>)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="number">3.141592653589793238462643383279502884e+00F</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">static</span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="identifier">get</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="identifier">construct_from_double</span><span class="special">>&)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="number">3.141592653589793238462643383279502884e+00</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">static</span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="identifier">get</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="identifier">construct_from_long_double</span><span class="special">>&)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="number">3.141592653589793238462643383279502884e+00L</span><span class="special">;</span>
<span class="special">}</span>
<span class="comment">// For very high precision that is nonetheless can be calculated at compile time:</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">int</span> <span class="identifier">N</span><span class="special">></span> <span class="keyword">static</span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="identifier">get</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="identifier">N</span><span class="special">>&</span> <span class="identifier">n</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">constant_initializer2</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">N</span><span class="special">,</span> <span class="special">&</span> <span class="identifier">constant_pi</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">template</span> <span class="identifier">compute</span><span class="special"><</span><span class="identifier">N</span><span class="special">></span> <span class="special">>::</span><span class="identifier">do_nothing</span><span class="special">();</span>
<span class="keyword">return</span> <span class="identifier">compute</span><span class="special"><</span><span class="identifier">N</span><span class="special">>();</span>
<span class="special">}</span>
<span class="comment">//For true arbitrary precision, which may well vary at runtime.</span>
<span class="keyword">static</span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="identifier">get</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="number">0</span><span class="special">>&)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">tools</span><span class="special">::</span><span class="identifier">digits</span><span class="special"><</span><span class="identifier">T</span><span class="special">>()</span> <span class="special">></span> <span class="identifier">max_string_digits</span> <span class="special">?</span> <span class="identifier">compute</span><span class="special"><</span><span class="number">0</span><span class="special">>()</span> <span class="special">:</span> <span class="identifier">get</span><span class="special">(</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="identifier">construct_from_string</span><span class="special">>());</span>
<span class="special">}</span>
<span class="special">};</span> <span class="comment">// template <class T> struct constant_pi</span>
<span class="special">}</span> <span class="comment">// namespace detail</span>
<span class="comment">// The actual forwarding function (including policy to control precision).</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Policy</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="identifier">pi</span><span class="special">(</span> <span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">detail</span><span class="special">::</span> <span class="identifier">constant_pi</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">get</span><span class="special">(</span><span class="keyword">typename</span> <span class="identifier">construction_traits</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">Policy</span><span class="special">>::</span><span class="identifier">type</span><span class="special">());</span>
<span class="special">}</span>
<span class="comment">// The actual forwarding function (using default policy to control precision).</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="identifier">pi</span><span class="special">()</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">pi</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">::</span><span class="identifier">policy</span><span class="special"><></span> <span class="special">>()</span>
<span class="special">}</span>
<span class="special">}</span> <span class="comment">// namespace constants</span>
<span class="comment">// Namespace specific versions, for the three built-in floats:</span>
<span class="keyword">namespace</span> <span class="identifier">float_constants</span>
<span class="special">{</span>
<span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">float</span> <span class="identifier">pi</span> <span class="special">=</span> <span class="number">3.141592653589793238462643383279502884e+00F</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">namespace</span> <span class="identifier">double_constants</span>
<span class="special">{</span>
<span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">double</span> <span class="identifier">pi</span> <span class="special">=</span> <span class="number">3.141592653589793238462643383279502884e+00</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">namespace</span> <span class="identifier">long_double_constants</span>
<span class="special">{</span>
<span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">long</span> <span class="keyword">double</span> <span class="identifier">pi</span> <span class="special">=</span> <span class="number">3.141592653589793238462643383279502884e+00L</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">namespace</span> <span class="identifier">constants</span><span class="special">{;</span>
<span class="special">}</span> <span class="comment">// namespace constants</span>
<span class="special">}</span> <span class="comment">// namespace math</span>
<span class="special">}</span> <span class="comment">// namespace boost</span>
</pre>
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<td align="right"><div class="copyright-footer">Copyright © 2006-2010, 2012-2014 Nikhar Agrawal,
Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, Hubert
Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Johan Råde, Gautam Sewani,
Benjamin Sobotta, Thijs van den Berg, Daryle Walker and Xiaogang Zhang<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
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