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    <p>At least if <a href="http://en.cppreference.com/w/cpp/types/numeric_limits/is_iec559" rel="noreferrer">std::numeric_limits::is_iec559</a> (IEEE 754) is true (which guarantees, that <a href="http://en.cppreference.com/w/cpp/types/numeric_limits/has_infinity" rel="noreferrer">std::numeric_limits::has_infinity</a> is also true), you can express positive and negative infinity values the way you already stated.</p> <p><strong>Short explanation of IEEE 754-1985 infinity values from <a href="http://en.wikipedia.org/wiki/IEEE_754-1985#Representation_of_non-numbers" rel="noreferrer">Wikipedia</a>:</strong></p> <blockquote> <p>......snip......</p> <p>The biased-exponent field is filled with all 1 bits to indicate either infinity or an invalid result of a computation.</p> <p><strong>Positive and negative infinity</strong></p> <p>Positive and negative infinity are represented thus:</p> <pre><code> sign = 0 for positive infinity, 1 for negative infinity. biased exponent = all 1 bits. fraction = all 0 bits. </code></pre> <p>......snip......</p> </blockquote> <p><strong>Assertions</strong></p> <p>The following example will either work as expected, or cause a compile time error in case the target platform does not support IEEE 754 floats.</p> <pre><code>#include &lt;cstdlib&gt; #include &lt;cmath&gt; #include &lt;cassert&gt; #include &lt;limits&gt; int main(void) { //Asserts floating point compatibility at compile time static_assert(std::numeric_limits&lt;float&gt;::is_iec559, "IEEE 754 required"); //C99 float negative_infinity1 = -INFINITY; float negative_infinity2 = -1 * INFINITY; float negative_infinity3 = -std::numeric_limits&lt;float&gt;::infinity(); float negative_infinity4 = -1 * std::numeric_limits&lt;float&gt;::infinity(); assert(std::isinf(negative_infinity1) &amp;&amp; negative_infinity1 &lt; std::numeric_limits&lt;float&gt;::lowest()); assert(std::isinf(negative_infinity2) &amp;&amp; negative_infinity2 &lt; std::numeric_limits&lt;float&gt;::lowest()); assert(std::isinf(negative_infinity3) &amp;&amp; negative_infinity3 &lt; std::numeric_limits&lt;float&gt;::lowest()); assert(std::isinf(negative_infinity4) &amp;&amp; negative_infinity4 &lt; std::numeric_limits&lt;float&gt;::lowest()); return EXIT_SUCCESS; } </code></pre>
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