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    <p><strong>Dynamic expressions</strong></p> <p>If you want to receive a string from the user and built an expression from that, maybe the <a href="http://partow.net/programming/exprtk/index.html" rel="noreferrer">C++ Mathematical Expression Library</a> fits your bill?</p> <pre><code>template&lt;typename T&gt; void trig_function() { std::string expression_string = "clamp(-1.0,sin(2 * pi * x) + cos(x / 2 * pi),+1.0)"; T x; exprtk::symbol_table&lt;T&gt; symbol_table; symbol_table.add_variable("x",x); symbol_table.add_constants(); exprtk::expression&lt;T&gt; expression; expression.register_symbol_table(symbol_table); exprtk::parser&lt;T&gt; parser; parser.compile(expression_string,expression); for (x = T(-5.0); x &lt;= T(+5.0); x += 0.001) { T y = expression.value(); printf("%19.15f\t%19.15f\n",x,y); } } </code></pre> <p>There are also the possibility embed a scripting language, such as <a href="http://www.lua.org" rel="noreferrer">Lua</a> or <a href="http://www.python.org" rel="noreferrer">Python</a>, which will give you (even) more power. This is something to consider if you're writing a game, since you'll likely want to script large parts of it.</p> <p>If you're using <a href="http://qt-project.org/" rel="noreferrer">Qt</a>, you can use <a href="http://qt-project.org/doc/qt-4.8/qtscript.html" rel="noreferrer">QtScript</a> (Javascript-ish) to run expressions that read (static or dynamic) properties from your QObject-derived objects.</p> <p>Using one of the above keeps you from having to write your own parser, AST and evaluator, however for a small set of operators it shouldn't be too hard to hack together something if you use <a href="http://boost-spirit.com/home/" rel="noreferrer">Boost.Spirit</a> or some other decent parsing library.</p> <p><strong>Static expressions</strong></p> <p>For selecting between a set of predefined expressions (i.e. known at compile time), you should store the expression in a polymorphic function object. </p> <p>For C++11, if that's available to you, use <code>std::function</code> and lambda expressions. </p> <pre><code>std::function&lt;bool (int, int)&gt; expr = [](int a, int b) { a*2 &lt; b }; </code></pre> <p>For earlier compilers, I recommend function and bind, either in <a href="http://www.boost.org" rel="noreferrer">Boost</a> (boost::) or C++0x TR1 (std::), depending on your compiler. Also, Boost.Lambda will be of help here, as it allows you to construct and store expressions for later evaluation. However, if you're not familiar with C++ and templates (or functional programming), it will likely scare you quite a bit.</p> <p>With that you could write</p> <pre><code>using namespace boost::lambda; boost::function&lt;bool (int, int)&gt; myexpr1 = (_1 + _2) &gt; 20; boost::function&lt;bool (int, int)&gt; myexpr2 = (_1 * _2) &gt; 42; std::cout &lt;&lt; myexpr1(4,7) &lt;&lt; " " &lt;&lt; myexpr2(2,5); </code></pre> <p>with bind, it'd look like:</p> <pre><code>boost::function&lt;bool (Player&amp;)&gt; check = bind(&amp;Player::getHealth, _1) &gt; 20; Player p1; if (check(p1)) { dostuff(); } check = bind(&amp;Player::getGold, _1) &lt; 42; if (check(p1)) { doOtherStuff(); } </code></pre>
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    1. POIs there any kind of "expression class" (C++)
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    1. COunfortunately, this is entirely impractical for arbitrary expressions. Which i think is what the poster really wants (despite his example being of "val1 op val2").
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    2. COIs it? Using boost::lambda you can combine all sorts of expressions, and even statements and all. What do you mean?
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    3. COwell, since the boost lambdas are constructed at compile time and just used by the caller. what would you do if the user entered an op combination that you haven't accounted for. Since there is an infinite number of combinations of math operations...certainly you can't statically account for them all.
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