Added examples/cxx-api/evaldemo.cc

diff --git a/examples/cxx-api/evaldemo.cc b/examples/cxx-api/evaldemo.cc
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+++ b/examples/cxx-api/evaldemo.cc
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+/* A simple Yosys plugin. (Copy&paste from http://stackoverflow.com/questions/32093541/how-does-the-yosys-consteval-api-work)
+
+Usage example:
+
+$ cat > evaldemo.v <<EOT
+module main(input [1:0] A, input [7:0] B, C, D, output [7:0] Y);
+  assign Y = A == 0 ? B : A == 1 ? C : A == 2 ? D : 42;
+endmodule
+EOT
+
+$ yosys-config --build evaldemo.so evaldemo.cc
+$ yosys -m evaldemo.so -p evaldemo evaldemo.v
+*/
+
+#include "kernel/yosys.h"
+#include "kernel/consteval.h"
+
+USING_YOSYS_NAMESPACE
+PRIVATE_NAMESPACE_BEGIN
+
+struct EvalDemoPass : public Pass
+{
+       EvalDemoPass() : Pass("evaldemo") { }
+
+       virtual void execute(vector<string>, Design *design)
+       {
+               Module *module = design->top_module();
+
+               if (module == nullptr)
+                       log_error("No top module found!\n");
+
+               Wire *wire_a = module->wire("\\A");
+               Wire *wire_y = module->wire("\\Y");
+
+               if (wire_a == nullptr)
+                       log_error("No wire A found!\n");
+
+               if (wire_y == nullptr)
+                       log_error("No wire Y found!\n");
+
+               ConstEval ce(module);
+               for (int v = 0; v < 4; v++) {
+                       ce.push();
+                       ce.set(wire_a, Const(v, GetSize(wire_a)));
+                       SigSpec sig_y = wire_y, sig_undef;
+                       if (ce.eval(sig_y, sig_undef))
+                               log("Eval results for A=%d: Y=%s\n", v, log_signal(sig_y));
+                       else
+                               log("Eval failed for A=%d: Missing value for %s\n", v, log_signal(sig_undef));
+                       ce.pop();
+               }
+       }
+} EvalDemoPass;
+
+PRIVATE_NAMESPACE_END