started advanced topics section in the docs

example/n1962/factorial.cpp

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+
+#include <boost/contract.hpp>
+#include <boost/detail/lightweight_test.hpp>
+
+// Assertion complexity classified relative their function body complexity.
+#define O_LESS_THAN_BODY        0
+#define O_SAME_AS_BODY          1
+#define O_GREATHER_THAN_BODY    2
+#define COMPLEXITY_MAX          O_SAME_AS_BODY
+
+int factorial(int n ) {
+    int result;
+    auto c = boost::contract::function()
+        .precondition([&] {
+            BOOST_CONTRACT_ASSERT(n >= 0); // Non-negative natural number.
+            BOOST_CONTRACT_ASSERT(n <= 12); // Max function input.
+        })
+        .postcondition([&] {
+            BOOST_CONTRACT_ASSERT(result >= 1);
+            if(n < 2) { // Select assertion.
+                BOOST_CONTRACT_ASSERT(result == 1);
+            } else {
+                // Following assertion introduce significant run-time overhead
+                // (same as the function body) so assertion can be selectively
+                // disabled by setting COMPLEXITY_MAX.
+                if(O_SAME_AS_BODY <= COMPLEXITY_MAX) {
+                    // Assertion automatically disable within other assertions.
+                    // Therefore, this postcondition can recursively call the
+                    // function without causing infinite recursion.
+                    BOOST_CONTRACT_ASSERT(n * factorial(n - 1));
+                }
+            }
+        })
+    ;
+
+    return n < 2 ? (result = 1) : (result = n * factorial(n - 1));
+}
+
+int main() {
+    BOOST_TEST_EQ(factorial(4), 24);
+    return boost::report_errors();
+}
+