started advanced topics section in the docs

example/features/static.cpp

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+
+#include <boost/contract.hpp>
+
+//[static
+template<class C>
+class instance_counter {
+public:
+    static void static_invariant() { // Static class invariants.
+        BOOST_CONTRACT_ASSERT(count() >= 0);
+    }
+
+    static int count() { // Static public function (check static invariants).
+        auto c = boost::contract::public_function<instance_counter>();
+        return count_; // Function body.
+    }
+
+    instance_counter() : obj_() {
+        auto old_count = BOOST_CONTRACT_OLDOF(count());
+        auto c = boost::contract::constructor(this)
+            .postcondition([&] {
+                BOOST_CONTRACT_OLDOF(count() == *old_count + 1);
+            })
+        ;
+
+        ++count_;
+    }
+
+    ~instance_counter() {
+        auto old_count = BOOST_CONTRACT_OLDOF(count());
+        auto c = boost::contract::destructor(this)
+            .postcondition([&] { // (An example of destructor postconditions.)
+                BOOST_CONTRACT_OLDOF(count() == *old_count - 1);
+            })
+        ;
+
+        --count_;
+    }
+    
+    /* ... */
+
+private:
+    C obj_;
+    static int count_;
+};
+
+template<class C>
+int instance_counter<C>::count_ = 0;
+//]
+