started advanced topics section in the docs

example/n1962/circle.cpp

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+
+#include <boost/contract.hpp>
+#include <boost/detail/lightweight_test.hpp>
+#include <cassert>
+
+class shape {
+public:
+    virtual ~shape() {}
+
+    virtual int compute_area(boost::contract::virtual_* v = 0) const = 0;
+};
+
+int shape::compute_area(boost::contract::virtual_* v) const {
+    int result;
+    auto c = boost::contract::public_function(v, result, this)
+        .postcondition([&] (int const& result) {
+            BOOST_CONTRACT_ASSERT(result > 0);
+        })
+    ;
+    assert(false); return result = -1; // Never gets here.
+}
+
+class circle
+    #define BASES public shape
+    : BASES
+{
+public:
+    typedef BOOST_CONTRACT_BASE_TYPES(BASES) base_types;
+    #undef BASES
+
+    int radius; // Make sure to set this...
+    static int const pi = 3; // Truncated to int from 3.14...
+
+    virtual int compute_area(boost::contract::virtual_* v = 0)
+            const /* override */ {
+        int result;
+        auto c = boost::contract::public_function<override_compute_area>(
+                v, result, &circle::compute_area, this)
+            .postcondition([&] (int const& result) {
+                BOOST_CONTRACT_ASSERT(result == pi * radius * radius);
+            })
+        ;
+        
+        return result = pi * radius * radius;
+    }
+    BOOST_CONTRACT_OVERRIDE(compute_area);
+};
+
+int main() {
+    circle c;
+    c.radius = 2;
+    BOOST_TEST_EQ(c.compute_area(), 12);
+    return boost::report_errors();
+}
+