diff --git a/test/world.cpp b/test/world.cpp
index 72295cf..2e7ca51 100644
--- a/test/world.cpp
+++ b/test/world.cpp
@@ -17,14 +17,17 @@
 #include <boost/test/unit_test.hpp>
 #include <vector>
 
-struct person; BOOST_TYPEOF_REGISTER_TYPE(person)
 struct person {};
+BOOST_TYPEOF_REGISTER_TYPE(person)
 
-struct world; BOOST_TYPEOF_REGISTER_TYPE(world)
 struct world {
     void add_person(person const& a_person);
+    size_t population(void) const { return persons_.size(); }
+
+private:
     std::vector<person> persons_;
 };
+BOOST_TYPEOF_REGISTER_TYPE(world)
 
 //[world
 void world::add_person(person const& a_person) {
@@ -46,7 +49,7 @@ BOOST_AUTO_TEST_CASE( test_world ) {
     world w;
     person p;
     w.add_person(p);
-    BOOST_CHECK( w.persons_.size() == 1 );
+    BOOST_CHECK( w.population() == 1 );
 }
 
 #else
diff --git a/test/world_checkpoint.cpp b/test/world_checkpoint.cpp
index 3549b23..d12cf4f 100644
--- a/test/world_checkpoint.cpp
+++ b/test/world_checkpoint.cpp
@@ -20,30 +20,23 @@
 #include <iostream>
 #include <sstream>
 
-class person; BOOST_TYPEOF_REGISTER_TYPE(person)
-class person {
-    friend class world;
-public:
+struct person {
     typedef unsigned int id_t;
     typedef unsigned int evolution_t;
+    
+    id_t id;
+    evolution_t evolution;
 
-    person(void) : id_(0), evolution_(0) {}
+    person(void) : id(0), evolution(0) {}
 
     friend std::ostream& operator<<(std::ostream& o, person const& p) {
-        return o << "person(" << p.id_ << ", " << p.evolution_ << ")";
+        return o << "person(" << p.id << ", " << p.evolution << ")";
     }
-private:
-    id_t id_;
-    evolution_t evolution_;
 };
+BOOST_TYPEOF_REGISTER_TYPE(person)
 
-class world; BOOST_TYPEOF_REGISTER_TYPE(world)
-class world {
-public:
-    typedef unsigned int id_t;
-
+struct world {
     world(void) : next_id_(1) {}
-
     void add_person(person const& a_person);
 
     friend std::ostream& operator<<(std::ostream& o, world const& w) {
@@ -53,10 +46,12 @@ public:
         }
         return o << "})";
     }
+
 private:
-    id_t next_id_;
+    person::id_t next_id_;
     std::vector<person> persons_;
 };
+BOOST_TYPEOF_REGISTER_TYPE(world)
 
 //[world_checkpoint
 void world::add_person(person const& a_person) {
@@ -64,28 +59,28 @@ void world::add_person(person const& a_person) {
 
     // This block must be no-throw.
     person& p = persons_.back();
-    person::evolution_t checkpoint = p.evolution_;
+    person::evolution_t checkpoint = p.evolution;
     BOOST_SCOPE_EXIT(checkpoint, &p, &persons_) {
-        if(checkpoint == p.evolution_) persons_.pop_back();
+        if(checkpoint == p.evolution) persons_.pop_back();
     } BOOST_SCOPE_EXIT_END
 
     // ...
 
-    checkpoint = ++p.evolution_;
+    checkpoint = ++p.evolution;
 
     // Assign new identifier to the person.
-    world::id_t const prev_id = p.id_;
-    p.id_ = next_id_++;
+    person::id_t const prev_id = p.id;
+    p.id = next_id_++;
     BOOST_SCOPE_EXIT(checkpoint, &p, &next_id_, prev_id) {
-        if(checkpoint == p.evolution_) {
-            next_id_ = p.id_;
-            p.id_ = prev_id;
+        if(checkpoint == p.evolution) {
+            next_id_ = p.id;
+            p.id = prev_id;
         }
     } BOOST_SCOPE_EXIT_END
 
     // ...
 
-    checkpoint = ++p.evolution_;
+    checkpoint = ++p.evolution;
 }
 //]
 
diff --git a/test/world_checkpoint_all.cpp b/test/world_checkpoint_all.cpp
index 61f528b..fc1ca8e 100644
--- a/test/world_checkpoint_all.cpp
+++ b/test/world_checkpoint_all.cpp
@@ -20,30 +20,23 @@
 #include <iostream>
 #include <sstream>
 
-class person; BOOST_TYPEOF_REGISTER_TYPE(person)
-class person {
-    friend class world;
-public:
+struct person {
     typedef unsigned int id_t;
     typedef unsigned int evolution_t;
+    
+    id_t id;
+    evolution_t evolution;
 
-    person(void) : id_(0), evolution_(0) {}
+    person(void) : id(0), evolution(0) {}
 
     friend std::ostream& operator<<(std::ostream& o, person const& p) {
-        return o << "person(" << p.id_ << ", " << p.evolution_ << ")";
+        return o << "person(" << p.id << ", " << p.evolution << ")";
     }
-private:
-    id_t id_;
-    evolution_t evolution_;
 };
+BOOST_TYPEOF_REGISTER_TYPE(person)
 
-class world; BOOST_TYPEOF_REGISTER_TYPE(world)
-class world {
-public:
-    typedef unsigned int id_t;
-
+struct world {
     world(void) : next_id_(1) {}
-
     void add_person(person const& a_person);
 
     friend std::ostream& operator<<(std::ostream& o, world const& w) {
@@ -53,10 +46,12 @@ public:
         }
         return o << "})";
     }
+
 private:
-    id_t next_id_;
+    person::id_t next_id_;
     std::vector<person> persons_;
 };
+BOOST_TYPEOF_REGISTER_TYPE(world)
 
 //[world_checkpoint_all
 void world::add_person(person const& a_person) {
@@ -64,28 +59,28 @@ void world::add_person(person const& a_person) {
 
     // This block must be no-throw.
     person& p = persons_.back();
-    person::evolution_t checkpoint = p.evolution_;
+    person::evolution_t checkpoint = p.evolution;
     BOOST_SCOPE_EXIT_ALL(&, checkpoint, this_) { // Capture all by ref (C++11).
-        if(checkpoint == p.evolution_) this_->persons_.pop_back();
+        if(checkpoint == p.evolution) this_->persons_.pop_back();
     } BOOST_SCOPE_EXIT_END
 
     // ...
 
-    checkpoint = ++p.evolution_;
+    checkpoint = ++p.evolution;
 
     // Assign new identifier to the person.
-    world::id_t const prev_id = p.id_;
-    p.id_ = next_id_++;
+    person::id_t const prev_id = p.id;
+    p.id = next_id_++;
     BOOST_SCOPE_EXIT_ALL(=, &p, this) { // Capture all by value, `this` (C++11).
-        if(checkpoint == p.evolution_) {
-            this->next_id_ = p.id_;
-            p.id_ = prev_id;
+        if(checkpoint == p.evolution) {
+            this->next_id_ = p.id;
+            p.id = prev_id;
         }
     }; // Use `;` instead of `SCOPE_EXIT_END` (C++11).
 
     // ...
 
-    checkpoint = ++p.evolution_;
+    checkpoint = ++p.evolution;
 }
 //]
 
diff --git a/test/world_checkpoint_all_seq.cpp b/test/world_checkpoint_all_seq.cpp
index b8be8ad..355e818 100644
--- a/test/world_checkpoint_all_seq.cpp
+++ b/test/world_checkpoint_all_seq.cpp
@@ -20,30 +20,23 @@
 #include <iostream>
 #include <sstream>
 
-class person; BOOST_TYPEOF_REGISTER_TYPE(person)
-class person {
-    friend class world;
-public:
+struct person {
     typedef unsigned int id_t;
     typedef unsigned int evolution_t;
 
-    person(void) : id_(0), evolution_(0) {}
+    id_t id;
+    evolution_t evolution;
+
+    person(void) : id(0), evolution(0) {}
 
     friend std::ostream& operator<<(std::ostream& o, person const& p) {
-        return o << "person(" << p.id_ << ", " << p.evolution_ << ")";
+        return o << "person(" << p.id << ", " << p.evolution << ")";
     }
-private:
-    id_t id_;
-    evolution_t evolution_;
 };
+BOOST_TYPEOF_REGISTER_TYPE(person)
 
-class world; BOOST_TYPEOF_REGISTER_TYPE(world)
-class world {
-public:
-    typedef unsigned int id_t;
-
+struct world {
     world(void) : next_id_(1) {}
-
     void add_person(person const& a_person);
 
     friend std::ostream& operator<<(std::ostream& o, world const& w) {
@@ -53,10 +46,12 @@ public:
         }
         return o << "})";
     }
+
 private:
-    id_t next_id_;
+    person::id_t next_id_;
     std::vector<person> persons_;
 };
+BOOST_TYPEOF_REGISTER_TYPE(world)
 
 //[world_checkpoint_all_seq
 void world::add_person(person const& a_person) {
@@ -64,28 +59,28 @@ void world::add_person(person const& a_person) {
 
     // This block must be no-throw.
     person& p = persons_.back();
-    person::evolution_t checkpoint = p.evolution_;
+    person::evolution_t checkpoint = p.evolution;
     BOOST_SCOPE_EXIT_ALL( (&) (checkpoint) (this_) ) {
-        if(checkpoint == p.evolution_) this_->persons_.pop_back();
+        if(checkpoint == p.evolution) this_->persons_.pop_back();
     } BOOST_SCOPE_EXIT_END
 
     // ...
 
-    checkpoint = ++p.evolution_;
+    checkpoint = ++p.evolution;
 
     // Assign new identifier to the person.
-    world::id_t const prev_id = p.id_;
-    p.id_ = next_id_++;
+    person::id_t const prev_id = p.id;
+    p.id = next_id_++;
     BOOST_SCOPE_EXIT_ALL( (=) (&p) (this) ) {
-        if(checkpoint == p.evolution_) {
-            this->next_id_ = p.id_;
-            p.id_ = prev_id;
+        if(checkpoint == p.evolution) {
+            this->next_id_ = p.id;
+            p.id = prev_id;
         }
     };
 
     // ...
 
-    checkpoint = ++p.evolution_;
+    checkpoint = ++p.evolution;
 }
 //]
 
diff --git a/test/world_checkpoint_seq.cpp b/test/world_checkpoint_seq.cpp
index aef0ef9..6448233 100644
--- a/test/world_checkpoint_seq.cpp
+++ b/test/world_checkpoint_seq.cpp
@@ -17,28 +17,22 @@
 #include <iostream>
 #include <sstream>
 
-class person; BOOST_TYPEOF_REGISTER_TYPE(person)
-class person {
-    friend class world;
-public:
+struct person {
     typedef unsigned int id_t;
     typedef unsigned int evolution_t;
 
-    person(void) : id_(0), evolution_(0) {}
+    id_t id;
+    evolution_t evolution;
+
+    person(void) : id(0), evolution(0) {}
 
     friend std::ostream& operator<<(std::ostream& o, person const& p) {
-        return o << "person(" << p.id_ << ", " << p.evolution_ << ")";
+        return o << "person(" << p.id << ", " << p.evolution << ")";
     }
-private:
-    id_t id_;
-    evolution_t evolution_;
 };
+BOOST_TYPEOF_REGISTER_TYPE(person)
 
-class world; BOOST_TYPEOF_REGISTER_TYPE(world)
-class world {
-public:
-    typedef unsigned int id_t;
-
+struct world {
     world(void) : next_id_(1) {}
 
     void add_person(person const& a_person);
@@ -50,10 +44,12 @@ public:
         }
         return o << "})";
     }
+
 private:
-    id_t next_id_;
+    person::id_t next_id_;
     std::vector<person> persons_;
 };
+BOOST_TYPEOF_REGISTER_TYPE(world)
 
 //[world_checkpoint_seq
 void world::add_person(person const& a_person) {
@@ -61,28 +57,28 @@ void world::add_person(person const& a_person) {
 
     // This block must be no-throw.
     person& p = persons_.back();
-    person::evolution_t checkpoint = p.evolution_;
+    person::evolution_t checkpoint = p.evolution;
     BOOST_SCOPE_EXIT( (checkpoint) (&p) (&persons_) ) { // Sequence, not commas.
-        if(checkpoint == p.evolution_) persons_.pop_back();
+        if(checkpoint == p.evolution) persons_.pop_back();
     } BOOST_SCOPE_EXIT_END
 
     // ...
 
-    checkpoint = ++p.evolution_;
+    checkpoint = ++p.evolution;
 
     // Assign new identifier to the person.
-    world::id_t const prev_id = p.id_;
-    p.id_ = next_id_++;
+    person::id_t const prev_id = p.id;
+    p.id = next_id_++;
     BOOST_SCOPE_EXIT( (checkpoint) (&p) (&next_id_) (prev_id) ) {
-        if(checkpoint == p.evolution_) {
-            next_id_ = p.id_;
-            p.id_ = prev_id;
+        if(checkpoint == p.evolution) {
+            next_id_ = p.id;
+            p.id = prev_id;
         }
     } BOOST_SCOPE_EXIT_END
 
     // ...
 
-    checkpoint = ++p.evolution_;
+    checkpoint = ++p.evolution;
 }
 //]
 
diff --git a/test/world_lambda.cpp b/test/world_lambda.cpp
index 90e716a..384f2d3 100644
--- a/test/world_lambda.cpp
+++ b/test/world_lambda.cpp
@@ -34,7 +34,7 @@ void world::add_person(person const& a_person) {
     bool commit = false;
 
     persons_.push_back(a_person);
-    scope_exit on_exit1([&commit, &persons_]() { // Use C++11 lambda.
+    scope_exit on_exit1([&commit, &persons_](void) { // Use C++11 lambda.
         if(!commit) persons_.pop_back();
     });
 
diff --git a/test/world_seq.cpp b/test/world_seq.cpp
index 26df773..a6b3a5c 100644
--- a/test/world_seq.cpp
+++ b/test/world_seq.cpp
@@ -14,14 +14,17 @@
 #include <boost/test/unit_test.hpp>
 #include <vector>
 
-struct person; BOOST_TYPEOF_REGISTER_TYPE(person)
 struct person {};
+BOOST_TYPEOF_REGISTER_TYPE(person)
 
-struct world; BOOST_TYPEOF_REGISTER_TYPE(world)
 struct world {
     void add_person(person const& a_person);
+    size_t population(void) const { return persons_.size(); }
+
+private:
     std::vector<person> persons_;
 };
+BOOST_TYPEOF_REGISTER_TYPE(world)
 
 //[world_seq
 void world::add_person(person const& a_person) {
@@ -42,6 +45,6 @@ BOOST_AUTO_TEST_CASE( test_world_seq ) {
     world w;
     person p;
     w.add_person(p);
-    BOOST_CHECK( w.persons_.size() == 1 );
+    BOOST_CHECK( w.population() == 1 );
 }
 
diff --git a/test/world_this.cpp b/test/world_this.cpp
index c608641..bf6549f 100644
--- a/test/world_this.cpp
+++ b/test/world_this.cpp
@@ -18,14 +18,17 @@
 #include <boost/config.hpp>
 #include <vector>
 
-struct person; BOOST_TYPEOF_REGISTER_TYPE(person)
 struct person {};
+BOOST_TYPEOF_REGISTER_TYPE(person)
 
-struct world; BOOST_TYPEOF_REGISTER_TYPE(world)
 struct world {
     void add_person(person const& a_person);
+    size_t population(void) const { return persons_.size(); }
+
+private:
     std::vector<person> persons_;
 };
+BOOST_TYPEOF_REGISTER_TYPE(world)
 
 void world::add_person(person const& a_person) {
     bool commit = false;
@@ -54,7 +57,7 @@ BOOST_AUTO_TEST_CASE( test_world_this ) {
     world w;
     person p;
     w.add_person(p);
-    BOOST_CHECK( w.persons_.size() == 1 );
+    BOOST_CHECK( w.population() == 1 );
 }
 
 #else
diff --git a/test/world_this_seq.cpp b/test/world_this_seq.cpp
index 6b976ae..de5692f 100644
--- a/test/world_this_seq.cpp
+++ b/test/world_this_seq.cpp
@@ -15,14 +15,17 @@
 #include <boost/config.hpp>
 #include <vector>
 
-struct person; BOOST_TYPEOF_REGISTER_TYPE(person)
 struct person {};
+BOOST_TYPEOF_REGISTER_TYPE(person)
 
-struct world; BOOST_TYPEOF_REGISTER_TYPE(world)
 struct world {
     void add_person(person const& a_person);
+    size_t population(void) const { return persons_.size(); }
+
+private:
     std::vector<person> persons_;
 };
+BOOST_TYPEOF_REGISTER_TYPE(world)
 
 void world::add_person(person const& a_person) {
     bool commit = false;
@@ -51,6 +54,6 @@ BOOST_AUTO_TEST_CASE( test_world_this_seq ) {
     world w;
     person p;
     w.add_person(p);
-    BOOST_CHECK( w.persons_.size() == 1 );
+    BOOST_CHECK( w.population() == 1 );
 }
 
diff --git a/test/world_tpl.cpp b/test/world_tpl.cpp
index 1099531..6a1dc6f 100644
--- a/test/world_tpl.cpp
+++ b/test/world_tpl.cpp
@@ -17,15 +17,18 @@
 #include <boost/test/unit_test.hpp>
 #include <vector>
 
-struct person; BOOST_TYPEOF_REGISTER_TYPE(peron)
 struct person {};
+BOOST_TYPEOF_REGISTER_TYPE(peron)
 
-template<typename Person> struct world; BOOST_TYPEOF_REGISTER_TEMPLATE(world, 1)
 template<typename Person>
 struct world {
     void add_person(Person const& a_person);
+    size_t population(void) const { return persons_.size(); }
+
+private:
     std::vector<Person> persons_;
 };
+BOOST_TYPEOF_REGISTER_TEMPLATE(world, 1)
 
 //[world_tpl
 template<typename Person>
@@ -47,7 +50,7 @@ BOOST_AUTO_TEST_CASE( test_world_tpl ) {
     world<person> w;
     person p;
     w.add_person(p);
-    BOOST_CHECK( w.persons_.size() == 1 );
+    BOOST_CHECK( w.population() == 1 );
 }
 
 #else
diff --git a/test/world_tpl_seq.cpp b/test/world_tpl_seq.cpp
index f734e8d..9863779 100644
--- a/test/world_tpl_seq.cpp
+++ b/test/world_tpl_seq.cpp
@@ -14,15 +14,18 @@
 #include <boost/test/unit_test.hpp>
 #include <vector>
 
-struct person; BOOST_TYPEOF_REGISTER_TYPE(person)
 struct person {};
+BOOST_TYPEOF_REGISTER_TYPE(person)
 
-template<typename Person> struct world; BOOST_TYPEOF_REGISTER_TEMPLATE(world, 1)
 template<typename Person>
 struct world {
     void add_person(Person const& a_person);
+    size_t population(void) const { return persons_.size(); }
+
+private:
     std::vector<Person> persons_;
 };
+BOOST_TYPEOF_REGISTER_TEMPLATE(world, 1)
 
 //[world_tpl_seq
 template<typename Person>
@@ -44,6 +47,6 @@ BOOST_AUTO_TEST_CASE( test_world_tpl_seq ) {
     world<person> w;
     person p;
     w.add_person(p);
-    BOOST_CHECK( w.persons_.size() == 1 );
+    BOOST_CHECK( w.population() == 1 );
 }
 
diff --git a/test/world_void.cpp b/test/world_void.cpp
index 0806ff1..ff4a7d9 100644
--- a/test/world_void.cpp
+++ b/test/world_void.cpp
@@ -14,8 +14,8 @@
 #include <boost/test/unit_test.hpp>
 #include <vector>
 
-struct person; BOOST_TYPEOF_REGISTER_TYPE(person)
 struct person {};
+BOOST_TYPEOF_REGISTER_TYPE(person)
 
 struct world_t; BOOST_TYPEOF_REGISTER_TYPE(world_t)
 //[world_void