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#include <iostream>
#include <vector>
#include <map>
#include <tuple>

enum OutputType { Int, Bool, String, NumOutputTypes, FirstOutputType = Int };

template <typename T> struct GetOutputType;
template <> struct GetOutputType<int> : std::integral_constant<OutputType, Int> { };
template <> struct GetOutputType<bool> : std::integral_constant<OutputType, Bool> { };
template <> struct GetOutputType<std::string> : std::integral_constant<OutputType, String> { };

template <typename T>
struct Observer {
    virtual void field_changed (T&, const std::string& field_name, OutputType) = 0;
};

template <typename T>
class Observable {
    std::vector<Observer<T>*> observers;
public:
    void notify_field_change (T& source, OutputType output_type, const std::string& field_name) {
        for (Observer<T>* obs : observers)
            obs->field_changed(source, field_name, output_type);
    }
    void subscribe (Observer<T>* f) { observers.push_back(f); }
    void unsubscribe (Observer<T>* f) { observers.erase(remove(observers.begin(), observers.end(), f), observers.end()); }
};

class Person : public Observable<Person> {
    int age, height;
    std::string name, country, zip_code;
    static const std::map<std::string, std::vector<std::pair<std::string, OutputType>>> dependenciesMap;  // Keeps a list of properties affected by a property.
public:
    Person (const std::string& n, int a, const std::string& c, const std::string& z) : name(n), age(a), country(c), zip_code(z) { }
    static const std::map<std::string, int(Person::*)() const> intMethodMap;
    static const std::map<std::string, bool(Person::*)() const> boolMethodMap;
    static const std::map<std::string, std::string(Person::*)() const> stringMethodMap;
    using TupleMaps = std::tuple<decltype(intMethodMap), decltype(boolMethodMap), decltype(stringMethodMap)>;
    using TupleMaps2 = std::tuple<std::map<std::string, int>, std::map<std::string, bool>, std::map<std::string, std::string>>;
    static const TupleMaps methodMaps;
    int get_age() const { return age; }
    void set_age (int a) { set_field(&Person::age, "age", a); }
    int get_height() const { return height; }
    std::string get_name() const { return name; }
    std::string nickname() const {
        if (country != "France")
            return name;
        return "Francois";
    }
    std::string get_country() const { return country; }
    void set_country (const std::string& c) { set_field(&Person::country, "country", c); }
    std::string get_zip_code() const { return zip_code; }
    void set_zip_code (const std::string& z) { set_field(&Person::zip_code, "zip_code", z); }
    bool can_vote() const { return age >= 16 && country == "USA"; }
    bool can_quit_school() const { return age >= 15 && country == "USA" && zip_code == "90210"; }
    int years_left() const {
        if (country == "France")
            return 10;
        return 80 - age;
    }
private:
    template <typename T> void set_field (T Person::*, const std::string& field_name, const T& value);  // T Person::* is pointer to data member.
    template <OutputType> void set_field_helper (TupleMaps2& trackedValuesAllTypes, const std::pair<std::string, OutputType>&);
    template <OutputType> void notify_other_field_changes (const TupleMaps2&);
};
const std::map<std::string, std::vector<std::pair<std::string, OutputType>>> Person::dependenciesMap = {
    {"age", {{"can_vote()", Bool}, {"can_quit_school()", Bool}, {"years_left()", Int}}},  // age affects can_vote(), can_quit_school(), and years_left().
    {"country", {{"can_vote()", Bool}, {"can_quit_school()", Bool}, {"years_left()", Int}, {"nickname", String}}},  // country affects can_vote, can_quit_school(), years_left(), and nickname.
    {"zip_code", {{"can_quit_school()", Bool}}}  // zip_code affects can_quit_school
 };
const std::map<std::string, int(Person::*)() const> Person::intMethodMap = {
    {"age", &Person::get_age}, {"height", &Person::get_height}, {"years_left()", &Person::years_left}
};
const std::map<std::string, bool(Person::*)() const> Person::boolMethodMap = {
    {"can_vote()", &Person::can_vote}, {"can_quit_school()", &Person::can_quit_school}
};
const std::map<std::string, std::string(Person::*)() const> Person::stringMethodMap = {
    {"name", &Person::get_name}, {"country", &Person::get_country}, {"zip_code", &Person::get_zip_code}, {"nickname", &Person::nickname}
};
const Person::TupleMaps Person::methodMaps = std::make_tuple(Person::intMethodMap, Person::boolMethodMap, Person::stringMethodMap);

struct ConsolePersonObserver : Observer<Person> {
    virtual void field_changed (Person& source, const std::string& field_name, OutputType output_type) override { field_changed_helper<FirstOutputType>(source, field_name, output_type); }
private:
    template <OutputType> void field_changed_helper (Person& source, const std::string& field_name, OutputType output_type);
};

template <OutputType N>
void ConsolePersonObserver::field_changed_helper (Person& source, const std::string& field_name, OutputType output_type) {
    if (output_type == N)
        std::cout << source.get_name() << "'s " << field_name << " has changed to " << (source.*std::get<N>(Person::methodMaps).at(field_name))() << ".\n";
    else
        field_changed_helper<static_cast<OutputType>(N+1)>(source, field_name, output_type);
}

template <>
void ConsolePersonObserver::field_changed_helper<NumOutputTypes> (Person&, const std::string&, OutputType) { }  // End of recursion

template <typename T>
void Person::set_field (T Person::*ptr, const std::string& field_name, const T& value) {
    if (value == this->*ptr)
        return;
    std::tuple<std::map<std::string, int>, std::map<std::string, bool>, std::map<std::string, std::string>> trackedValuesAllTypes;
    for (const std::pair<std::string, OutputType>& pair : dependenciesMap.at(field_name))
        set_field_helper<FirstOutputType>(trackedValuesAllTypes, pair);
    this->*ptr = value;
    const OutputType type = GetOutputType<typename std::remove_reference<decltype(this->*ptr)>::type>::value;
    notify_field_change(*this, type, field_name);
    notify_other_field_changes<FirstOutputType>(trackedValuesAllTypes);
}

template <OutputType N>
void Person::set_field_helper (TupleMaps2& trackedValuesAllTypes, const std::pair<std::string, OutputType>& pair) {
    if (pair.second == N)
        std::get<N>(trackedValuesAllTypes)[pair.first] = (this->*std::get<N>(Person::methodMaps).at(pair.first))();
    else
        set_field_helper<static_cast<OutputType>(N+1)>(trackedValuesAllTypes, pair);
}

template <>
void Person::set_field_helper<NumOutputTypes> (TupleMaps2&, const std::pair<std::string, OutputType>&) { }  // End of recursion.

template <OutputType N>
void Person::notify_other_field_changes (const TupleMaps2& trackedValuesAllTypes) {
    for (const auto& pair : std::get<N>(trackedValuesAllTypes)) {
        if (pair.second != (this->*std::get<N>(methodMaps).at(pair.first))())
            notify_field_change(*this, N, pair.first);
    }
    notify_other_field_changes<static_cast<OutputType>(N+1)>(trackedValuesAllTypes);
}

template <>
void Person::notify_other_field_changes<NumOutputTypes> (const TupleMaps2&) { }  // End of recursion.

int main() {
    Person p("Tom", 14, "USA", "90210");
    ConsolePersonObserver cpo;
    p.subscribe(&cpo);
    p.set_age(15);
    p.set_age(16);
    p.set_zip_code("999999");
    p.set_country("France");
}

#include <iostream>
#include <vector>
#include <map>
#include <tuple>

enum OutputType { Int, Bool, String, NumOutputTypes, FirstOutputType = Int };

template <typename T> struct GetOutputType;
template <> struct GetOutputType<int> : std::integral_constant<OutputType, Int> { };
template <> struct GetOutputType<bool> : std::integral_constant<OutputType, Bool> { };
template <> struct GetOutputType<std::string> : std::integral_constant<OutputType, String> { };

template <typename T>
struct Observer {
    virtual void field_changed (T&, const std::string& field_name, OutputType) = 0;
};

template <typename T>
class Observable {
    std::vector<Observer<T>*> observers;
public:
    void notify_field_change (T& source, OutputType output_type, const std::string& field_name) {
        for (Observer<T>* obs : observers)
            obs->field_changed(source, field_name, output_type);
    }
    void subscribe (Observer<T>* f) { observers.push_back(f); }
    void unsubscribe (Observer<T>* f) { observers.erase(remove(observers.begin(), observers.end(), f), observers.end()); }
};

class Person : public Observable<Person> {
    int age, height;
    std::string name, country, zip_code;
    static const std::map<std::string, std::vector<std::pair<std::string, OutputType>>> dependenciesMap;  // Keeps a list of properties affected by a property.
public:
    Person (const std::string& n, int a, const std::string& c, const std::string& z) : name(n), age(a), country(c), zip_code(z) { }
    static const std::map<std::string, int(Person::*)() const> intMethodMap;
    static const std::map<std::string, bool(Person::*)() const> boolMethodMap;
    static const std::map<std::string, std::string(Person::*)() const> stringMethodMap;
    using TupleMaps = std::tuple<decltype(intMethodMap), decltype(boolMethodMap), decltype(stringMethodMap)>;
    using TupleMaps2 = std::tuple<std::map<std::string, int>, std::map<std::string, bool>, std::map<std::string, std::string>>;
    static const TupleMaps methodMaps;
    int get_age() const { return age; }
    void set_age (int a) { set_field(&Person::age, "age", a); }
    int get_height() const { return height; }
    std::string get_name() const { return name; }
    std::string nickname() const {
        if (country != "France")
            return name;
        return "Francois";
    }
    std::string get_country() const { return country; }
    void set_country (const std::string& c) { set_field(&Person::country, "country", c); }
    std::string get_zip_code() const { return zip_code; }
    void set_zip_code (const std::string& z) { set_field(&Person::zip_code, "zip_code", z); }
    bool can_vote() const { return age >= 16 && country == "USA"; }
    bool can_quit_school() const { return age >= 15 && country == "USA" && zip_code == "90210"; }
    int years_left() const {
        if (country == "France")
            return 10;
        return 80 - age;
    }
private:
    template <typename T> void set_field (T Person::*, const std::string& field_name, const T& value);  // T Person::* is pointer to data member.
    template <OutputType> void set_field_helper (TupleMaps2& trackedValuesAllTypes, const std::pair<std::string, OutputType>&);
    template <OutputType> void notify_other_field_changes (const TupleMaps2&);
};
const std::map<std::string, std::vector<std::pair<std::string, OutputType>>> Person::dependenciesMap = {
    {"age", {{"can_vote()", Bool}, {"can_quit_school()", Bool}, {"years_left()", Int}}},  // age affects can_vote(), can_quit_school(), and years_left().
    {"country", {{"can_vote()", Bool}, {"can_quit_school()", Bool}, {"years_left()", Int}, {"nickname", String}}},  // country affects can_vote, can_quit_school(), years_left(), and nickname.
    {"zip_code", {{"can_quit_school()", Bool}}}  // zip_code affects can_quit_school
 };
const std::map<std::string, int(Person::*)() const> Person::intMethodMap = {
    {"age", &Person::get_age}, {"height", &Person::get_height}, {"years_left()", &Person::years_left}
};
const std::map<std::string, bool(Person::*)() const> Person::boolMethodMap = {
    {"can_vote()", &Person::can_vote}, {"can_quit_school()", &Person::can_quit_school}
};
const std::map<std::string, std::string(Person::*)() const> Person::stringMethodMap = {
    {"name", &Person::get_name}, {"country", &Person::get_country}, {"zip_code", &Person::get_zip_code}, {"nickname", &Person::nickname}
};
const Person::TupleMaps Person::methodMaps = std::make_tuple(Person::intMethodMap, Person::boolMethodMap, Person::stringMethodMap);

struct ConsolePersonObserver : Observer<Person> {
    virtual void field_changed (Person& source, const std::string& field_name, OutputType output_type) override { field_changed_helper<FirstOutputType>(source, field_name, output_type); }
private:
    template <OutputType> void field_changed_helper (Person& source, const std::string& field_name, OutputType output_type);
};

template <OutputType N>
void ConsolePersonObserver::field_changed_helper (Person& source, const std::string& field_name, OutputType output_type) {
    if (output_type == N)
        std::cout << source.get_name() << "'s " << field_name << " has changed to " << (source.*std::get<N>(Person::methodMaps).at(field_name))() << ".\n";
    else
        field_changed_helper<static_cast<OutputType>(N+1)>(source, field_name, output_type);
}

template <>
void ConsolePersonObserver::field_changed_helper<NumOutputTypes> (Person&, const std::string&, OutputType) { }  // End of recursion

template <typename T>
void Person::set_field (T Person::*ptr, const std::string& field_name, const T& value) {
    if (value == this->*ptr)
        return;
    TupleMaps2 trackedValuesAllTypes;
    for (const std::pair<std::string, OutputType>& pair : dependenciesMap.at(field_name))
        set_field_helper<FirstOutputType>(trackedValuesAllTypes, pair);
    this->*ptr = value;
    const OutputType type = GetOutputType<typename std::remove_reference<decltype(this->*ptr)>::type>::value;
    notify_field_change(*this, type, field_name);
    notify_other_field_changes<FirstOutputType>(trackedValuesAllTypes);
}

template <OutputType N>
void Person::set_field_helper (TupleMaps2& trackedValuesAllTypes, const std::pair<std::string, OutputType>& pair) {
    if (pair.second == N)
        std::get<N>(trackedValuesAllTypes)[pair.first] = (this->*std::get<N>(Person::methodMaps).at(pair.first))();
    else
        set_field_helper<static_cast<OutputType>(N+1)>(trackedValuesAllTypes, pair);
}

template <>
void Person::set_field_helper<NumOutputTypes> (TupleMaps2&, const std::pair<std::string, OutputType>&) { }  // End of recursion.

template <OutputType N>
void Person::notify_other_field_changes (const TupleMaps2& trackedValuesAllTypes) {
    for (const auto& pair : std::get<N>(trackedValuesAllTypes)) {
        if (pair.second != (this->*std::get<N>(methodMaps).at(pair.first))())
            notify_field_change(*this, N, pair.first);
    }
    notify_other_field_changes<static_cast<OutputType>(N+1)>(trackedValuesAllTypes);
}

template <>
void Person::notify_other_field_changes<NumOutputTypes> (const TupleMaps2&) { }  // End of recursion.

int main() {
    Person p("Tom", 14, "USA", "90210");
    ConsolePersonObserver cpo;
    p.subscribe(&cpo);
    p.set_age(15);
    p.set_age(16);
    p.set_zip_code("999999");
    p.set_country("France");
}

Here is my extended code to handle the general case when the output types from the methods obtained by dependenciesMap are different (I removed the template argument of notify_field_changed to make this work):

Here is my extended code to handle the general case when the output types from the methods obtained by dependenciesMap are different (I removed the template argument of notify_field_change to make this work). For example, country affects can_vote, can_quit_school(), years_left(), and nickname, which return bool, bool, int, and std::string respectively. But I have a feeling there is a cleaner way to handle this than my code below:

Here is my extended code to handle the general case when the output types from the methods obtained by dependenciesMap are different (I removed the template argument of notify_field_changed to make this work):

#include <iostream>
#include <vector>
#include <map>
#include <tuple>

enum OutputType { Int, Bool, String, NumOutputTypes, FirstOutputType = Int };

template <typename T> struct GetOutputType;
template <> struct GetOutputType<int> : std::integral_constant<OutputType, Int> { };
template <> struct GetOutputType<bool> : std::integral_constant<OutputType, Bool> { };
template <> struct GetOutputType<std::string> : std::integral_constant<OutputType, String> { };

template <typename T>
struct Observer {
    virtual void field_changed (T&, const std::string& field_name, OutputType) = 0;
};

template <typename T>
class Observable {
    std::vector<Observer<T>*> observers;
public:
    void notify_field_change (T& source, OutputType output_type, const std::string& field_name) {
        for (Observer<T>* obs : observers)
            obs->field_changed(source, field_name, output_type);
    }
    void subscribe (Observer<T>* f) { observers.push_back(f); }
    void unsubscribe (Observer<T>* f) { observers.erase(remove(observers.begin(), observers.end(), f), observers.end()); }
};

class Person : public Observable<Person> {
    int age, height;
    std::string name, country, zip_code;
    static const std::map<std::string, std::vector<std::pair<std::string, OutputType>>> dependenciesMap;  // Keeps a list of properties affected by a property.
public:
    Person (const std::string& n, int a, const std::string& c, const std::string& z) : name(n), age(a), country(c), zip_code(z) { }
    static const std::map<std::string, int(Person::*)() const> intMethodMap;
    static const std::map<std::string, bool(Person::*)() const> boolMethodMap;
    static const std::map<std::string, std::string(Person::*)() const> stringMethodMap;
    using TupleMaps = std::tuple<decltype(intMethodMap), decltype(boolMethodMap), decltype(stringMethodMap)>;
    using TupleMaps2 = std::tuple<std::map<std::string, int>, std::map<std::string, bool>, std::map<std::string, std::string>>;
    static const TupleMaps methodMaps;
    int get_age() const { return age; }
    void set_age (int a) { set_field(&Person::age, "age", a); }
    int get_height() const { return height; }
    std::string get_name() const { return name; }
    std::string nickname() const {
        if (country != "France")
            return name;
        return "Francois";
    }
    std::string get_country() const { return country; }
    void set_country (const std::string& c) { set_field(&Person::country, "country", c); }
    std::string get_zip_code() const { return zip_code; }
    void set_zip_code (const std::string& z) { set_field(&Person::zip_code, "zip_code", z); }
    bool can_vote() const { return age >= 16 && country == "USA"; }
    bool can_quit_school() const { return age >= 15 && country == "USA" && zip_code == "90210"; }
    int years_left() const {
        if (country == "France")
            return 10;
        return 80 - age;
    }
private:
    template <typename T> void set_field (T Person::*, const std::string& field_name, const T& value);  // T Person::* is pointer to data member.
    template <OutputType> void set_field_helper (TupleMaps2& trackedValuesAllTypes, const std::pair<std::string, OutputType>&);
    template <OutputType> void notify_other_field_changes (const TupleMaps2&);
};
const std::map<std::string, std::vector<std::pair<std::string, OutputType>>> Person::dependenciesMap = {
    {"age", {{"can_vote()", Bool}, {"can_quit_school()", Bool}, {"years_left()", Int}}},  // age affects can_vote(), can_quit_school(), and years_left().
    {"country", {{"can_vote()", Bool}, {"can_quit_school()", Bool}, {"years_left()", Int}, {"nickname", String}}},  // country affects can_vote, can_quit_school(), years_left(), and nickname.
    {"zip_code", {{"can_quit_school()", Bool}}}  // zip_code affects can_quit_school
 };
const std::map<std::string, int(Person::*)() const> Person::intMethodMap = {
    {"age", &Person::get_age}, {"height", &Person::get_height}, {"years_left()", &Person::years_left}
};
const std::map<std::string, bool(Person::*)() const> Person::boolMethodMap = {
    {"can_vote()", &Person::can_vote}, {"can_quit_school()", &Person::can_quit_school}
};
const std::map<std::string, std::string(Person::*)() const> Person::stringMethodMap = {
    {"name", &Person::get_name}, {"country", &Person::get_country}, {"zip_code", &Person::get_zip_code}, {"nickname", &Person::nickname}
};
const Person::TupleMaps Person::methodMaps = std::make_tuple(Person::intMethodMap, Person::boolMethodMap, Person::stringMethodMap);

struct ConsolePersonObserver : Observer<Person> {
    virtual void field_changed (Person& source, const std::string& field_name, OutputType output_type) override { field_changed_helper<FirstOutputType>(source, field_name, output_type); }
private:
    template <OutputType> void field_changed_helper (Person& source, const std::string& field_name, OutputType output_type);
};

template <OutputType N>
void ConsolePersonObserver::field_changed_helper (Person& source, const std::string& field_name, OutputType output_type) {
    if (output_type == N)
        std::cout << source.get_name() << "'s " << field_name << " has changed to " << (source.*std::get<N>(Person::methodMaps).at(field_name))() << ".\n";
    else
        field_changed_helper<static_cast<OutputType>(N+1)>(source, field_name, output_type);
}

template <>
void ConsolePersonObserver::field_changed_helper<NumOutputTypes> (Person&, const std::string&, OutputType) { }  // End of recursion

template <typename T>
void Person::set_field (T Person::*ptr, const std::string& field_name, const T& value) {
    if (value == this->*ptr)
        return;
    std::tuple<std::map<std::string, int>, std::map<std::string, bool>, std::map<std::string, std::string>> trackedValuesAllTypes;
    for (const std::pair<std::string, OutputType>& pair : dependenciesMap.at(field_name))
        set_field_helper<FirstOutputType>(trackedValuesAllTypes, pair);
    this->*ptr = value;
    const OutputType type = GetOutputType<typename std::remove_reference<decltype(this->*ptr)>::type>::value;
    notify_field_change(*this, type, field_name);
    notify_other_field_changes<FirstOutputType>(trackedValuesAllTypes);
}

template <OutputType N>
void Person::set_field_helper (TupleMaps2& trackedValuesAllTypes, const std::pair<std::string, OutputType>& pair) {
    if (pair.second == N)
        std::get<N>(trackedValuesAllTypes)[pair.first] = (this->*std::get<N>(Person::methodMaps).at(pair.first))();
    else
        set_field_helper<static_cast<OutputType>(N+1)>(trackedValuesAllTypes, pair);
}

template <>
void Person::set_field_helper<NumOutputTypes> (TupleMaps2&, const std::pair<std::string, OutputType>&) { }  // End of recursion.

template <OutputType N>
void Person::notify_other_field_changes (const TupleMaps2& trackedValuesAllTypes) {
    for (const auto& pair : std::get<N>(trackedValuesAllTypes)) {
        if (pair.second != (this->*std::get<N>(methodMaps).at(pair.first))())
            notify_field_change(*this, N, pair.first);
    }
    notify_other_field_changes<static_cast<OutputType>(N+1)>(trackedValuesAllTypes);
}

template <>
void Person::notify_other_field_changes<NumOutputTypes> (const TupleMaps2&) { }  // End of recursion.

int main() {
    Person p("Tom", 14, "USA", "90210");
    ConsolePersonObserver cpo;
    p.subscribe(&cpo);
    p.set_age(15);
    p.set_age(16);
    p.set_zip_code("999999");
    p.set_country("France");
}

Output:

Tom's age has changed to 15.
Tom's years_left() has changed to 65.
Tom's can_quit_school() has changed to 1.
Tom's age has changed to 16.
Tom's years_left() has changed to 64.
Tom's can_vote() has changed to 1.
Tom's zip_code has changed to 999999.
Tom's can_quit_school() has changed to 0.
Tom's country has changed to France.
Tom's years_left() has changed to 10.
Tom's can_vote() has changed to 0.
Tom's nickname has changed to Francois.