Example code of the C++11 features

feature.cpp

/// This is a demonstration of a few nice C++11 features: auto, the ':'
/// iterator, initializer lists, std::function, and lambda expressions.  In
/// many cases, it's possible to achieve the same effect as I present using
/// even less code, but by being explicit I hope I have made the code and
/// comments easy to follow

#include <functional>
#include <iostream>
#include <map>
#include <string>
#include <vector>

using namespace std; // I'm being lazy here... should explicitly use cout,
                     // endl, function, map, pair, string, and vector


/// wrapped_map_t extends map<string, string>.  We start with a map of
/// key/value pairs, where both the key and value are strings, and we add a
/// new method to it.  The new method, apply_to_all, lets us apply a lambda
/// to (a copy of) every key/value pair that is in the map.
///
/// my claim is that lambdas fundamentally change the art of data structure
/// design.  To support my claim, I will show how this one simple function
/// obviates many public functions that one might want int a data structure
/// (count, print, extract_keys, find_matching_keys, etc).
struct wrapped_map_t : public map<string, string>
{
    /// the apply_to_all function simply iterates through the key/value pairs
    /// of the map, calling lambda(key, value) for each pair.  Strictly
    /// speaking, std::map is a red/black tree, and this will do a pre-order
    /// traversal
    void apply_to_all(function<void(const string, const string)>& lambda)
    {
        // note the use of C++11 'auto' keyword to avoid having to specify
        // the type of the iterator
        for (auto i = begin(), e = end(); i != e; ++i)
            lambda(i->first, i->second);
    }
};

int main()
{
    // declare an instance of the extended map
    wrapped_map_t kvpairs;

    // populate the map with some keys and values.  We're going to use some
    // great C++ magic here: initializer lists.  We don't even have to know
    // the type of the list, but the compiler will deduce that it's something
    // like pair<string,string>[]
    kvpairs.insert({{"donald", "duck"}, {"mickey", "mouse"},
                    {"minnie", "mouse"}, {"bat", "man"},
                    {"super", "man"}, {"milo", "otis"},
                    {"mighty", "mouse"}, {"rocky", "squirrel"},
                    {"bullwinkle", "moose"}});

    // 'func' is a reference to a function that takes two strings as its
    // parameters, and returns nothing.  This is a lot nicer than the old C
    // syntax in use before C++11 (i.e., void (*func)(string, string)).
    function<void(string, string)> func;

    // let's assign a new lambda to func.  the lambda will simply print its
    // parameters
    func = [](string k, string v) { cout << "{" << k << ", " << v << "} "; };
    // now let's apply func to every element in the set.  No need for a
    // "print_in_sorted_key_order" function in our class.  It would be
    // trivial to add "print keys" and "print values" features without
    // changing the data structure... we'd just need to pass different
    // lambdas
    cout << "KVPairs = ";
    kvpairs.apply_to_all(func);
    cout << endl;

    // The last example was pretty boring.  A function pointer could have
    // done that easily.  Now let's pass in a function that has a captured
    // reference to a variable that is local to this function.  In that way,
    // each time the lambda is called, it can access the function 'my_count'
    // that we declare *right here*.  The net result is that we can count the
    // elements in the map, without requiring map to provide a "count()"
    // function.  Note that the parameters to the lambda aren't used, and
    // that's OK.
    int my_count = 0;
    func = [&my_count](string k, string v) { my_count++; };
    kvpairs.apply_to_all(func);
    cout << "Elements in map = " << my_count << endl;

    // OK, so we could have achieved that effect with an apply_to_all that
    // took three parameters: string, string, int&.  But would we really want
    // to do that for every possible pass-by-reference type?  For example,
    // here we pass a vector of strings so that we can copy out all keys from
    // the map
    vector<string> keys;
    func = [&keys](string k, string v) {keys.push_back(k);};
    kvpairs.apply_to_all(func);
    // print the keys, to show that it worked
    cout << "Keys in map = {";
    for (auto i : keys)
        cout << i << ",";
    cout << "}" << endl;

    // note, too, that we can use the lambda to capture multiple local
    // variables, and that some can be pass by value while others are pass by
    // reference.  Here 'pre' is pass by value, 'keys' is pass by reference,
    // and our function copies into 'keys' all keys in the map whose values
    // begin with 'mo'
    string pre = "mo";
    keys.clear();
    func = [&keys, pre](string k, string v) { if (v.find(pre) == 0) keys.push_back(k);};
    kvpairs.apply_to_all(func);
    cout << "Keys whose values start with 'mo' = {";
    for (auto i : keys)
        cout << i << ",";
    cout << "}" << endl;

    // this example is much like the prior one.  Here we extract all
    // key/value pairs where the key begins with 'mi'
    pre = "mi";
    vector<pair<string, string>> pairs;
    func = [&pairs, pre] (string k, string v)
        { if (k.find(pre) == 0) pairs.push_back(make_pair(k, v)); };
    kvpairs.apply_to_all(func);
    cout << "Elements whose key begins with 'mi' = {";
    for (auto i : pairs)
        cout << "{"<<i.first<<", "<<i.second<<"} ";
    cout << "}" << endl;

    // the point of this example is to illustrate that when the lambda is
    // called with the key and value, the function we are executing *cannot*
    // modify the internals of the map.  Put another way, the lambda does not
    // become a member function of the class.  So, for example, if we wanted
    // to add a suffix to every key, then the only way we could do it would
    // be by explicitly updating the k/v pair in the map.
    string suf = " <3";
    // note: we need to capture *kvpairs itself* by reference
    func = [&kvpairs, suf] (string k, string v) { kvpairs[k] = v+suf; };
    kvpairs.apply_to_all(func);
    // use a lambda to print it out
    func = [](string k, string v) { cout << "{" << k << ", " << v << "} "; };
    cout << "Modified kvpairs = ";
    kvpairs.apply_to_all(func);
    cout << endl;
}