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C++11 Features Every C++ Programmer Should Know

Recently I've been practicing LeetCode problems for technical interview. LeetCode OJ supports C++11 and I feel great writing code with it. While there are articles introducing C++11 features (for example this one), most of them are too deep. I don't believe you need to understand move semantics before you write a well-formed C++ program.

To clarify, I'm not saying that advanced features like move semantics are not important. Rather, I find some C++11 features which not only are easy to learn but can greatly improve programming experience using C++. So, this article is about those features that every C++ programmer should absolutely check out.

Before we start: if you want to compile the examples, please make sure your favorite compiler supports C++11 and set the option -std=c++11. (The latest MSVC, g++ and clang has support for most of the C++11 features. Check here for a compatibility table)

initializer list

C++, and also many old languages, lack support for literals for non-native types like vector and map. With C++11 initializer list, you can initialize them like this:

vector<int> iv = {1, 2, 3, 4};
map<int, string> employees = {{1, "John"}, {2, "Mary"}};

Note as map (and also unordered_map, see below) stores key-value pairs internally, the syntax is like initializing a set of pairs.

What's more, the initializer list can be nested:

map<int, vector<char>> number_pad = {
    {1, {'a', 'b', 'c'}},
    {2, {'d', 'e', 'f'}},
    ...
};

This not only gives you more concise code, but more importantly shows your intention better.

Advanced Tip: you can actually use std::initializer_list to define literals for your own class.

right angle bracket

Did you notice in the previous example we omitted the space between the two > characters? Compilers with C++11 support will not treat that as shift operator any more. One more pitfall fixed.

type inference (the auto keyword)

Remember the last time you wrote this:

for(vector<int>::iterator it = iv.begin();it != iv.end();++it) {}

Have you ever wondered why the compiler forces you to specify type of it even though it can infer the type? (it is unambiguous that vector<int>::begin returns vector<int>::iterator)

Not any more! With C++11, you can let the compiler infer the type itself with the auto keyword:

vector<int> iv = {1, 2, 3};
for(auto it = iv.begin();it != iv.end();++it) {
    cout << (*it) << endl;
}

This handy feature can reduce verbosity greatly. For example, instead of writing:

void DoSomething(const multimap<string, vector<int>>& myMap)
{
    const pair<multimap<string, vector<int>>::const_iterator,
               multimap<string, vector<int>>::const_iterator> eq =
        myMap.equal_range("String!");
    for (multimap<string, vector<int>>::const_iterator itr = eq.first;
        itr != eq.second; ++itr)
        cout << itr->size() << endl;
}

You can write:

void DoSomething(const multimap<string, vector<int>>& myMap)
{
    const auto eq = myMap.equal_range("String!");
    for(auto itr = eq.first; itr != eq.second; ++itr)
        cout << itr->size() << endl;
}

(example taken from Keith Schwarz's CS106L Course Reader)

That's not only good for your fingers but also good for your eyes.

Advanced Tip: you will be able to use auto keyword to do function return type deduction in C++14.

range-based for loop

Yes, C++ finally got language level support for range-based for loop! With this, you can write the previous example as

vector<int> iv = {1, 2, 3};
for(auto i : iv) { // type of i is int
    cout << i << endl;
}

Same behavior, much less code!

When copying is not desired, you can also use reference:

list<HeavyClass> queue = {...};
for(auto &o : queue) { // type of o is HeavyClass&
    // manipulate o
}

Advanced Tip: In order for ranged-based for loop to work, the object provided to it must satisfy some properties. As a result, while you can do range-based loop on most STL collections (including string), you cannot directly range-based loop over raw types like char *. So, it turns out, C++11 range-based for loop is actually a syntactic sugar.

hashtable containers

C++11 introduces four new containers: unordered_set, unordered_multiset, unordered_map and unordered_multimap. Each has the exactly same interface as their counterparts without the unordered_ prefix. The difference is that these new containers are based on hash table instead of red-black trees.

See this discussion for choosing map or unordered_map.

Note: the standard committee chose the unordered_ prefix to ensure backward compatibility with programs using non-standard hashtable implementation (which is available in many C++ libraries as extension).

object construction improvement

Object construction in C++03 used to have several restrictions. The most unintuitive one may be that you cannot delegate construction to constructors of the same class. Although you can often solve such problem with default parameter(s) or a common initialize function, sometimes you just can't. C++11 removes such restriction:

class SomeType  {
    int number;

public:
    SomeType(int new_number) : number(new_number) {}
    SomeType() : SomeType(42) {}
};

The second improvement is that you can use base class's constructor directly, instead of writing a wrapper constructor.

class BaseClass {
public:
    BaseClass(int value);
};

class DerivedClass : public BaseClass {
public:
    // C++03
    DerivedClass(int value): BaseClass(value) {}
    // C++11
    using BaseClass::BaseClass;
};

The third improvement is more handy: you can initialize non-static member at declaration.

class Coordinate {
    int x = 0;
    int y = 0;
public:
    Coordinate() {}
    Coordinate(int _x, int _y): x(_x), y(_y) {}
};

In the above example, if the constructor with no parameter is called, the two members x and y will have the default value of 0.

Note that initialization at declaration is only possible for non-static members. For static members, you still have to initialize them outside the class.

lambda functions

Functional programming (FP) has become more and more popular in recent years. Anonymous functions, which may not be an essential feature of FP, is so handy that many programmers love it. Before C++11, STL uses functors to emulate anonymous functions.

Consider the following program:

struct AbsCompare {
    bool operator() (int i, int j) {
        return abs(i) < abs(j);
    }
};

int main() {
    vector<int> iv = {8, -4, 3, -2, 5, 1, -7, 9, -6};

    sort(iv.begin(), iv.end(), AbsCompare());

    for(int i : iv)
        cout << i << endl;
}

With C++11, you can write as this:

int main() {
    vector<int> iv = {8, -4, 3, -2, 5, 1, -7, 9, -6};

    sort(iv.begin(), iv.end(), [](int i, int j) {
        return abs(i) < abs(j);
    });

    for(int i : iv)
        cout << i << endl;
}

Not surprisingly, C++11 anonymous functions support closures. You can capture variables outside the lambda's scope with []. A simple example is to find number of values less than threshold:

vector<int> numbers = {1, 2, 3, 4, 5, 6};
int threshold = 4, count = 0;
for_each(numbers.begin(), numbers.end(), [threshold, &count](int i) {
    if (i < threshold) ++count;
});
cout << count << endl;

This is a pedagogical example, you can actually use count_if, but it demonstrates how to capture variables anyway. threshold is captured by value, while count is captured by reference - that's why we can change value of count inside the anonymous function. Actually, if you are lazy enough, you can use [&] or [=] to implicitly capture any external variable by reference or not. But such practice is not recommended.

What may surprise you is that anonymous functions have a different type from plain functions. They are actually specialization of type std::function. For example, the anonymous function used in the previous example is a std::function<void(int)> and, as a result, it cannot be assignment to variable of void (*)(int) type. If you want your function to accept both traditional and anonymous functions, remember to use templates (like what std::sort does).

regular expressions

The last feature I want to introduce is regular expression facility. A simple example would serve the purpose:

#include <regex>
#include <string>
#include <iostream>

int main() {
    std::string haystack = "The United States is a developed country and has the world's largest national economy, with an estimated GDP in 2013 of $16.7 trillion – 23% of global nominal GDP and 19% at purchasing-power parity.";
    std::regex num_rgx("(^|\\b)\\d+($|\\b)");
    std::smatch match;

    while (std::regex_search(haystack, match, num_rgx)) {
        std::cout << "find number: " << match[0] << std::endl;
        haystack = match.suffix().str();
    }

    return 0;
}

Here we first constructed regular expression of numbers and a match object (it's prefixed s because it's for string, there's also a cmatch class for const char*). Then we use regex_search for searching in haystack for numbers. If there's a match, as you would expect, group 0 is the whole matched string, group 1 is the preceding boundary ((^|\\b)) and group 2 is the succeeding boundary (($|\\b)). You can access each group using [] operator.

Note in this simple example, in order to advance the matching, we use match.suffix().str() to get the unmatched part of haystack, which is rather inefficient. For performance, use std::regex_iterator instead.

I also found a cheat sheet for C++11 regex.

There are actually tons of nice new features like threading, uniform initialization, strongly typed enums, tuples, raw string literals and on and on and on. C++11 is really a whole new language. For reference and a full list of C++11 new features, see this Wikipedia article (some examples in this article are from the article).

With coming C++14, the trend is that, the language is becoming more dynamic (at least in appearance) and programmer-friendly.