The Style of C++ 11

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C++11 status
(Some) New language features
(Some) New library features
Modern C++ style

• After more than a decade of contemplation . . .
• ISO C++11 Standard was published in September 2011
• Feature list: http://en.wikipedia.org/wiki/C%2B%2B11

• The Standard Library is part of the C++11 standard
• Some new features are upstream merges from TR1

• Visual Studio 2010: Some features are supported
• Visual Studio 2012: Some more features are supported

Visual Studio 2012

Not supported yet
Visual Studio 2010

Automatic Concurrency Variadic
variables, library templates
decltype Memory Custom
Rvalue model literals
references Delegating
Lambda constructors
functions
• Comparison chart between many other compilers:
http://s.sashag.net/rpST0u

Implicit variable declaration: The compiler knows what
you mean
(Almost) necessary for anonymous types
Very convenient for complex templates
Easily abused by lazy programmers!

std::map<...> M;
auto iter = M.begin(); //what’s the type of iter?
auto pair = std::make_pair(iter, M.key_range(...));
auto lambda = []() { ... }; //lambdas have an anonymous type
auto ptr = condition ? new class1 : new class2; //ERROR
auto x = 15; auto s = (string)"Hello"; //try to avoid...

Automatic iterator over arrays and STL collections
Your collection will work – provide begin(), end(), and an
input iterator over the elements
Up to VC11 Beta: for each … in, a non-standard
Microsoft extension

int numbers[] = ...;
for (int n : numbers) std::cout << n;
std::map<std::string,std::list<int>> M;
for (const auto& pair : M)
for (auto n : pair.second)
std::cout << pair.first << ' ' << pair.second;

Use a compile-time expression instead of a type name
Can take the type of any expression
Very useful for templates, forwarding etc.

float arr[15];
decltype(arr[0]) flt_ref = arr[0];
decltype(arr[1]+7) flt;
decltype(rand()) n = rand();

decltype(i+j) sum = i+j;

Your function can return a decltype
Requires special syntax in these examples because the return
type depends on parameter types

template <typename T1, typename T2>
auto multiply(const T1& t1, const T2& t2) -> decltype(t1+t2) {
return t1 * t2;
}

template <typename InputIterator>
auto nth_element(InputIterator iter, int n) -> decltype(*iter) {
while (--n > 0) ++iter;
return *iter;
}

Initialize arrays, lists, vectors, other containers—and
your own containers—with a natural syntax
Not yet supported by Visual Studio 2012

vector<int> v { 1, 2, 3, 4 };
list<string> l = { “Tel-Aviv”, “Jerusalem” };
my_cont c { 42, 43, 44 };

class my_cont {
public: my_cont(std::initializer_list<int> list) {
for (auto it = list.begin(); it != list.end(); ++it) . . .
}
};

int main() {
[](){}();
[]{}();
} //this is legal C++,
//although not useful

The current state of function objects and operations on
them leaves much to be desired
Must use arcane binding functions, placeholders, and
rules to construct composite functors

class employee {
public: void bonus(float commission, int vacation);
};
vector<int> employees;
std::for_each(employees.begin(), employees.end(),
std::bind(std::mem_fn(&employee::bonus), _1, 0.25f, 3));

TR1 makes it somewhat easier to manipulate functors
(functions and classes with operator())
Doesn’t make it easier to create functors

std::function<bool(int,int)> g = greater<int>();
std::function<int(int,char**)> m = main;

std::function<bool(int)> greater_than17 = std::bind(g, _1, 17);
std::function<void(X*)> f = &X::foo; //foo is a member function

Inline methods in other methods (closures)
Compile to an anonymous class that serves as a function object
Rich capture semantics by value and by reference

auto print_num = [](int n) { std::cout << n; };
std::list<int> ns = ...;
std::for_each(ns.begin(), ns.end(), print_num);
int even = std::count_if(ns.begin(), ns.end(), [](int n) { return n&1==0;
});

int x = 5;
[&x]() { ++x; }(); //capture by reference
[ x]() { ++x; }(); //capture by value. doesn’t compile!!

Default capture (use at your own risk)
Mutable lambdas
Explicit return value
int fib1 = 1, fib2 = 1;
auto next_step = [&]() { //default capture by reference
int temp = fib2; fib2 = fib2 + fib1; fib1 = temp;
};
for (int i = 0; i < 20; ++i) next_step();

int n = 10;
auto say_yes_n_times = [=]() mutable ->bool { //default capture by value,
return (--n > 0); //mutable and returns bool
};

std::function<...> to the rescue
Freely manipulate lambdas as objects

auto identity = [](int x) {
return [x]() { return x; };
};
auto next = [](const std::function<int(void)>& lambda) {
return [&lambda]() { return lambda() + 1; };
};
auto _1 = identity(1);
auto _2 = next(_1);
auto _3 = next(_2);
std::cout << _1() << _2() << _3();

Recall our contrived bind(mem_fn(…)) example
Use a lambda instead of composite functors
Design your APIs with lambdas in mind
std::bind(memfn(&employee::bonus), _1, 0.25f, 3));
[](const employee& e) { e.bonus(0.25f, 3); });

template <typename Callback>
void enum_windows(const string& title, Callback callback) {
. . . callback(current_window);
}
//or, use const std::function<void(const window&)>& as parameter

Lvalues are values that have a name
Can appear on the left-hand-side of an assignment
Rvalues are the rest
int x;
x = 42; //OK, x has a name, it’s an lvalue
42 = x; //Obviously wrong, 42 does not have a name, it’s an rvalue
x + 2 = 42; //Also wrong, x + 2 returns a temporary, it’s an rvalue
x++ = 42; //Also wrong, x++ returns a temporary, it’s an rvalue

int& foo();
int* goo();
--foo(); //OK, foo() returns an lvalue
++(*goo()); //OK, a dereferenced pointer is an lvalue

Turns out, this “standard” approach to references limits
the performance of the language
In this example, the contents of the vectors are COPIED
void init_vector(vector<int>& v);

vector<int> v, w;
init_vector(v); //no copy, we were careful to pass a reference
init_vector(w); //no copy, we were careful to pass a reference

swap(v, w);
//internally, swap will copy v to temp, w to v, temp to w, for a total
//of THREE MEMORY ALLOCATIONS AND DEALLOCATIONS!
//but how can we tell swap (and vector) to MOVE the contents around?

Rvalue references are references to rvalues!
Standard references are to lvalues, const references may refer to
temporary rvalues
Enable move construction and assignment

my_array(const my_array& other) { //copy ctor
dataptr_ = new T[size_ = other.size_];
memcpy_s(dataptr_, size_*sizeof(T), other.dataptr_, size_*sizeof(T));
}
my_array& operator=(const my_array& other) { /*same deal*/ }
my_array& operator=(my_array&& other) { //move assignment
dataptr_ = other.dataptr_; size_ = other.size_;
other.dataptr_ = nullptr; other.size_ = 0;
}
my_array(my_array&& other) { //move ctor
*this = std::move(other); //NOTE: && is lvalue in the method body
}

• Much fewer copies of temporary objects float around
– E.g. consider std::vector<T> with reallocation
– Huge performance boost when your types are used in STL
– Huge performance boost when using strings and other types
with inner state that is expensive to copy

• Use auto, for each, initializer lists ubiquitously
• Don’t be afraid of returning objects by value
– RVO, NRVO, and move constructors will minimize copies
• OK to design algorithms that require predicates,
projections, and other functors
– They will be easy to use—with lambda functions
• Use STL algorithms more widely with lambdas

Four standard unordered containers which use hash
tables as their implementation
unordered_map, unordered_set,
unordered_multimap, unordered_multiset

set<string> names = { “Mike”, “Adam” };
assert(*names.begin() == “Adam”);

unordered_set<string> names = { “John”, “Abe” };
for (auto name : names)
cout << name; //alphabetic order is NOT guaranteed

PERL-style regular expression facility offered by
std::regex class and associated functions

regex version("(d+).(d+).(d+)");
string text = "2.0.50727";

cmatch captures;
if (regex_search(text.c_str(), captures, version)) {
cout << "Major: " << captures[0] << endl;
cout << "Build: " << captures[2] << endl;
}

//there’s also regex_replace for obvious purposes

• The standard library now has three types of smart
pointers, eliminating the need to ever use delete

• If you are the sole owner of the object, use
unique_ptr to make sure it’s deleted when the
pointer dies (RAII)
• If you want to share the object with others, use
shared_ptr—it will perform smart reference
counting
• If you got yourself a cycle, use weak_ptr to break it!

Sole owner of an object
Supports move semantics, but not copy semantics
Replaces auto_ptr (which can’t move!)
unique_ptr<expensive_thing> create() {
unique_ptr<expensive_thing> p(new expensive_thing);
//...do some initialization, exceptions are covered by RAII
return p;
}

unique_ptr<expensive_thing> p = create(); //move constructor used!

//another example is storing pointers in containers:
vector<unique_ptr<string>> v = { new string(“A”), new string(“B”) };

Thread-safe reference-counted pointer to an object
with shared ownership
When the last pointer dies, the object is deleted

struct file_handle {
HANDLE handle;
file_handle(const string& filename) ...
~file_handle() ... //closes the handle
};
class file {
shared_ptr<file_handle> _handle;
public:
file(const string& filename) : _handle(new file_handle(filename)) {}
file(shared_ptr<file_handle> fh) : _handle(fh) {}
}; //can have multiple file objects over the same file_handle

Points to a shared object but does not keep it alive
(does not affect reference count)
The object may be destroyed “under our nose” at any time
Breaks cycles between shared_ptrs

class employee {
weak_ptr<employee> _manager;
vector<shared_ptr<employee>> _direct_reports;
public:
void beg_for_vacation(int days) {
if (auto mgr = _manager.lock()) { mgr->beg(days); } //mgr is shared_ptr
else { /* your manager has been eliminated :-) */ }
}
};

• Use smart pointers—no reason to have a delete
statement in your code
– If you’re the only owner, use unique_ptr
– If you’re sharing the object, use shared_ptr
– Create shared_ptrs with make_shared()
– To prevent cycles, use weak_ptr
• Use the non-member begin() and end() functions
– They work on arrays, and can be overloaded for types you
don’t control

• Bjarne Stroustrup’s FAQ: http://s.sashag.net/vWT1eI
• C++11 Wikipedia article: http://s.sashag.net/vdSCW3
• What’s New in VC++ 10: http://s.sashag.net/tb1fnr
• What’s New in VC++ 11: http://s.sashag.net/sXy26y
• More on rvalue references: http://s.sashag.net/uVLJ23
• STL11 preliminary docs: http://s.sashag.net/vWR7sW
• C++ memory model: http://s.sashag.net/rqsoDW
• Modern C++ style: http://s.sashag.net/rP5DFl

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