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C++11

Overview

Many of these descriptions and examples come from various resources (see Acknowledgements section), summarized in my own words.

C++11 includes the following new language features:

C++11 includes the following new library features:

C++11 Language Features

Move semantics

Move semantics is mostly about performance optimization: the ability to move an object without the expensive overhead of copying. The difference between a copy and a move is that a copy leaves the source unchanged, and a move will leave the source either unchanged or radically different -- depending on what the source is. For plain old data, a move is the same as a copy.

To move an object means to transfer ownership of some resource it manages to another object. You could think of this as changing pointers held by the source object to be moved, or now held, by the destination object; the resource remains in its location in memory. Such an inexpensive transfer of resources is extremely useful when the source is an rvalue, where the potentially dangerous side-effect of changing the source after the move is redundant since the source is a temporary object that won't be accessible later.

Moves also make it possible to transfer objects such as std::unique_ptrs, smart pointers that are designed to hold a pointer to a unique object, from one scope to another.

See the sections on: rvalue references, defining move special member functions, std::move, std::forward.

Rvalue references

C++11 introduces a new reference termed the rvalue reference. An rvalue reference to A is created with the syntax A&&. This enables two major features: move semantics; and perfect forwarding, the ability to pass arguments while maintaining information about them as lvalues/rvalues in a generic way.

auto type deduction with lvalues and rvalues:

int x = 0; // `x` is an lvalue of type `int`
int& xl = x; // `xl` is an lvalue of type `int&`
int&& xr = x; // compiler error -- `x` is an lvalue
int&& xr2 = 0; // `xr2` is an lvalue of type `int&&`
auto& al = x; // `al` is an lvalue of type `int&`
auto&& al2 = x; // `al2` is an lvalue of type `int&`
auto&& ar = 0; // `ar` is an lvalue of type `int&&`

See also: std::move, std::forward.

Variadic templates

The ... syntax creates a parameter pack or expands one. A template parameter pack is a template parameter that accepts zero or more template arguments (non-types, types, or templates). A template with at least one parameter pack is called a variadic template.

template <typename... T>
struct arity {
  constexpr static int value = sizeof...(T);
};
static_assert(arity<>::value == 0);
static_assert(arity<char, short, int>::value == 3);

Initializer lists

A lightweight array-like container of elements created using a "braced list" syntax. For example, { 1, 2, 3 } creates a sequences of integers, that has type std::initializer_list<int>. Useful as a replacement to passing a vector of objects to a function.

int sum(const std::initializer_list<int>& list) {
  int total = 0;
  for (auto& e : list) {
    total += e;
  }

  return total;
}

auto list = { 1, 2, 3 };
sum(list); // == 6
sum({ 1, 2, 3 }); // == 6
sum({}); // == 0

Static assertions

Assertions that are evaluated at compile-time.

constexpr int x = 0;
constexpr int y = 1;
static_assert(x == y, "x != y");

auto

auto-typed variables are deduced by the compiler according to the type of their initializer.

auto a = 3.14; // double
auto b = 1; // int
auto& c = b; // int&
auto d = { 0 }; // std::initializer_list<int>
auto&& e = 1; // int&&
auto&& f = b; // int&
auto g = new auto(123); // int*
const auto h = 1; // const int
auto i = 1, j = 2, k = 3; // int, int, int
auto l = 1, m = true, n = 1.61; // error -- `l` deduced to be int, `m` is bool
auto o; // error -- `o` requires initializer

Extremely useful for readability, especially for complicated types:

std::vector<int> v = ...;
std::vector<int>::const_iterator cit = v.cbegin();
// vs.
auto cit = v.cbegin();

Functions can also deduce the return type using auto. In C++11, a return type must be specified either explicitly, or using decltype like so:

template <typename X, typename Y>
auto add(X x, Y y) -> decltype(x + y) {
  return x + y;
}
add(1, 2); // == 3
add(1, 2.0); // == 3.0
add(1.5, 1.5); // == 3.0

The trailing return type in the above example is the declared type (see section on decltype) of the expression x + y. For example, if x is an integer and y is a double, decltype(x + y) is a double. Therefore, the above function will deduce the type depending on what type the expression x + y yields. Notice that the trailing return type has access to its parameters, and this when appropriate.

Lambda expressions

A lambda is an unnamed function object capable of capturing variables in scope. It features: a capture list; an optional set of parameters with an optional trailing return type; and a body. Examples of capture lists:

  • [] - captures nothing.
  • [=] - capture local objects (local variables, parameters) in scope by value.
  • [&] - capture local objects (local variables, parameters) in scope by reference.
  • [this] - capture this pointer by value.
  • [a, &b] - capture objects a by value, b by reference.
int x = 1;

auto getX = [=]{ return x; };
getX(); // == 1

auto addX = [=](int y) { return x + y; };
addX(1); // == 2

auto getXRef = [&]() -> int& { return x; };
getXRef(); // int& to `x`

By default, value-captures cannot be modified inside the lambda because the compiler-generated method is marked as const. The mutable keyword allows modifying captured variables. The keyword is placed after the parameter-list (which must be present even if it is empty).

int x = 1;

auto f1 = [&x] { x = 2; }; // OK: x is a reference and modifies the original

auto f2 = [x] { x = 2; }; // ERROR: the lambda can only perform const-operations on the captured value
// vs.
auto f3 = [x] () mutable { x = 2; }; // OK: the lambda can perform any operations on the captured value

decltype

decltype is an operator which returns the declared type of an expression passed to it. Examples of decltype:

int a = 1; // `a` is declared as type `int`
decltype(a) b = a; // `decltype(a)` is `int`
const int& c = a; // `c` is declared as type `const int&`
decltype(c) d = a; // `decltype(c)` is `const int&`
decltype(123) e = 123; // `decltype(123)` is `int`
int&& f = 1; // `f` is declared as type `int&&`
decltype(f) g = 1; // `decltype(f) is `int&&`
decltype((a)) h = g; // `decltype((a))` is int&
template <typename X, typename Y>
auto add(X x, Y y) -> decltype(x + y) {
  return x + y;
}
add(1, 2.0); // `decltype(x + y)` => `decltype(3.0)` => `double`

Template aliases

Semantically similar to using a typedef however, template aliases with using are easier to read and are compatible with templates.

template <typename T>
using Vec = std::vector<T>;
Vec<int> v{}; // std::vector<int>

using String = std::string;
String s{"foo"};

nullptr

C++11 introduces a new null pointer type designed to replace C's NULL macro. nullptr itself is of type std::nullptr_t and can be implicitly converted into pointer types, and unlike NULL, not convertible to integral types except bool.

void foo(int);
void foo(char*);
foo(NULL); // error -- ambiguous
foo(nullptr); // calls foo(char*)

Strongly-typed enums

Type-safe enums that solve a variety of problems with C-style enums including: implicit conversions, inability to specify the underlying type, scope pollution.

// Specifying underlying type as `unsigned int`
enum class Color : unsigned int { Red = 0xff0000, Green = 0xff00, Blue = 0xff };
// `Red`/`Green` in `Alert` don't conflict with `Color`
enum class Alert : bool { Red, Green };
Color c = Color::Red;

Attributes

Attributes provide a universal syntax over __attribute__(...), __declspec, etc.

// `noreturn` attribute indicates `f` doesn't return.
[[ noreturn ]] void f() {
  throw "error";
}

constexpr

Constant expressions are expressions evaluated by the compiler at compile-time. Only non-complex computations can be carried out in a constant expression. Use the constexpr specifier to indicate the variable, function, etc. is a constant expression.

constexpr int square(int x) {
  return x * x;
}

int square2(int x) {
  return x * x;
}

int a = square(2);  // mov DWORD PTR [rbp-4], 4

int b = square2(2); // mov edi, 2
                    // call square2(int)
                    // mov DWORD PTR [rbp-8], eax

constexpr values are those that the compiler can evaluate at compile-time:

const int x = 123;
constexpr const int& y = x; // error -- constexpr variable `y` must be initialized by a constant expression

Constant expressions with classes:

struct Complex {
  constexpr Complex(double r, double i) : re(r), im(i) { }
  constexpr double real() { return re; }
  constexpr double imag() { return im; }

private:
  double re;
  double im;
};

constexpr Complex I(0, 1);

Delegating constructors

Constructors can now call other constructors in the same class using an initializer list.

struct Foo {
  int foo;
  Foo(int foo) : foo(foo) {}
  Foo() : Foo(0) {}
};

Foo foo{};
foo.foo; // == 0

User-defined literals

User-defined literals allow you to extend the language and add your own syntax. To create a literal, define a T operator "" X(...) { ... } function that returns a type T, with a name X. Note that the name of this function defines the name of the literal. Any literal names not starting with an underscore are reserved and won't be invoked. There are rules on what parameters a user-defined literal function should accept, according to what type the literal is called on.

Converting Celsius to Fahrenheit:

// `unsigned long long` parameter required for integer literal.
long long operator "" _celsius(unsigned long long tempCelsius) {
  return std::llround(tempCelsius * 1.8 + 32);
}
24_celsius; // == 75

String to integer conversion:

// `const char*` and `std::size_t` required as parameters.
int operator "" _int(const char* str, std::size_t) {
  return std::stoi(str);
}

"123"_int; // == 123, with type `int`

Explicit virtual overrides

Specifies that a virtual function overrides another virtual function. If the virtual function does not override a parent's virtual function, throws a compiler error.

struct A {
  virtual void foo();
  void bar();
};

struct B : A {
  void foo() override; // correct -- B::foo overrides A::foo
  void bar() override; // error -- A::bar is not virtual
  void baz() override; // error -- B::baz does not override A::baz
};

Final specifier

Specifies that a virtual function cannot be overridden in a derived class or that a class cannot be inherited from.

struct A {
  virtual void foo();
};

struct B : A {
  virtual void foo() final;
};

struct C : B {
  virtual void foo(); // error -- declaration of 'foo' overrides a 'final' function
};

Class cannot be inherited from.

struct A final {

};

struct B : A { // error -- base 'A' is marked 'final'

};

Default functions

A more elegant, efficient way to provide a default implementation of a function, such as a constructor.

struct A {
  A() = default;
  A(int x) : x(x) {}
  int x{ 1 };
};
A a{}; // a.x == 1
A a2{ 123 }; // a.x == 123

With inheritance:

struct B {
  B() : x(1);
  int x;
};

struct C : B {
  // Calls B::B
  C() = default;
};

C c{}; // c.x == 1

Deleted functions

A more elegant, efficient way to provide a deleted implementation of a function. Useful for preventing copies on objects.

class A {
  int x;

public:
  A(int x) : x(x) {};
  A(const A&) = delete;
  A& operator=(const A&) = delete;
};

A x{ 123 };
A y = x; // error -- call to deleted copy constructor
y = x; // error -- operator= deleted

Range-based for loops

Syntactic sugar for iterating over a container's elements.

std::array<int, 5> a{ 1, 2, 3, 4, 5 };
for (int& x : a) x *= 2;
// a == { 2, 4, 6, 8, 10 }

Note the difference when using int as opposed to int&:

std::array<int, 5> a{ 1, 2, 3, 4, 5 };
for (int x : a) x *= 2;
// a == { 1, 2, 3, 4, 5 }

Special member functions for move semantics

The copy constructor and copy assignment operator are called when copies are made, and with C++11's introduction of move semantics, there is now a move constructor and move assignment operator for moves.

struct A {
  std::string s;
  A() : s("test") {}
  A(const A& o) : s(o.s) {}
  A(A&& o) : s(std::move(o.s)) {}
  A& operator=(A&& o) {
   s = std::move(o.s);
   return *this;
  }
};

A f(A a) {
  return a;
}

A a1 = f(A{}); // move-constructed from rvalue temporary
A a2 = std::move(a1); // move-constructed using std::move
A a3 = A{};
a2 = std::move(a3); // move-assignment using std::move
a1 = f(A{}); // move-assignment from rvalue temporary

Converting constructors

Converting constructors will convert values of braced list syntax into constructor arguments.

struct A {
  A(int) {}
  A(int, int) {}
  A(int, int, int) {}
};

A a{0, 0}; // calls A::A(int, int)
A b(0, 0); // calls A::A(int, int)
A c = {0, 0}; // calls A::A(int, int)
A d{0, 0, 0}; // calls A::A(int, int, int)

Note that the braced list syntax does not allow narrowing:

struct A {
  A(int) {}
};

A a(1.1); // OK
A b{1.1}; // Error narrowing conversion from double to int

Note that if a constructor accepts a std::initializer_list, it will be called instead:

struct A {
  A(int) {}
  A(int, int) {}
  A(int, int, int) {}
  A(std::initializer_list<int>) {}
};

A a{0, 0}; // calls A::A(std::initializer_list<int>)
A b(0, 0); // calls A::A(int, int)
A c = {0, 0}; // calls A::A(std::initializer_list<int>)
A d{0, 0, 0}; // calls A::A(std::initializer_list<int>)

Explicit conversion functions

Conversion functions can now be made explicit using the explicit specifier.

struct A {
  operator bool() const { return true; }
};

struct B {
  explicit operator bool() const { return true; }
};

A a{};
if (a); // OK calls A::operator bool()
bool ba = a; // OK copy-initialization selects A::operator bool()

B b{};
if (b); // OK calls B::operator bool()
bool bb = b; // error copy-initialization does not consider B::operator bool()

Inline namespaces

All members of an inline namespace are treated as if they were part of its parent namespace, allowing specialization of functions and easing the process of versioning. This is a transitive property, if A contains B, which in turn contains C and both B and C are inline namespaces, C's members can be used as if they were on A.

namespace Program {
  namespace Version1 {
    int getVersion() { return 1; }
    bool isFirstVersion() { return true; }
  }
  inline namespace Version2 {
    int getVersion() { return 2; }
  }
}

int version {Program::getVersion()};              // Uses getVersion() from Version2
int oldVersion {Program::Version1::getVersion()}; // Uses getVersion() from Version1
bool firstVersion {Program::isFirstVersion()};    // Does not compile when Version2 is added

Non-static data member initializers

Allows non-static data members to be initialized where they are declared, potentially cleaning up constructors of default initializations.

// Default initialization prior to C++11
class Human {
    Human() : age(0) {}
  private:
    unsigned age;
};
// Default initialization on C++11
class Human {
  private:
    unsigned age{0};
};

Right angle Brackets

C++11 is now able to infer when a series of right angle brackets is used as an operator or as a closing statement of typedef, without having to add whitespace.

typedef std::map<int, std::map <int, std::map <int, int> > > cpp98LongTypedef;
typedef std::map<int, std::map <int, std::map <int, int>>>   cpp11LongTypedef;

C++11 Library Features

std::move

std::move indicates that the object passed to it may be moved, or in other words, moved from one object to another without a copy. The object passed in should not be used after the move in certain situations.

A definition of std::move (performing a move is nothing more than casting to an rvalue):

template <typename T>
typename remove_reference<T>::type&& move(T&& arg) {
  return static_cast<typename remove_reference<T>::type&&>(arg);
}

Transferring std::unique_ptrs:

std::unique_ptr<int> p1{ new int };
std::unique_ptr<int> p2 = p1; // error -- cannot copy unique pointers
std::unique_ptr<int> p3 = std::move(p1); // move `p1` into `p2`
                                         // now unsafe to dereference object held by `p1`

std::forward

Returns the arguments passed to it as-is, either as an lvalue or rvalue references, and includes cv-qualification. Useful for generic code that need a reference (either lvalue or rvalue) when appropriate, e.g factories. Forwarding gets its power from template argument deduction:

  • T& & becomes T&
  • T& && becomes T&
  • T&& & becomes T&
  • T&& && becomes T&&

A definition of std::forward:

template <typename T>
T&& forward(typename remove_reference<T>::type& arg) {
  return static_cast<T&&>(arg);
}

An example of a function wrapper which just forwards other A objects to a new A object's copy or move constructor:

struct A {
  A() = default;
  A(const A& o) { std::cout << "copied" << std::endl; }
  A(A&& o) { std::cout << "moved" << std::endl; }
};

template <typename T>
A wrapper(T&& arg) {
  return A{ std::forward<T>(arg) };
}

wrapper(A{}); // moved
A a{};
wrapper(a); // copied
wrapper(std::move(a)); // moved

std::to_string

Converts a numeric argument to a std::string.

std::to_string(1.2); // == "1.2"
std::to_string(123); // == "123"

Type traits

Type traits defines a compile-time template-based interface to query or modify the properties of types.

static_assert(std::is_integral<int>::value);
static_assert(std::is_same<int, int>::value);
static_assert(std::is_same<std::conditional<true, int, double>::type, int>::value);

Smart pointers

C++11 introduces new smart(er) pointers: std::unique_ptr, std::shared_ptr, std::weak_ptr. std::auto_ptr now becomes deprecated and then eventually removed in C++17.

std::unique_ptr is a non-copyable, movable smart pointer that properly manages arrays and STL containers. Note: Prefer using the std::make_X helper functions as opposed to using constructors. See the sections for std::make_unique and std::make_shared.

std::unique_ptr<Foo> p1 { new Foo{} };  // `p1` owns `Foo`
if (p1) p1->bar();

{
  std::unique_ptr<Foo> p2 { std::move(p1) };  // Now `p2` owns `Foo`
  f(*p2);

  p1 = std::move(p2);  // Ownership returns to `p1` -- `p2` gets destroyed
}

if (p1) p1->bar();
// `Foo` instance is destroyed when `p1` goes out of scope

A std::shared_ptr is a smart pointer that manages a resource that is shared across multiple owners. A shared pointer holds a control block which has a few components such as the managed object and a reference counter. All control block access is thread-safe, however, manipulating the managed object itself is not thread-safe.

void foo(std::shared_ptr<T> t) {
  // Do something with `t`...
}

void bar(std::shared_ptr<T> t) {
  // Do something with `t`...
}

void baz(std::shared_ptr<T> t) {
  // Do something with `t`...
}

std::shared_ptr<T> p1 { new T{} };
// Perhaps these take place in another threads?
foo(p1);
bar(p1);
baz(p1);

std::chrono

The chrono library contains a set of utility functions and types that deal with durations, clocks, and time points. One use case of this library is benchmarking code:

std::chrono::time_point<std::chrono::steady_clock> start, end;
start = std::chrono::steady_clock::now();
// Some computations...
end = std::chrono::steady_clock::now();

std::chrono::duration<double> elapsed_seconds = end-start;

elapsed_seconds.count(); // t number of seconds, represented as a `double`

Tuples

Tuples are a fixed-size collection of heterogeneous values. Access the elements of a std::tuple by unpacking using std::tie, or using std::get.

// `playerProfile` has type `std::tuple<int, std::string, std::string>`.
auto playerProfile = std::make_tuple(51, "Frans Nielsen", "NYI");
std::get<0>(playerProfile); // 51
std::get<1>(playerProfile); // "Frans Nielsen"
std::get<2>(playerProfile); // "NYI"

std::tie

Creates a tuple of lvalue references. Useful for unpacking std::pair and std::tuple objects. Use std::ignore as a placeholder for ignored values. In C++17, structured bindings should be used instead.

// With tuples...
std::string playerName;
std::tie(std::ignore, playerName, std::ignore) = std::make_tuple(91, "John Tavares", "NYI");

// With pairs...
std::string yes, no;
std::tie(yes, no) = std::make_pair("yes", "no");

std::array

std::array is a container built on top of a C-style array. Supports common container operations such as sorting.

std::array<int, 3> a = {2, 1, 3};
std::sort(a.begin(), a.end()); // a == { 1, 2, 3 }
for (int& x : a) x *= 2; // a == { 2, 4, 6 }

Unordered containers

These containers maintain average constant-time complexity for search, insert, and remove operations. In order to achieve constant-time complexity, sacrifices order for speed by hashing elements into buckets. There are four unordered containers:

  • unordered_set
  • unordered_multiset
  • unordered_map
  • unordered_multimap

std::make_shared

std::make_shared is the recommended way to create instances of std::shared_ptrs due to the following reasons:

  • Avoid having to use the new operator.
  • Prevents code repetition when specifying the underlying type the pointer shall hold.
  • It provides exception-safety. Suppose we were calling a function foo like so:
foo(std::shared_ptr<T>{ new T{} }, function_that_throws(), std::shared_ptr<T>{ new T{} });

The compiler is free to call new T{}, then function_that_throws(), and so on... Since we have allocated data on the heap in the first construction of a T, we have introduced a leak here. With std::make_shared, we are given exception-safety:

foo(std::make_shared<T>(), function_that_throws(), std::make_shared<T>());
  • Prevents having to do two allocations. When calling std::shared_ptr{ new T{} }, we have to allocate memory for T, then in the shared pointer we have to allocate memory for the control block within the pointer.

See the section on smart pointers for more information on std::unique_ptr and std::shared_ptr.

Memory model

C++11 introduces a memory model for C++, which means library support for threading and atomic operations. Some of these operations include (but aren't limited to) atomic loads/stores, compare-and-swap, atomic flags, promises, futures, locks, and condition variables.

Acknowledgements

Author

Anthony Calandra

Content Contributors

See: https://github.com/AnthonyCalandra/modern-cpp-features/graphs/contributors

License

MIT