C++17 Language Features
Template argument deduction for class templates
Automatic template argument deduction much like how it's done for functions, but now including class constructors.
template <typename T = float>
struct MyContainer {
T val;
MyContainer() : val() {}
MyContainer(T val) : val(val) {}
// ...
};
MyContainer c1{ 1 }; // OK MyContainer<int>
MyContainer c2; // OK MyContainer<float>
Declaring non-type template parameters with auto
Following the deduction rules of auto, while respecting the non-type template parameter list of allowable types[*], template arguments can be deduced from the types of its arguments:
template <auto ... seq>
struct my_integer_sequence {
// Implementation here ...
};
// Explicitly pass type `int` as template argument.
auto seq = std::integer_sequence<int, 0, 1, 2>();
// Type is deduced to be `int`.
auto seq2 = my_integer_sequence<0, 1, 2>();
Folding expressions
A fold expression performs a fold of a template parameter pack over a binary operator.
An expression of the form (... op e) or (e op ...), where op is a fold-operator and e is an unexpanded parameter pack, are called unary folds.
An expression of the form (e1 op ... op e2), where op are fold-operators, is called a binary fold. Either e1 or e2 are unexpanded parameter packs, but not both.
template<typename... Args>
bool logicalAnd(Args... args) {
// Binary folding.
return (true && ... && args);
}
bool b = true;
bool& b2 = b;
logicalAnd(b, b2, true); // == true
template<typename... Args>
auto sum(Args... args) {
// Unary folding.
return (... + args);
}
sum(1.0, 2.0f, 3); // == 6.0
New rules for auto deduction from braced-init-list
Changes to auto deduction when used with the uniform initialization syntax. Previously, auto x{ 3 }; deduces a std::initializer_list
auto x1{ 1, 2, 3 }; // error: not a single element
auto x2 = { 1, 2, 3 }; // decltype(x2) is std::initializer_list<int>
auto x3{ 3 }; // decltype(x3) is int
auto x4{ 3.0 }; // decltype(x4) is double
constexpr lambda
Compile-time lambdas using constexpr.
auto identity = [] (int n) constexpr { return n; };
static_assert(identity(123) == 123);
constexpr auto add = [] (int x, int y) {
auto L = [=] { return x; };
auto R = [=] { return y; };
return [=] { return L() + R(); };
};
static_assert(add(1, 2)() == 3);
constexpr int addOne(int n) {
return [n] { return n + 1; }();
}
static_assert(addOne(1) == 2);
Lambda capture this by value
Capturing this in a lambda's environment was previously reference-only. An example of where this is problematic is asynchronous code using callbacks that require an object to be available, potentially past its lifetime. *this (C++17) will now make a copy of the current object, while this (C++11) continues to capture by reference.
struct MyObj {
int value{ 123 };
auto getValueCopy() {
return [*this] { return value; };
}
auto getValueRef() {
return [this] { return value; };
}
};
MyObj mo;
auto valueCopy = mo.getValueCopy();
auto valueRef = mo.getValueRef();
mo.value = 321;
valueCopy(); // 123
valueRef(); // 321
Inline variables
The inline specifier can be applied to variables as well as to functions. A variable declared inline has the same semantics as a function declared inline.
// Disassembly example using compiler explorer.
struct S { int x; };
inline S x1 = S{321}; // mov esi, dword ptr [x1]
// x1: .long 321
S x2 = S{123}; // mov eax, dword ptr [.L_ZZ4mainE2x2]
// mov dword ptr [rbp - 8], eax
// .L_ZZ4mainE2x2: .long 123
Nested namespaces
Using the namespace resolution operator to create nested namespace definitions.
namespace A {
namespace B {
namespace C {
int i;
}
}
}
// vs.
namespace A::B::C {
int i;
}
Structured bindings
A proposal for de-structuring initialization, that would allow writing auto [ x, y, z ] = expr; where the type of expr was a tuple-like object, whose elements would be bound to the variables x, y, and z (which this construct declares). Tuple-like objects include std::tuple, std::pair, std::array, and aggregate structures.
using Coordinate = std::pair<int, int>;
Coordinate origin() {
return Coordinate{0, 0};
}
const auto [ x, y ] = origin();
x; // == 0
y; // == 0
Selection statements with initializer
New versions of the if and switch statements which simplify common code patterns and help users keep scopes tight.
{
std::lock_guard<std::mutex> lk(mx);
if (v.empty()) v.push_back(val);
}
// vs.
if (std::lock_guard<std::mutex> lk(mx); v.empty()) {
v.push_back(val);
}
Foo gadget(args);
switch (auto s = gadget.status()) {
case OK: gadget.zip(); break;
case Bad: throw BadFoo(s.message());
}
// vs.
switch (Foo gadget(args); auto s = gadget.status()) {
case OK: gadget.zip(); break;
case Bad: throw BadFoo(s.message());
}
constexpr if
// Write code that is instantiated depending on a compile-time condition.
template <typename T>
constexpr bool isIntegral() {
if constexpr (std::is_integral<T>::value) {
return true;
} else {
return false;
}
}
static_assert(isIntegral<int>() == true);
static_assert(isIntegral<char>() == true);
static_assert(isIntegral<double>() == false);
struct S {};
static_assert(isIntegral<S>() == false);
UTF-8 Character Literals
A character literal that begins with u8 is a character literal of type char. The value of a UTF-8 character literal is equal to its ISO 10646 code point value.
char x = u8'x';
Direct List Initialization of Enums
Enums can now be initialized using braced syntax.
enum byte : unsigned char {};
byte b{0}; // OK
byte c{-1}; // ERROR
byte d = byte{1}; // OK
byte e = byte{256}; // ERROR
C++17 Library Features
std::variant
The class template std::variant represents a type-safe union. An instance of std::variant at any given time holds a value of one of its alternative types (it's also possible for it to be valueless).
std::variant<int, double> v{ 12 };
std::get<int>(v); // == 12
std::get<0>(v); // == 12
v = 12.0;
std::get<double>(v); // == 12.0
std::get<1>(v); // == 12.0
std::optional
The class template std::optional manages an optional contained value, i.e. a value that may or may not be present. A common use case for optional is the return value of a function that may fail.
std::optional<std::string> create(bool b) {
if (b) {
return "Godzilla";
} else {
return {};
}
}
create(false).value_or("empty"); // == "empty"
create(true).value(); // == "Godzilla"
// optional-returning factory functions are usable as conditions of while and if
if (auto str = create(true)) {
// ...
}
std::any
A type-safe container for single values of any type.
std::any x{ 5 };
x.has_value() // == true
std::any_cast<int>(x) // == 5
std::any_cast<int&>(x) = 10;
std::any_cast<int>(x) // == 10
std::string_view
A non-owning reference to a string. Useful for providing an abstraction on top of strings (e.g. for parsing).
// Regular strings.
std::string_view cppstr{ "foo" };
// Wide strings.
std::wstring_view wcstr_v{ L"baz" };
// Character arrays.
char array[3] = {'b', 'a', 'r'};
std::string_view array_v(array, sizeof array);
std::string str{ " trim me" };
std::string_view v{ str };
v.remove_prefix(std::min(v.find_first_not_of(" "), v.size()));
str; // == " trim me"
v; // == "trim me"
std::invoke
Invoke a Callable object with parameters. Examples of Callable objects are std::function or std::bind where an object can be called similarly to a regular function.
template <typename Callable>
class Proxy {
Callable c;
public:
Proxy(Callable c): c(c) {}
template <class... Args>
decltype(auto) operator()(Args&&... args) {
// ...
return std::invoke(c, std::forward<Args>(args)...);
}
};
auto add = [] (int x, int y) {
return x + y;
};
Proxy<decltype(add)> p{ add };
p(1, 2); // == 3
std::apply
Invoke a Callable object with a tuple of arguments.
auto add = [] (int x, int y) {
return x + y;
};
std::apply(add, std::make_tuple( 1, 2 )); // == 3
Splicing for maps and sets
Moving nodes and merging containers without the overhead of expensive copies, moves, or heap allocations/deallocations.
// Moving elements from one map to another:
std::map<int, string> src{ { 1, "one" }, { 2, "two" }, { 3, "buckle my shoe" } };
std::map<int, string> dst{ { 3, "three" } };
dst.insert(src.extract(src.find(1))); // Cheap remove and insert of { 1, "one" } from `src` to `dst`.
dst.insert(src.extract(2)); // Cheap remove and insert of { 2, "two" } from `src` to `dst`.
// dst == { { 1, "one" }, { 2, "two" }, { 3, "three" } };
// Inserting an entire set:
std::set<int> src{1, 3, 5};
std::set<int> dst{2, 4, 5};
dst.merge(src);
// src == { 5 }
// dst == { 1, 2, 3, 4, 5 }
// Inserting elements which outlive the container:
auto elementFactory() {
std::set<...> s;
s.emplace(...);
return s.extract(s.begin());
}
s2.insert(elementFactory());
// Changing the key of a map element:
std::map<int, string> m{ { 1, "one" }, { 2, "two" }, { 3, "three" } };
auto e = m.extract(2);
e.key() = 4;
m.insert(std::move(e));
// m == { { 1, "one" }, { 3, "three" }, { 4, "two" } }