Cpp Notes

cpp_17

C++17 features

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 
struct MyContainer {
  T val;
  MyContainer() : val{} {}
  MyContainer(T val) : val{val} {}
  // ...
};
MyContainer c1 {1}; // OK MyContainer
MyContainer c2; // OK MyContainer

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 
struct my_integer_sequence {
  // Implementation here ...
};

// Explicitly pass type `int` as template argument.
auto seq = std::integer_sequence();
// Type is deduced to be `int`.
auto seq2 = my_integer_sequence<0, 1, 2>();

* For example, you cannot use a double as a template parameter type, which also makes this an invalid deduction using auto.

Fold1 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 is an unexpanded parameter pack, but not both.
template 
bool logicalAnd(Args... args) {
    // Binary folding.
    return (true && ... && args);
}
bool b = true;
bool& b2 = b;
logicalAnd(b, b2, true); // == true
template 
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<int>, which now deduces to int.

auto x1 {1, 2, 3}; // error: not a single element
auto x2 = {1, 2, 3}; // x2 is std::initializer_list
auto x3 {3}; // x3 is int
auto x4 {3.0}; // 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

It can also be used to declare and define a static member variable, such that it does not need to be initialized in the source file.

struct S {
  S() : id{count++} {}
  ~S() { count--; }
  int id;
  static inline int count{0}; // declare and initialize count to 0 within the class
};

Nested namespaces 

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;
Coordinate origin() {
  return Coordinate{0, 0};
}

const auto [ x, y ] = origin();
x; // == 0
y; // == 0
std::unordered_map mapping {
  {"a", 1},
  {"b", 2},
  {"c", 3}
};

// Destructure by reference.
for (const auto& [key, value] : mapping) {
  // Do something with key and value
}

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 lk(mx);
  if (v.empty()) v.push_back(val);
}
// vs.
if (std::lock_guard 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 
constexpr bool isIntegral() {
  if constexpr (std::is_integral::value) {
    return true;
  } else {
    return false;
  }
}
static_assert(isIntegral() == true);
static_assert(isIntegral() == true);
static_assert(isIntegral() == false);
struct S {};
static_assert(isIntegral() == 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

[[fallthrough]], [[nodiscard]], [[maybe_unused]] attributes

C++17 introduces three new attributes: [[fallthrough]], [[nodiscard]] and [[maybe_unused]].

  • [[fallthrough]] indicates to the compiler that falling through in a switch statement is intended behavior. This attribute may only be used in a switch statement, and must be placed before the next case/default label.
switch (n) {
  case 1:
    // ...
    [[fallthrough]];
  case 2:
    // ...
    break;
  case 3:
    // ...
    [[fallthrough]];
  default:
    // ...
}
  • [[nodiscard]] issues a warning when either a function or class has this attribute and its return value is discarded.
[[nodiscard]] bool do_something() {
  return is_success; // true for success, false for failure
}

do_something(); // warning: ignoring return value of 'bool do_something()',
                // declared with attribute 'nodiscard'
// Only issues a warning when `error_info` is returned by value.
struct [[nodiscard]] error_info {
  // ...
};

error_info do_something() {
  error_info ei;
  // ...
  return ei;
}

do_something(); // warning: ignoring returned value of type 'error_info',
                // declared with attribute 'nodiscard'
  • [[maybe_unused]] indicates to the compiler that a variable or parameter might be unused and is intended.
void my_callback(std::string msg, [[maybe_unused]] bool error) {
  // Don't care if `msg` is an error message, just log it.
  log(msg);
}

__has_include

__has_include (operand) operator may be used in #if and #elif expressions to check whether a header or source file (operand) is available for inclusion or not.

One use case of this would be using two libraries that work the same way, using the backup/experimental one if the preferred one is not found on the system.

#ifdef __has_include
#  if __has_include()
#    include 
#    define have_optional 1
#  elif __has_include()
#    include 
#    define have_optional 1
#    define experimental_optional
#  else
#    define have_optional 0
#  endif
#endif

It can also be used to include headers existing under different names or locations on various platforms, without knowing which platform the program is running on, OpenGL headers are a good example for this which are located in OpenGL\ directory on macOS and GL\ on other platforms.

#ifdef __has_include
#  if __has_include()
#    include 
#    include 
#  elif __has_include()
#    include 
#    include 
#  else
#    error No suitable OpenGL headers found.
# endif
#endif

Class template argument deduction

Class template argument deduction (CTAD) allows the compiler to deduce template arguments from constructor arguments.

std::vector v{ 1, 2, 3 }; // deduces std::vector

std::mutex mtx;
auto lck = std::lock_guard{ mtx }; // deduces to std::lock_guard

auto p = new std::pair{ 1.0, 2.0 }; // deduces to std::pair

For user-defined types, deduction guides can be used to guide the compiler how to deduce template arguments if applicable:

template 
struct container {
  container(T t) {}

  template 
  container(Iter beg, Iter end);
};

// deduction guide
template 
container(Iter b, Iter e) -> container::value_type>;

container a{ 7 }; // OK: deduces container

std::vector v{ 1.0, 2.0, 3.0 };
auto b = container{ v.begin(), v.end() }; // OK: deduces container

container c{ 5, 6 }; // ERROR: std::iterator_traits::value_type is not a type
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