C++17 is now [available](http://mirror.codeforces.com/blog/entry/57646) on codeforces, community [wants](http://mirror.codeforces.com/blog/entry/15643?#comment-413401) new edition of [C++ tricks](http://mirror.codeforces.com/blog/entry/15643) by [user:Swift,2018-02-139], so, let's start! ↵
Disclaimer: I have done only few examples of new features, which in my opinion are related to competitive programming. Feel free to comment and provide more real-world examples or ask to elaborate some features with more examples or explanations.↵
↵
### Fold expressions↵
↵
* I think that everybody knows, what reduce or fold means, but a c++11 example:↵
↵
```↵
vector<int> v = {1, 3, 5, 7};↵
int res = accumulate(v.begin(), v.end(), 0, [](int a, int b) { return a + b; });↵
cout << res; // 16↵
```↵
↵
* In C++17 there is also folding support for a template parameters list. It has the following syntax:↵
↵
```↵
(pack op ...)↵
(... op pack)↵
(pack op ... op init)↵
(init op ... op pack)↵
```↵
↵
* For example, implement a template function that takes a variable number of parameters and calculates their sum. ↵
↵
[cut]↵
↵
Before C++17 we cannot do this without explicit first argument:↵
↵
```↵
//C++14↵
auto Sum()↵
{↵
return 0;↵
}↵
↵
template<typename Arg, typename... Args>↵
auto Sum(Arg first, Args... rest)↵
{↵
return first + Sum(rest...);↵
}↵
↵
cout << Sum(1, 2, 3, 4, 5); // 15↵
```↵
↵
```↵
//C++17↵
template<typename... Args>↵
auto Func(Args... args)↵
{↵
return (args + ...);↵
}↵
↵
cout << Func(1, 2, 3, 4, 5); // 15↵
```↵
↵
* This is useful, when we use comma as `op`:↵
↵
```↵
// C++17↵
template<typename T, typename... Args>↵
void pushToVector(vector<T>& v, Args&&... args)↵
{↵
(v.push_back(forward<Args>(args)), ...);↵
//This code is expanded into a sequence of expressions separated by commas as follows:↵
// v.push_back(forward<Args_1>(arg1)),↵
// v.push_back(forward<Args_2>(arg2)),↵
// ....↵
}↵
↵
vector<int> v;↵
pushToVector(v, 1, 4, 5, 8);↵
```↵
↵
* And my favourite example:↵
↵
```↵
//C++17↵
template<typename... Args>↵
void readln(Args&... args)↵
{↵
((cin >> args), ...);↵
}↵
↵
template<typename... Args>↵
void writeln(Args... args)↵
{↵
((cout << args << " "), ...);↵
}↵
↵
int x;↵
double y;↵
readln(x, y); // enter 100 500.1234↵
writeln(x, "some string", y); // 100 some string 500.1234↵
```↵
↵
* **Note**: brackets are meaningfull↵
↵
### Class template argument deduction↵
↵
```↵
template<typename T>↵
struct point↵
{↵
T x;↵
T y;↵
point(T x, T y) : x(x), y(y) {}↵
};↵
↵
//C++11↵
pair<int, double> p1 = {14, 17.0}↵
point<int> u = {1, 2};↵
↵
//C++17↵
pair p2 = {14, 17.0}↵
point v = {1, 2};↵
```↵
↵
If struct is complex, there is a possibility to write deduction guides ourselves, for instance:↵
↵
```↵
template<typename T, typename U>↵
struct S↵
{↵
T first;↵
U second;↵
};↵
↵
// My deduction guide↵
template<typename T, typename U>↵
S(const T &first, const U &second) -> S<T, U>;↵
↵
```↵
**Note**: the compiler is able to create deduction guide automatically from a constructor, but in this example, the structure S has no constructor, so, we define deduction guide manually.↵
↵
### `*this` capture in lambda expressions↵
↵
I don't think this is useful in CP, but who knows:↵
↵
```↵
struct someClass↵
{↵
int x = 0;↵
↵
void f() const↵
{↵
cout << x << '\n';↵
}↵
↵
void g()↵
{↵
x++;↵
}↵
↵
// C++14↵
void func()↵
{↵
auto lambda1 = [self = *this]() { self.f(); };↵
auto lambda2 = [self = *this]() mutable { self.g(); };↵
lambda1();↵
lambda2();↵
}↵
↵
// C++17↵
void funcNew()↵
{↵
auto lambda1 = [*this]() { f(); };↵
auto lambda2 = [*this]() mutable { g(); };↵
lambda1();↵
lambda2();↵
}↵
};↵
↵
```↵
[Article](https://arne-mertz.de/2017/10/mutable/) about `mutable` keyword.↵
↵
### Structured bindings↵
↵
* The most useful syntax sugar for decomposition of objects.↵
↵
```↵
template<typename T>↵
struct point↵
{↵
T x;↵
T y;↵
point(T x, T y) : x(x), y(y) {}↵
};↵
↵
vector<point<int>> points = {{0, 0}, {1, 0}, {1, 1}, {1, 0}};↵
//C++11↵
for (auto& point : points)↵
{↵
int x, y;↵
tie(x, y) = point;↵
//...Some compex logic with x and y↵
}↵
↵
//C++17↵
for (auto& [x, y] : points)↵
{↵
//...Some compex logic with x and y↵
}↵
```↵
↵
* Iterating over map:↵
↵
```↵
map<int, string> m;↵
for (auto [key, value] : m)↵
cout << "key: " << key << '\n' << "value: " << value << '\n';↵
```↵
↵
* A good example of usage is problem [problem:938D]. Code with structured bindings (Dijkstra algo) is much more readable and understandable: compare [submission:35474147] and [submission:35346635].↵
↵
```↵
while (!q.empty())↵
{↵
auto [dist, u] = *q.begin();↵
q.erase(q.begin());↵
used[u] = true;↵
for (auto& [w, v] : g[u])↵
if (!used[v] && d[v] > dist + 2 * w)↵
q.erase({d[v], v}),↵
d[v] = dist + 2 * w,↵
q.insert({d[v], v});↵
}↵
```↵
↵
### Initializer in `if` and `switch`↵
↵
```↵
set<int> s;↵
↵
if (auto [iter, ok] = s.insert(42); ok)↵
{↵
//...↵
}↵
else↵
{↵
//`ok` and `iter` are available here↵
}↵
//But not here↵
```↵
↵
### New attributes↵
↵
* `[[fallthrough]]` attribute indicates that the break operator inside a case block is missing intentionally:↵
↵
```↵
int requests, type;↵
cin >> requests;↵
for (int q = 0; q < requests; ++q)↵
switch (cin >> type; type) //Initializer in switch↵
{↵
case 1:↵
int l, r;↵
cin >> l >> r;↵
//proceed request of first type↵
break;↵
case 2:↵
[[fallthrough]];↵
//Compiler warning will be supressed↵
case 3:↵
int value;↵
cin >> value;↵
//Proceed requests of second and third types.↵
}↵
```↵
↵
* `[[nodiscard]]` attribute is used to indicate that the return value of the function should not be ignored and can be also applied to data types.↵
↵
### std::optional↵
↵
```↵
optional<int> findPath(graph g, int from, int to)↵
{↵
//Find path from `from` to `to`↵
if (d[to] != INF)↵
return d[to];↵
return {}↵
}↵
↵
//We can check if value exists↵
if (auto dist = findPath(...); dist.hasValue())↵
cout << dist.value(); //And get it↵
else↵
cout << -1;↵
↵
//Or use defaultValue if value is not set↵
cout << findPath(...).value_or(-1); //Prints distance if path exists and -1 otherwise↵
```↵
↵
### Non-constant string::data↵
↵
For C-lovers:↵
↵
```↵
string str = "hello";↵
char *p = str.data();↵
p[0] = 'H';↵
cout << str; // Hello↵
```↵
↵
### Free functions std::size, std::data and std::empty↵
↵
In addition to the already existing free functions std::begin, std::end and others, some new free functions appeared, such as: std::size, std::data and std::empty:↵
↵
```↵
vector<int> v = { 3, 2, 5, 1, 7, 6 };↵
↵
size_t sz = size(v);↵
bool empty = empty(v);↵
auto ptr = data(v);↵
```↵
↵
### std::clamp↵
↵
Returns `x` if it is in the interval `[low, high]` or, otherwise, the nearest value:↵
↵
```↵
cout << clamp(7, 0, 10); //7↵
cout << clamp(7, 0, 5); //5↵
cout << clamp(7, 10, 50); //10↵
```↵
↵
I think that it is convenient function, but it'll be difficult to call it in mind during contest :)↵
↵
### GCD and LCM!↵
↵
```↵
cout << gcd(24, 60); // 12↵
cout << lcm(8, 10); // 40↵
```↵
↵
### The return value from `emplace_back`↵
↵
```↵
vector<int> v = { 1, 2, 3 };↵
↵
auto &r = v.emplace_back(10);↵
r = 42;↵
//v now contains {1, 2, 3, 42}↵
```↵
↵
### std::map functions:↵
↵
* Extract (and even change key!!!)↵
↵
```↵
map<int, string> myMap{ { 1, "Gennady" }, { 2, "Petr" }, { 3, "Makoto" } };↵
auto node = myMap.extract(2);↵
node.key() = 42;↵
myMap.insert(move(node));↵
↵
// myMap: {{1, "Gennady"}, {42, "Petr"}, {3, "Makoto"}};↵
```↵
↵
**Note**: Extract is the only way to change a key of a map element without reallocation↵
↵
Complexity: ↵
extract(key): $O(\log(N))$ [doc](http://en.cppreference.com/w/cpp/container/map/extract) ↵
extract(iterator): $O(1)$ amortized [doc](http://en.cppreference.com/w/cpp/container/map/extract)↵
↵
* Merge↵
↵
```↵
map<int, string> m1{ { 1, "aa" }, { 2, "bb" }, { 3, "cc" } }; ↵
map<int, string> m2{ { 4, "dd" }, { 5, "ee" }, { 6, "ff" } };↵
m1.merge(m2);↵
// m1: { {1, "aa"}, {2, "bb"}, {3, "cc"}, {4, "dd"}, {5, "ee"}, {6, "ff"} }↵
// m2: {}↵
```↵
↵
Compexity: $O(N \log(N + M))$ [doc](http://en.cppreference.com/w/cpp/container/map/merge)↵
↵
* To figure out if the insert or update occurred, we had to first look for the element, and then apply the operator[]. Now we had insert_or_assign:↵
↵
```↵
map<int, string> m;↵
m.emplace(1, "aaa");↵
m.emplace(2, "bbb");↵
m.emplace(3, "ccc");↵
↵
auto [it1, inserted1] = m.insert_or_assign(3, "ddd");↵
cout << inserted1; // 0↵
↵
auto [it2, inserted2] = m.insert_or_assign(4, "eee");↵
cout << inserted2; // 1↵
```↵
↵
Complexity: $O(\log(N))$ [doc](http://en.cppreference.com/w/cpp/container/map/emplace)↵
↵
### More rigorous evaluation order of expressions↵
↵
And in general c++17 introduces new rules, defining more strictly the evaluation order of expressions:↵
↵
* Postfix expressions are evaluated from left to right (including function calls and access to objects members)↵
* Assignment expressions are evaluated from right to left.↵
* Operands of operators << and >> are evaluated from left to right.↵
↵
Thus, as it is mentioned in the proposal for the standard, in the following expressions a is now guaranteed to be evaluated first, then b, then c:↵
↵
```↵
a.b↵
a->b↵
a->*b↵
a(b1, b2, b3)↵
b @= a↵
a[b]↵
a << b << c↵
a >> b >> c↵
```↵
↵
**Note**: the evaluation order between b1, b2, b3 is still not defined.↵
↵
P.S.: All materials are adopted with my examples from [here](https://www.viva64.com/en/b/0533) ↵
P.P.S.: I don't think my english is [poor](http://mirror.codeforces.com/blog/entry/57479?#comment-411601), but please PM me about grammar or other mistakes to make this article better!
Disclaimer: I have done only few examples of new features, which in my opinion are related to competitive programming. Feel free to comment and provide more real-world examples or ask to elaborate some features with more examples or explanations.↵
↵
### Fold expressions↵
↵
* I think that everybody knows, what reduce or fold means, but a c++11 example:↵
↵
```↵
vector<int> v = {1, 3, 5, 7};↵
int res = accumulate(v.begin(), v.end(), 0, [](int a, int b) { return a + b; });↵
cout << res; // 16↵
```↵
↵
* In C++17 there is also folding support for a template parameters list. It has the following syntax:↵
↵
```↵
(pack op ...)↵
(... op pack)↵
(pack op ... op init)↵
(init op ... op pack)↵
```↵
↵
* For example, implement a template function that takes a variable number of parameters and calculates their sum. ↵
↵
[cut]↵
↵
Before C++17 we cannot do this without explicit first argument:↵
↵
```↵
//C++14↵
auto Sum()↵
{↵
return 0;↵
}↵
↵
template<typename Arg, typename... Args>↵
auto Sum(Arg first, Args... rest)↵
{↵
return first + Sum(rest...);↵
}↵
↵
cout << Sum(1, 2, 3, 4, 5); // 15↵
```↵
↵
```↵
//C++17↵
template<typename... Args>↵
auto Func(Args... args)↵
{↵
return (args + ...);↵
}↵
↵
cout << Func(1, 2, 3, 4, 5); // 15↵
```↵
↵
* This is useful, when we use comma as `op`:↵
↵
```↵
// C++17↵
template<typename T, typename... Args>↵
void pushToVector(vector<T>& v, Args&&... args)↵
{↵
(v.push_back(forward<Args>(args)), ...);↵
//This code is expanded into a sequence of expressions separated by commas as follows:↵
// v.push_back(forward<Args_1>(arg1)),↵
// v.push_back(forward<Args_2>(arg2)),↵
// ....↵
}↵
↵
vector<int> v;↵
pushToVector(v, 1, 4, 5, 8);↵
```↵
↵
* And my favourite example:↵
↵
```↵
//C++17↵
template<typename... Args>↵
void readln(Args&... args)↵
{↵
((cin >> args), ...);↵
}↵
↵
template<typename... Args>↵
void writeln(Args... args)↵
{↵
((cout << args << " "), ...);↵
}↵
↵
int x;↵
double y;↵
readln(x, y); // enter 100 500.1234↵
writeln(x, "some string", y); // 100 some string 500.1234↵
```↵
↵
* **Note**: brackets are meaningfull↵
↵
### Class template argument deduction↵
↵
```↵
template<typename T>↵
struct point↵
{↵
T x;↵
T y;↵
point(T x, T y) : x(x), y(y) {}↵
};↵
↵
//C++11↵
pair<int, double> p1 = {14, 17.0}↵
point<int> u = {1, 2};↵
↵
//C++17↵
pair p2 = {14, 17.0}↵
point v = {1, 2};↵
```↵
↵
If struct is complex, there is a possibility to write deduction guides ourselves, for instance:↵
↵
```↵
template<typename T, typename U>↵
struct S↵
{↵
T first;↵
U second;↵
};↵
↵
// My deduction guide↵
template<typename T, typename U>↵
S(const T &first, const U &second) -> S<T, U>;↵
↵
```↵
**Note**: the compiler is able to create deduction guide automatically from a constructor, but in this example, the structure S has no constructor, so, we define deduction guide manually.↵
↵
### `*this` capture in lambda expressions↵
↵
I don't think this is useful in CP, but who knows:↵
↵
```↵
struct someClass↵
{↵
int x = 0;↵
↵
void f() const↵
{↵
cout << x << '\n';↵
}↵
↵
void g()↵
{↵
x++;↵
}↵
↵
// C++14↵
void func()↵
{↵
auto lambda1 = [self = *this]() { self.f(); };↵
auto lambda2 = [self = *this]() mutable { self.g(); };↵
lambda1();↵
lambda2();↵
}↵
↵
// C++17↵
void funcNew()↵
{↵
auto lambda1 = [*this]() { f(); };↵
auto lambda2 = [*this]() mutable { g(); };↵
lambda1();↵
lambda2();↵
}↵
};↵
↵
```↵
[Article](https://arne-mertz.de/2017/10/mutable/) about `mutable` keyword.↵
↵
### Structured bindings↵
↵
* The most useful syntax sugar for decomposition of objects.↵
↵
```↵
template<typename T>↵
struct point↵
{↵
T x;↵
T y;↵
point(T x, T y) : x(x), y(y) {}↵
};↵
↵
vector<point<int>> points = {{0, 0}, {1, 0}, {1, 1}, {1, 0}};↵
//C++11↵
for (auto& point : points)↵
{↵
int x, y;↵
tie(x, y) = point;↵
//...Some compex logic with x and y↵
}↵
↵
//C++17↵
for (auto& [x, y] : points)↵
{↵
//...Some compex logic with x and y↵
}↵
```↵
↵
* Iterating over map:↵
↵
```↵
map<int, string> m;↵
for (auto [key, value] : m)↵
cout << "key: " << key << '\n' << "value: " << value << '\n';↵
```↵
↵
* A good example of usage is problem [problem:938D]. Code with structured bindings (Dijkstra algo) is much more readable and understandable: compare [submission:35474147] and [submission:35346635].↵
↵
```↵
while (!q.empty())↵
{↵
auto [dist, u] = *q.begin();↵
q.erase(q.begin());↵
used[u] = true;↵
for (auto& [w, v] : g[u])↵
if (!used[v] && d[v] > dist + 2 * w)↵
q.erase({d[v], v}),↵
d[v] = dist + 2 * w,↵
q.insert({d[v], v});↵
}↵
```↵
↵
### Initializer in `if` and `switch`↵
↵
```↵
set<int> s;↵
↵
if (auto [iter, ok] = s.insert(42); ok)↵
{↵
//...↵
}↵
else↵
{↵
//`ok` and `iter` are available here↵
}↵
//But not here↵
```↵
↵
### New attributes↵
↵
* `[[fallthrough]]` attribute indicates that the break operator inside a case block is missing intentionally:↵
↵
```↵
int requests, type;↵
cin >> requests;↵
for (int q = 0; q < requests; ++q)↵
switch (cin >> type; type) //Initializer in switch↵
{↵
case 1:↵
int l, r;↵
cin >> l >> r;↵
//proceed request of first type↵
break;↵
case 2:↵
[[fallthrough]];↵
//Compiler warning will be supressed↵
case 3:↵
int value;↵
cin >> value;↵
//Proceed requests of second and third types.↵
}↵
```↵
↵
* `[[nodiscard]]` attribute is used to indicate that the return value of the function should not be ignored and can be also applied to data types.↵
↵
### std::optional↵
↵
```↵
optional<int> findPath(graph g, int from, int to)↵
{↵
//Find path from `from` to `to`↵
if (d[to] != INF)↵
return d[to];↵
return {}↵
}↵
↵
//We can check if value exists↵
if (auto dist = findPath(...); dist.hasValue())↵
cout << dist.value(); //And get it↵
else↵
cout << -1;↵
↵
//Or use defaultValue if value is not set↵
cout << findPath(...).value_or(-1); //Prints distance if path exists and -1 otherwise↵
```↵
↵
### Non-constant string::data↵
↵
For C-lovers:↵
↵
```↵
string str = "hello";↵
char *p = str.data();↵
p[0] = 'H';↵
cout << str; // Hello↵
```↵
↵
### Free functions std::size, std::data and std::empty↵
↵
In addition to the already existing free functions std::begin, std::end and others, some new free functions appeared, such as: std::size, std::data and std::empty:↵
↵
```↵
vector<int> v = { 3, 2, 5, 1, 7, 6 };↵
↵
size_t sz = size(v);↵
bool empty = empty(v);↵
auto ptr = data(v);↵
```↵
↵
### std::clamp↵
↵
Returns `x` if it is in the interval `[low, high]` or, otherwise, the nearest value:↵
↵
```↵
cout << clamp(7, 0, 10); //7↵
cout << clamp(7, 0, 5); //5↵
cout << clamp(7, 10, 50); //10↵
```↵
↵
I think that it is convenient function, but it'll be difficult to call it in mind during contest :)↵
↵
### GCD and LCM!↵
↵
```↵
cout << gcd(24, 60); // 12↵
cout << lcm(8, 10); // 40↵
```↵
↵
### The return value from `emplace_back`↵
↵
```↵
vector<int> v = { 1, 2, 3 };↵
↵
auto &r = v.emplace_back(10);↵
r = 42;↵
//v now contains {1, 2, 3, 42}↵
```↵
↵
### std::map functions:↵
↵
* Extract (and even change key!!!)↵
↵
```↵
map<int, string> myMap{ { 1, "Gennady" }, { 2, "Petr" }, { 3, "Makoto" } };↵
auto node = myMap.extract(2);↵
node.key() = 42;↵
myMap.insert(move(node));↵
↵
// myMap: {{1, "Gennady"}, {42, "Petr"}, {3, "Makoto"}};↵
```↵
↵
**Note**: Extract is the only way to change a key of a map element without reallocation↵
↵
Complexity: ↵
extract(key): $O(\log(N))$ [doc](http://en.cppreference.com/w/cpp/container/map/extract) ↵
extract(iterator): $O(1)$ amortized [doc](http://en.cppreference.com/w/cpp/container/map/extract)↵
↵
* Merge↵
↵
```↵
map<int, string> m1{ { 1, "aa" }, { 2, "bb" }, { 3, "cc" } }; ↵
map<int, string> m2{ { 4, "dd" }, { 5, "ee" }, { 6, "ff" } };↵
m1.merge(m2);↵
// m1: { {1, "aa"}, {2, "bb"}, {3, "cc"}, {4, "dd"}, {5, "ee"}, {6, "ff"} }↵
// m2: {}↵
```↵
↵
Compexity: $O(N \log(N + M))$ [doc](http://en.cppreference.com/w/cpp/container/map/merge)↵
↵
* To figure out if the insert or update occurred, we had to first look for the element, and then apply the operator[]. Now we had insert_or_assign:↵
↵
```↵
map<int, string> m;↵
m.emplace(1, "aaa");↵
m.emplace(2, "bbb");↵
m.emplace(3, "ccc");↵
↵
auto [it1, inserted1] = m.insert_or_assign(3, "ddd");↵
cout << inserted1; // 0↵
↵
auto [it2, inserted2] = m.insert_or_assign(4, "eee");↵
cout << inserted2; // 1↵
```↵
↵
Complexity: $O(\log(N))$ [doc](http://en.cppreference.com/w/cpp/container/map/emplace)↵
↵
### More rigorous evaluation order of expressions↵
↵
And in general c++17 introduces new rules, defining more strictly the evaluation order of expressions:↵
↵
* Postfix expressions are evaluated from left to right (including function calls and access to objects members)↵
* Assignment expressions are evaluated from right to left.↵
* Operands of operators << and >> are evaluated from left to right.↵
↵
Thus, as it is mentioned in the proposal for the standard, in the following expressions a is now guaranteed to be evaluated first, then b, then c:↵
↵
```↵
a.b↵
a->b↵
a->*b↵
a(b1, b2, b3)↵
b @= a↵
a[b]↵
a << b << c↵
a >> b >> c↵
```↵
↵
**Note**: the evaluation order between b1, b2, b3 is still not defined.↵
↵
P.S.: All materials are adopted with my examples from [here](https://www.viva64.com/en/b/0533) ↵
P.P.S.: I don't think my english is [poor](http://mirror.codeforces.com/blog/entry/57479?#comment-411601), but please PM me about grammar or other mistakes to make this article better!
