C++17 is now available on codeforces, community wants new edition of C++ tricks by HosseinYousefi, 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. 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 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 938D - Buy a Ticket. Code with structured bindings (Dijkstra algo) is much more readable and understandable: compare 35474147 and 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): 
doc
extract(iterator): O(1) amortized doc
- 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: 
doc
- 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: doc
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
P.P.S.: I don't think my english is poor, but please PM me about grammar or other mistakes to make this article better!
