Вывод шаблонов аргументов класса (CTAD) (с C++17)

std::pair p(2, 4.5);     // deduces to std::pair<int, double> p(2, 4.5);
std::tuple t(4, 3, 2.5); // same as auto t = std::make_tuple(4, 3, 2.5);
std::less l;             // same as std::less<void> l;

template<class T>
struct A
{
    A(T, T);
};
 
auto y = new A{1, 2}; // allocated type is A<int>

auto lck = std::lock_guard(mtx);     // deduces to std::lock_guard<std::mutex>
std::copy_n(vi1, 3,
    std::back_insert_iterator(vi2)); // deduces to std::back_insert_iterator<T>,
                                     // where T is the type of the container vi2
std::for_each(vi.begin(), vi.end(),
    Foo([&](int i) {...}));          // deduces to Foo<T>,
                                     // where T is the unique lambda type

template<class T>
struct UniquePtr
{
    UniquePtr(T* t);
};
 
UniquePtr dp{new auto(2.0)};
 
// One declared constructor:
// C1: UniquePtr(T*);
 
// Set of implicitly-generated deduction guides:
 
// F1: template<class T>
//     UniquePtr<T> F(T* p);
 
// F2: template<class T> 
//     UniquePtr<T> F(UniquePtr<T>); // copy deduction candidate
 
// imaginary class to initialize:
// struct X
// {
//     template<class T>
//     X(T* p);         // from F1
//     
//     template<class T>
//     X(UniquePtr<T>); // from F2
// };
 
// direct-initialization of an X object
// with "new double(2.0)" as the initializer
// selects the constructor that corresponds to the guide F1 with T = double
// For F1 with T=double, the return type is UniquePtr<double>
 
// result:
// UniquePtr<double> dp{new auto(2.0)}

template<class T>
struct S
{
    template<class U>
    struct N
    {
        N(T);
        N(T, U);
 
        template<class V>
        N(V, U);
    };
};
 
S<int>::N x{2.0, 1};
 
// the implicitly-generated deduction guides are (note that T is already known to be int)
 
// F1: template<class U>
//     S<int>::N<U> F(int);
 
// F2: template<class U>
//     S<int>::N<U> F(int, U);
 
// F3: template<class U, class V>
//     S<int>::N<U> F(V, U);
 
// F4: template<class U>
//     S<int>::N<U> F(S<int>::N<U>); (copy deduction candidate)
 
// Overload resolution for direct-list-init with "{2.0, 1}" as the initializer
// chooses F3 with U=int and V=double.
// The return type is S<int>::N<int>
 
// result:
// S<int>::N<int> x{2.0, 1};

// declaration of the template
template<class T>
struct container
{
    container(T t) {}
 
    template<class Iter>
    container(Iter beg, Iter end);
};
 
// additional deduction guide
template<class Iter>
container(Iter b, Iter e) -> container<typename std::iterator_traits<Iter>::value_type>;
 
// uses
container c(7); // OK: deduces T=int using an implicitly-generated guide
std::vector<double> v = {/* ... */};
auto d = container(v.begin(), v.end()); // OK: deduces T=double
container e{5, 6}; // Error: there is no std::iterator_traits<int>::value_type

template<class T>
struct A
{
    explicit A(const T&, ...) noexcept; // #1
    A(T&&, ...);                        // #2
};
 
int i;
A a1 = {i, i}; // error: cannot deduce from rvalue reference in #2,
               // and #1 is explicit, and not considered in copy-initialization.
A a2{i, i};    // OK, #1 deduces to A<int> and also initializes
A a3{0, i};    // OK, #2 deduces to A<int> and also initializes
A a4 = {0, i}; // OK, #2 deduces to A<int> and also initializes
 
template<class T>
A(const T&, const T&) -> A<T&>; // #3
 
template<class T>
explicit A(T&&, T&&)  -> A<T>;  // #4
 
A a5 = {0, 1}; // error: #3 deduces to A<int&>
               // and #1 & #2 result in same parameter constructors.
A a6{0, 1};    // OK, #4 deduces to A<int> and #2 initializes
A a7 = {0, i}; // error: #3 deduces to A<int&>
A a8{0, i};    // error: #3 deduces to A<int&>

template<class T>
struct B
{
    template<class U>
    using TA = T;
 
    template<class U>
    B(U, TA<U>); // #1
};
 
// Implicit deduction guide generated from #1 is the equivalent of
//     template<class T, class U>
//     B(U, T) -> B<T>;
// rather than
//     template<class T, class U>
//     B(U, typename B<T>::template TA<U>) -> B<T>;
// which would not have been deducible
 
B b{(int*)0, (char*)0}; // OK, deduces B<char*>

std::tuple t1(1, 2, 3);                // OK: deduction
std::tuple<int, int, int> t2(1, 2, 3); // OK: all arguments are provided
 
std::tuple<> t3(1, 2, 3);    // Error: no matching constructor in tuple<>.
                             //        No deduction performed.
std::tuple<int> t4(1, 2, 3); // Error

template<class T>
struct S
{
    S(T);
};
S(char const*) -> S<std::string>;
 
S s{"hello"}; // deduced to S<std::string>

template<class T>
struct X
{
    X(T) {}
 
    template<class Iter>
    X(Iter b, Iter e) {}
 
    template<class Iter>
    auto foo(Iter b, Iter e)
    {
        return X(b, e); // no deduction: X is the current X<T>
    }
 
    template<class Iter>
    auto bar(Iter b, Iter e)
    {
        return X<typename Iter::value_type>(b, e); // must specify what we want
    }
 
    auto baz()
    {
        return ::X(0); // not the injected-class-name; deduced to be X<int>
    }
};

template<class T>
struct A
{
    A(T, int*);     // #1
    A(A<T>&, int*); // #2
 
    enum { value };
};
 
template<class T, int N = T::value>
A(T&&, int*) -> A<T>; //#3
 
A a{1, 0}; // uses #1 to deduce A<int> and initializes with #1
A b{a, 0}; // uses #2 (more specialized than #3) to deduce A<int> and initializes with #2

template<class T>
struct A
{
    using value_type = T;
 
    A(value_type); // #1
    A(const A&);   // #2
    A(T, T, int);  // #3
 
    template<class U>
    A(int, T, U);  // #4
};                 // #5, the copy deduction candidate A(A);
 
A x(1, 2, 3); // uses #3, generated from a non-template constructor
 
template<class T>
A(T) -> A<T>; // #6, less specialized than #5
 
A a(42); // uses #6 to deduce A<int> and #1 to initialize
A b = a; // uses #5 to deduce A<int> and #2 to initialize
 
template<class T>
A(A<T>) -> A<A<T>>; // #7, as specialized as #5
 
A b2 = a; // uses #7 to deduce A<A<int>> and #1 to initialize

template<class T>
struct A
{
    template<class U>
    A(T&&, U&&, int*); // #1: T&& is not a forwarding reference
                       //     U&& is a forwarding reference
 
    A(T&&, int*);      // #2: T&& is not a forwarding reference
};
 
template<class T>
A(T&&, int*) -> A<T>; // #3: T&& is a forwarding reference
 
int i, *ip;
A a{i, 0, ip};  // error, cannot deduce from #1
A a0{0, 0, ip}; // uses #1 to deduce A<int> and #1 to initialize
A a2{i, ip};    // uses #3 to deduce A<int&> and #2 to initialize

std::tuple t1{1};  //std::tuple<int>
std::tuple t2{t1}; //std::tuple<int>, not std::tuple<std::tuple<int>>
 
std::vector v1{1, 2};   // std::vector<int>
std::vector v2{v1};     // std::vector<int>, not std::vector<std::vector<int>> (P0702R1)
std::vector v3{v1, v2}; // std::vector<std::vector<int>>

std::vector v1{1, 2}; // std::vector<int>
 
std::vector v2(v1.begin(), v1.end()); // std::vector<int>
std::vector v3{v1.begin(), v1.end()}; // std::vector<std::vector<int>::iterator>

std::tuple p1{1, 1.0};             //std::tuple<int, double>, using deduction
auto p2 = std::make_tuple(1, 1.0); //std::tuple<int, double>, pre-C++17