In order to instantiate aclass template, every template argument must be known, but not every template argument has to be specified. In the following contexts the compiler will deduce the template arguments from the type of the initializer:

• anydeclaration that specifies initialization of a variable and variable template, whose declared type is the class template (possiblycv-qualified):

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;

• new-expressions:

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

• function-style cast expressions:

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 vi2std::for_each(vi.begin(), vi.end(),    Foo([&](int i){...}));// deduces to Foo<T>,// where T is the unique lambda type

template<class T>struct X{constexpr X(T){}}; template<X x>struct Y{}; Y<0> y;// OK, Y<X<int>(0)>

[edit]Deduction for class templates

[edit]Implicitly-generated deduction guides

When, in a function-style cast or in a variable's declaration, the type specifier consists solelyof the name of a primary class templateC (i.e., there is no accompanying template argument list), candidates for deduction are formed as follows:

• IfC is defined, for each constructor (or constructor template)Ci declared in the named primary template, a fictional function templateFi, is constructed, such that all following conditions are satisfied:

• The template parameters ofFi are the template parameters ofC followed (ifCi is a constructor template) by the template parameters ofCi (default template arguments are included too).

• Theparameter list ofFi is the parameter list ofCi.
• The return type ofFi isC followed by the template parameters of the class template enclosed in<>.

• IfC is not defined or does not declare any constructors, an additional fictional function template is added, derived as above from a hypothetical constructorC().

• In any case, an additional fictional function template derived as above from a hypothetical constructorC(C) is added, called the copy deduction candidate.

• For eachuser-defined deduction guideGi, a fictional function or function templateFi, is constructed, such that all following conditions are satisfied:

• The parameter list ofFi is the parameter list ofGi.
• The return type ofFi is the simple template identifier ofGi.
• IfGi has template parameters (syntax(2)),Fi is a function template, and its template parameter list is the template parameter list ofGi. Otherwise,Fi is a function.

template<class T>struct A{    T t; struct{long a, b;} u;}; A a{1,2,3};// aggregate deduction candidate://   template<class T>//   A<T> F(T, long, long); template<class...Args>struct B:std::tuple<Args...>, Args...{}; B b{std::tuple<std::any,std::string>{},std::any{}};// aggregate deduction candidate://   template<class... Args>//   B<Args...> F(std::tuple<Args...>, Args...); // type of b is deduced as B<std::any, std::string>

Template argument deduction andoverload resolution is then performed for initialization of a fictional object of hypothetical class type, whose constructor signatures match the guides (except for return type) for the purpose of forming an overload set, and the initializer is provided by the context in which class template argument deduction was performed, except that the first phase oflist-initialization (considering initializer-list constructors) is omitted if the initializer list consists of a single expression of type (possibly cv-qualified)U, whereU is a specialization ofC or a class derived from a specialization ofC.

These fictional constructors are public members of the hypothetical class type. They are explicit if the guide was formed from an explicit constructor. If overload resolution fails, the program is ill-formed. Otherwise, the return type of the selectedF template specialization becomes the deduced class template specialization.

template<class T>struct UniquePtr{    UniquePtr(T* t);}; UniquePtr dp{newauto(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)}

Or, for a more complex example (note: "S::N" would not compile: scope resolution qualifiers are not something that can be deduced):

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};

[edit]User-defined deduction guides

The syntax of a user-defined deduction guide is the syntax of a function (template) declaration with a trailing return type, except that it uses the name of a class template as the function name:

User-defined deduction guides must name a class template and must be introduced within the same semantic scope of the class template (which could be namespace or enclosing class) and, for a member class template, must have the same access, but deduction guides do not become members of that scope.

A deduction guide is not a function and does not have a body. Deduction guides are not found by name lookup and do not participate in overload resolution except for theoverload resolution against other deduction guides when deducing class template arguments. Deduction guides cannot be redeclared in the same translation unit for the same class template.

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

The fictional constructors for the purpose of overload resolution (described above) are explicit if they correspond to an implicitly-generated deduction guide formed from an explicit constructor or to a user-defined deduction guide that is declaredexplicit. As always, such constructors are ignored in copy-initialization context:

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 initializesA a3{0, i};// OK, #2 deduces to A<int> and also initializesA 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 initializesA a7={0, i};// error: #3 deduces to A<int&>A a8{0, i};// error: #3 deduces to A<int&> // Note: check https://github.com/cplusplus/CWG/issues/647, claiming that// examples a7 and a8 are incorrect, to be possibly replaced as//A a7 = {0, i}; // error: #2 and #3 both match, overload resolution fails//A a8{i,i};     // error: #3 deduces to A<int&>,//               //        #1 and #2 declare same constructor

Using a member typedef or alias template in a constructor or constructor template's parameter list does not, by itself, render the corresponding parameter of the implicitly generated guide a non-deduced context.

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*>

template<class T>class unique_ptr{/* ... */}; template<class T>class unique_ptr<T[]>{/* ... */}; template<class T>unique_ptr(T*)-> unique_ptr<T>;// #1 template<class T>unique_ptr(T*)-> unique_ptr<T[]>;// #2 template<class T>concept NonArray=!std::is_array_v<T>; template<NonArray A>using unique_ptr_nonarray= unique_ptr<A>; template<class A>using unique_ptr_array= unique_ptr<A[]>; // generated guide for unique_ptr_nonarray: // from #1 (deduction of unique_ptr<T> from unique_ptr<A> yields T = A):// template<class A>//     requires(argument_of_unique_ptr_nonarray_is_deducible_from<unique_ptr<A>>)// auto F(A*) -> unique_ptr<A>; // from #2 (deduction of unique_ptr<T[]> from unique_ptr<A> yields nothing):// template<class T>//     requires(argument_of_unique_ptr_nonarray_is_deducible_from<unique_ptr<T[]>>)// auto F(T*) -> unique_ptr<T[]>; // where argument_of_unique_ptr_nonarray_is_deducible_from can be defined as // template<class>// class AA; // template<NonArray A>// class AA<unique_ptr_nonarray<A>> {}; // template<class T>// concept argument_of_unique_ptr_nonarray_is_deducible_from =//     requires { sizeof(AA<T>); }; // generated guide for unique_ptr_array: // from #1 (deduction of unique_ptr<T> from unique_ptr<A[]> yields T = A[]):// template<class A>//     requires(argument_of_unique_ptr_array_is_deducible_from<unique_ptr<A[]>>)// auto F(A(*)[]) -> unique_ptr<A[]>; // from #2 (deduction of unique_ptr<T[]> from unique_ptr<A[]> yields T = A):// template<class A>//     requires(argument_of_unique_ptr_array_is_deducible_from<unique_ptr<A[]>>)// auto F(A*) -> unique_ptr<A[]>; // where argument_of_unique_ptr_array_is_deducible_from can be defined as // template<class>// class BB; // template<class A>// class BB<unique_ptr_array<A>> {}; // template<class T>// concept argument_of_unique_ptr_array_is_deducible_from =//     requires { sizeof(BB<T>); }; // Use:unique_ptr_nonarray p(newint);// deduced to unique_ptr<int>// deduction guide generated from #1 returns unique_ptr<int>// deduction guide generated from #2 returns unique_ptr<int[]>, which is ignored because//   argument_of_unique_ptr_nonarray_is_deducible_from<unique_ptr<int[]>> is unsatisfied unique_ptr_array q(newint[42]);// deduced to unique_ptr<int[]>// deduction guide generated from #1 fails (cannot deduce A in A(*)[] from new int[42])// deduction guide generated from #2 returns unique_ptr<int[]>

[edit]Notes

Class template argument deduction is only performed if no template argument list is present. If a template argument list is specified, deduction does not take place.

std::tuple t1(1,2,3);// OK: deductionstd::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 A,class B>struct Agg{    A a;    B b;};// implicitly-generated guides are formed from default, copy, and move constructors template<class A,class B>Agg(A a, B b)-> Agg<A, B>;// ^ This deduction guide can be implicitly generated in C++20 Agg agg{1,2.0};// deduced to Agg<int, double> from the user-defined guide template<class...T>array(T&&...t)-> array<std::common_type_t<T...>, sizeof...(T)>;auto a= array{1,2, 5u};// deduced to array<unsigned, 3> from the user-defined guide

User-defined deduction guides do not have to be templates:

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

Within the scope of a class template, the name of the template without a parameter list is an injected class name, and can be used as a type. In that case, class argument deduction does not happen and template parameters must be supplied explicitly:

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>}};

Inoverload resolution, partial ordering takes precedence over whether a function template is generated from a user-defined deduction guide: if the function template generated from the constructor is more specialized than the one generated from the user-defined deduction guide, the one generated from the constructor is chosen. Because the copy deduction candidate is typically more specialized than a wrapping constructor, this rule means that copying is generally preferred over wrapping.

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 #1A b{a,0};// uses #2 (more specialized than #3) to deduce A<int> and initializes with #2

When earlier tiebreakers, including partial ordering, failed to distinguish between two candidate function templates, the following rules apply:

• A function template generated from a user-defined deduction guide is preferred over one implicitly generated from a constructor or constructor template.
• The copy deduction candidate is preferred over all other function templates implicitly generated from a constructor or constructor template.
• A function template implicitly generated from a non-template constructor is preferred over a function template implicitly generated from a constructor template.

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 initializeA 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

An rvalue reference to a cv-unqualified template parameter is not aforwarding reference if that parameter is a class template parameter:

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 #1A a0{0,0, ip};// uses #1 to deduce A<int> and #1 to initializeA a2{i, ip};// uses #3 to deduce A<int&> and #2 to initialize

When initializing from a single argument of a type that is a specialization of the class template at issue, copying deduction is generally preferred over wrapping by default:

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>>

Outside the special case for copying vs. wrapping, the strong preference for initializer-list constructors in list-initialization remains intact.

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>

Before class template argument deduction was introduced, a common approach to avoiding explicitly specifying arguments is to use a function template:

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

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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