CODEWITHSUNDEEP
c++templateslanguage-lawyertemplate-argument-deductionclass-template
Vlad from Moscow
Vlad from Moscow

Reputation: 99

Class template argument deduction doesn't work as function template argument deduction

I am learning about class template argument deduction and I could not understand why statement C<> c(5); fails unlike the call func<>(5) . I mean I expected that in the statement C<> c(5);, T should have been deduced to int but to my surprise the program generated an error saying error: wrong number of template arguments (0, should be 1).

template<typename T>
void func(T t)
{
   
}
template<typename T>
struct C
{
    C(T t)
    {
       
    }
};

int main()
{
    func<>(5);  //works as expected with T = int
   
   
    C<> c(5);  //why this fails? I mean why T is not being deduced from the argument 5's type?
   
    C   b(5);  //works as expected with T = int
}

So I want to know why does the statement C<> c(5); fail to compile.

Upvotes: 1

Views: 655

Answers (1)

Alan
Alan

Reputation: 1

Because class template argument deduction(CTAD) doesn't work if/when you explicitly use/specify the template argument list <> even if it is empty!

This can be seen from CTAD's documentation which states:

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.

(emphasis mine)

This means that since you've explicitly specified the template argument list <>, deduction won't happen here.

Upvotes: 1

Related Questions