CODEWITHSUNDEEP
c++templates
Victor Miller
Victor Miller

Reputation: 451

C++ template argument deduction/substition failed

I'm using a template library in which class B is a templated class parametrized by class A. I have a declaration

template <class A, template <class A> class B>
void foo(){
   B<A> x;
}

later on I want to invoke this as

foo<A, B>();

where X is a concrete class in the library and Y is a particular templated concrete class in the library. However, I get the titled error abour template argument deduction/substitution failed. If I change the declaration of foo to remove the templates and subsitute in X and Y, then it all works ok. I also tried

foo<X, Y<X> >();

which failed with the same message. Can someone explain why this is happening?

I'm using gcc 5.3.0

Here's a complete example which gives the indicated behavior

#include <vector>
template <class A, template <class A> class B>
void foo() {
   B<A> x;
}
void bar() {
   foo<int, std::vector>();
}

Upvotes: 0

Views: 170

Answers (2)

Alex Zywicki
Alex Zywicki

Reputation: 2313

You have two issues that I can see.

First is that std::vector takes more than one template argument so the template template parameter template<class A> class B will never match std::vector.

Second and slightly less of an issue is that the class A here template<class A> class B shadows the previous class A here template <class A/*here*/, template <class A> class B>

To fix both of these issues you can declare the second template argument as an nameless variadic template like so: template <class...> class B.

Combining everything you get:

#include <vector>
template <class A, template <class...> class B>
void foo() {
    B<A> x;
}
void bar() {
    foo<int, std::vector>();
}

Edit:

If you would like to only use B in the form of B<A> you could do one of a few things:

template <class A, template <class...> class B>
void foo() {
    using C = B<A>;
    C x;
}

OR:

template <class A, template <class...> class B, class C = B<A>>
void foo() {
    C x;
}

OR (depending on the larger point of your code) you could just accept the whole thing as one template parameter:

template <class A>
void foo() {
    A x;
}
void bar() {
    foo<std::vector<int>>();
}

Upvotes: 3

dascandy
dascandy

Reputation: 7302

std::vector is not templated on one class, so it does not match your type. You can coerce it to match by

template <class X> using MyVector = std::vector<X>;

and that will work. C++17 will just plain fix this to work as you expected it to.

Upvotes: 1

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