CODEWITHSUNDEEP
c++templatesgccusing
Sydius
Sydius

Reputation: 14267

Can the "using" declaration be used with templates?

Is it possible to use the "using" declaration with template base classes? I have read it isn't here but is that because of a technical reason or is it against the C++ standard, and does it apply to gcc or other compilers? If it is not possible, why not?

Example code (from the link above):

struct A {
    template<class T> void f(T);
};

struct B : A {
    using A::f<int>;
};

Upvotes: 9

Views: 14580

Answers (1)

Johannes Schaub - litb
Johannes Schaub - litb

Reputation: 507055

What you linked to is a using directive. A using declaration can be used fine with templated base classes (haven't looked it up in the standard, but just tested it with a compiler):

template<typename T> struct c1 { 
    void foo() { std::cout << "empty" << std::endl; } 
}; 

template<typename T> struct c2 : c1<T> { 
    using c1<T>::foo; 
    void foo(int) { std::cout << "int" << std::endl; } 
}; 

int main() { 
    c2<void> c;
    c.foo();
    c.foo(10); 
}

The compiler correctly finds the parameter-less foo function because of our using-declaration re-declaring it into the scope of c2, and outputs the expected result.

Edit: updated the question. here is the updated answer:

The article is right about that you are not allowed to use a template-id (template name and arguments). But you can put a template name:

struct c1 { 
    template<int> void foo() { std::cout << "empty" << std::endl; } 
}; 

struct c2 : c1 { 
    using c1::foo; // using c1::foo<10> is not valid
    void foo(int) { std::cout << "int" << std::endl; } 
}; 

int main() { 
    c2 c;
    c.foo<10>();
    c.foo(10); 
}

Upvotes: 5

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