c++templatesc++14template-templatesvariable-templates

Can a variable template be passed as a template template argument?


The following nonsensical example does not compile, but is there some other way to pass a variable template as a template template argument?

template<typename T>
constexpr auto zero = T{0};

template<typename T, template<typename> auto VariableTemplate>
constexpr auto add_one()
{
    return VariableTemplate<T> + T{1};
}

int main()
{
    return add_one<int, zero>();
}

Try on Compiler Explorer


Solution

  • Short answer: No.

    Long answer: Yes you can using some indirection through a class template:

    template<typename T>
    constexpr auto zero = T{0};
    
    template<typename T>
    struct zero_global {
        static constexpr auto value = zero<T>;
    };
    
    template<typename T, template<typename> class VariableTemplate>
    constexpr auto add_one()
    {
        return VariableTemplate<T>::value + T{1};
    }
    
    int main()
    {
        return add_one<int, zero_global>();
    }
    

    Live example