Template in C++, why have to use enum

user_1465437 0
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I have a quick question about item 48 in Scott Meyers' "Effective C++". I just don't understand the code copied from the book below,

    #include <iostream>
    using namespace std;

    template <unsigned n>
    struct Factorial
    {
       enum { value=n*Factorial<n-1>::value };
    };

    template <>
    struct Factorial<0>
    {
        enum { value=1};
    };

    int main()
    {
        cout<<Factorial<5>::value<<endl;
        cout<<Factorial<10>::value<<endl;
    }

Why do I have to use enum in template programming? Is there an alternative way to do this? Thanks for the help in advance.

effective-c++ c++
5 Answers

You can use an int instead of static const it for this as follows:


template<int n>
struct Factorial
{
    int val{ n*Factorial<n - 1>().val };
};

template<>
struct Factorial<0>
{
    int val{1};
};

int main()
{
    cout << "Factorial 5: " << Factorial<5>().val << endl;
}

You can also use function template instead of struct/class template:

template<int n>
int fact()
{
    return n*fact<n - 1>();
}

template <>
int fact<0>()
{
    return 1;
}

int main()
{
    cout << "Fact 5: " << fact<5>() << endl;
}
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