Question

In < Modern C++ Design >,it introduces a way to check if a type fundamental type by introducing the so called type list. but what if I don’t want to include so many loki code and just want a simple function to implement that? What is the simplest way to do that?

Answer 1

You can use template specialization to get what you want.

// General template
template<typename T>
struct IsFundamentalType { enum { result = false }; };

// Template specializations for each fundamental type
template<>
struct IsFundamentalType<char> { enum { result = true }; };

template<>
struct IsFundamentalType<int> { enum { result = true }; };

template<>
struct IsFundamentalType<short> { enum { result = true }; };

template<>
struct IsFundamentalType<float> { enum { result = true }; };

// And so on for other fundamental types ...

class NonFundamentalType
{
};

template<typename T>
void DoSomething(const T& var)
{
    if(IsFundamentalType<T>::result)
    {
        printf("I'm fundamental type!\n");
    }
    else
    {
        printf("I'm not a fundamental type!\n");
    }
}

int main()
{
    int i = 42;
    char c = 42;
    short s = 42;
    float f = 42.0f;
    NonFundamentalType nft;
    DoSomething(i);
    DoSomething(c);
    DoSomething(s);
    DoSomething(f);
    DoSomething(nft);
}

In this code, if you pass in a type such as int or char, the compiler will use the specialization of IsFundamentalType (given that you’ve defined the specializations for all fundamental types). Otherwise, the compiler will use the general template, as it is the case for the NonFundamentalType class. The important thing is that the specialized ones have a result member defined as true, while the general template also has a result member defined as false. You can then use the result member for the if statement. Optimizing compilers should be able to elide the if statement seeing that the expression reduces to a constant true/false value, so doing something like this should not impose a runtime penalty.