C++ Templates access a function with different types

Question

So What I'm trying to do is something like this.

I have a template struct like this one:

template <typename T>
struct TemplateTest
{
    void test(T a)
    {
        switch (typeid(boost::any_cast<T>(a)))
        {
        case typeid(int):
            {
                std::cout << "INT";
                break;
            }
        case typeid(float):
            {
                std::cout << "FLOAT";
                break;
            }
        case typeid(double):
            {
                std::cout << "DOUBLE";
                break;
            }
        default:
            {
                std::cout << "OTHER";
                break;
            };
        }
    }
};

And I want to make a vector with different types of that struct and after just use a for-loop to iterate over all my elements and call this function form all of them.

What I would like to have will be something like this:

typedef boost::variant<TemplateTest<float>(), TemplateTest<double>(), TemplateTest<int>() > variant;
typedef std::vector<variant> TestVar;
TestVar d;
d.push_back(new TemplateTest<float>());
d.push_back(new TemplateTest<double>());
d.push_back(new TemplateTest<int>());
for (auto value : d)
{
    value.test(5);
}

Is out there any way to do this without using a specific cast of my type before my function call?

No disrespect, but your code sample looks like you are guessing at syntax. — StoryTeller - Unslander Monica, Aug 30 '18 at 09:36
Ofc... because of the code from "What I would like to have will be something like this:" is not working, that's something I want to achieve. That syntax is not going to run — Vali, Aug 30 '18 at 09:38
*"I have a template struct like this one"* - Does not imply the code sample is illustrative. And I was aiming more at the `boost::any_cast(a)` - Which is a very inefficient way to get at a `typeid(T)`. But no matter. That's beside the point. — StoryTeller - Unslander Monica, Aug 30 '18 at 09:42
You cannot have a vector with different types of elements. So much for the great idea. You may want to stop whatever you are doing and read about inheritance and polymorphism and virtual functions and the rest of the OO works. — n. m. could be an AI, Aug 30 '18 at 09:57
You might want something like [that](http://coliru.stacked-crooked.com/a/83a95562f6187da0) — Jarod42, Aug 30 '18 at 09:58

score 1 · Accepted Answer · answered Aug 30 '18 at 10:00

If you want a single function doing different things for a fixed number of types, you want to use overloaded functions, not templates.

void test(int)
{
   std::cout << "INT";
}

If you have a sane default that works for more than one type, you can use template specialization. You already have a struct template wrapping the functions, which is generally a good idea.

template <typename T>
struct TemplateTest
{
    void test(T a)
    {
        std::cout << "OTHER";
    }
};

template<>
struct TemplateTest<int>
{
    void test(int a)
    {
        std::cout << "INT";
    }
};

If, as in this very specific case of oversimplification all that is different is the string, you can use variable templates:

template<typename T>
constexpr char* Typename = "DEFAULT";

template<>
constexpr char* Typename<int> = "INT";

template <typename T>
struct TemplateTest
{
    void test(T a)
    {
        std::cout << Typename<T>;
    }
};

Live example of the last one here. You see that you probably wanted the test function to be static.

C++ Templates access a function with different types

1 Answers1