C++ how to check the signature of a template parameter class at compile time

Question

In the code below, I am trying to check the signature of the class that is passed as the second template argument to WTrajectory. In the current implementation, the constructor of WTrajectory compares the types of the template argument T and the template argument of the type that is passed to it as the second argument.

The current implementation can perform the check. However, I would prefer to perform it at compile time, if it is possible. Moreover, I would also like to check if the template argument TWPoint has a member function returnTimeTypeID, also at compile time (a solution that performs this check at run time can be found here: link).

template<typename T>
struct WPoint
{
    const std::type_info& returnTimeTypeID(void) const
        {return typeid(T);}
};

template<typename T, typename TWPoint>
struct WTrajectory
{
    WTrajectory(const TWPoint& wp)
        {
            compare_types(wp);
        }

    void compare_types(const TWPoint& wp)
        {
            if (typeid(T) != wp.returnTimeTypeID())
                throw std::runtime_error("Error");
        }
};

What are you trying to check exactly? The wording makes it seem like you're just trying to check that `TWPoint` is `WPoint`? — Barry, May 19 '15 at 18:47

score 1 · Accepted Answer · answered May 19 '15 at 18:41

Since returnTimeTypeID is non-virtual the compiler will know the dynamic type of TWPoint at compile time. So instead of doing a runtime check just change your template:

template<typename T>
struct WTrajectory
{
    typedef T TWPoint;
    ...

The best way to check whether a template type has a perticular method at compile time is to just call the method. You'll get a compilation error if it doesn't provide the needed functionality.

score 0 · Answer 2 · answered May 19 '15 at 18:55

If WPoint contains more than just the type info, then the following code will work

template<typename T>
struct WPoint
{
    // ... stuff not related to type checking
};

template<typename T>
struct WTrajectory
{
    WTrajectory(const WPoint<T>& wp)
        {
        }
};

You could remove WPoint if it does not contain anything else besides the type info.

C++ how to check the signature of a template parameter class at compile time

2 Answers2