Question

I have read that C++11 has sufficient static checking (compile time) so as to implement a big part of what should have been Concepts checking of C++11(removed). (I’ve read this in the comments of a recent question about Concepts removed… – that question was fastly closed as not constructive).

The C++03 code below only check the presence of a member function in a class (on which my template class wants to work). Here are 4 member functions that are searched for, and I always use the same pattern:

• a typedef to define a typedef of the prototype of the function
• a call to static_cast that breaks compilation if ever the typename TExtension doesn’t define such a member function, or if it has different prototype

Here is the code:

template <typename TExtension>
class
{
...
    void checkTemplateConcept()
    {
        typedef unsigned long (TExtension::*memberfunctionRequestedId)();
        static_cast<memberfunctionRequestedId>(&TExtension::getRequestId);

        typedef eDirection (TExtension::*memberfunctionDirection)();
        static_cast<memberfunctionDirection>(&TExtension::getDirection);

        typedef eDriveWay (TExtension::*memberfunctionDriveWay)();
        static_cast<memberfunctionDriveWay>(&TExtension::getDriveWay);

        typedef unsigned long (TExtension::*memberfunctionCycleId)();
        static_cast<memberfunctionCycleId>(&TExtension::getCycleId);
    }
}

This was in some part of my code, but it was completely based on C++03. I would rewrite it gladly, with those new C++11 features… what should be used here instead ?

Answer 1

With C++11, you could make the compiler to print the good error messages with static_assert as:

typedef unsigned long (TExtension::*required_type)();
typedef decltype(&TExtension::getRequestId) actual_type;

static_assert(std::is_same<required_type, actual_type>::value,
     "The type of getRequestId must be unsigned long (C::*)()");

Now if the type of member function doesn’t match, the compiler will print this helpful message:

"The type of getRequestId must be unsigned long (C::*)()"

You could make it more descriptive if you want to. 🙂