Does the function override the base function?

Question

I have three different compilers that I am using to compile this code. One of them (the one that I trust the least) warns that the function in Derived hides the function in Base. The other compilers (one is Visual C++) does not warn. Visual C++ doesn't even give a warning about it if I enable /Wall or /W4.

I tend to believe that this is a bug in the compiler that's giving a warning, since it compiles the code. If it really didn't override the base function, then it should give an error when I create an instance of the derived template.

Can anyone confirm how this should behave?

struct Base
{
   virtual void Func(float f) = 0;
};

template <typename T>
struct Derived : Base
{
   virtual void Func(T f){}
};

int main()
{
   Derived<float> d;
   d.Func(0);
   return 0;
}

---

When Derived is instantiated with float I get the unexpected warning. When Derived is instantiated with int I get an error, as expected.

Answer 1

It is indeed overridden. You can easily convince yourself in C++11 using the override keyword, which won't allow code to compile if the function is not overridden:

struct Base
{
   virtual void Func(float f) = 0;
   virtual ~Base() = default; // to silence warnings
};

template <typename T>
struct Derived : Base
{
   void Func(T f) override {} // will fail to compile if not overriding
};

int main()
{
   Derived<float> d;
   d.Func(0);
   return 0;
}

Live example here.

Note that in pre C++11, you can accidentally hide a virtual base function via changing its signature in a derived class, so even if you mark the derived function virtual the code still compiles, but does is not polymorphic anymore, see such an example here. Unfortunately g++ doesn't provide any warnings, even with -Wall -Wextra. That's why override is a much safer way of actually enforcing true override at compile time.

Answer 2

I don't believe you should be given a warning.
This is the same as:

struct Derived : Base
{
  virtual void Func(float f) { };
};

When your template parameter is float.

There is no hiding, only implementation of an abstract function.