Optional templated parameters for a function

Question

I have a function in a class I want to use, whose definition is:

 template <typename T>
 class FooClass
 {
 /* [...] */
 public:
     template <typename TT>
     void myFunction(int a, FooClass& b, int c,
                     int d, TT optional_e = static_cast<TT>(1));
 }

I tried to call it this way:

obj.myFunction(a, b, c, d);

With all the arguments matching the types they should have.

However, Visual Studio throws an error at compile time:

C2783 : could not deduce template argument for TT

But if I try to call the function this way, it compiles without errors:

obj.myFunction(a, b, c, d, 0);

---

My question is, why can't I call the function without the optional parameter? How to do this?

Answer 1

Becuase template argument deduction can't be done by default argument; TT can't be deduced.

Type template parameter cannot be deduced from the type of a function default argument

You could specify the template argument explicitly:

obj.myFunction<int>(a, b, c, d);

Or give the template parameter TT a default type too. e.g.

template <typename TT = T>
void myFunction(int a, FooClass& b, int c,
                int d, TT optional_e = static_cast<TT>(1));

Note you still could specify the type explicitly for it.