Can a template function be called with missing template parameters in C++ ?

Question

This is an interview question, which has been done.

Which line has error ?

  #include
  template void foo(T op1, T op2)
         


        

Answer 1

It's called type deducition.

On Line 1, the type of T can be deduced because parameters op1 and op2 are both int, making T an int.

Whereas on Line 2, you are passing both an int and a double while the function accepts both parameters as T, the compiler has no clue whether T should be a double or an int.

Line 3 is fine because you specify int specialization and pass ints in as well (making the specialization redundant but perfectly OK).

Line 4 is OK because you declare T to be an int, then casting the char value of '3' to its numeric int value.

Line 5 is an error because you're accessing a function that gets its type from the templated struct it's in, and type deduction only works for functions.