Finally it works

src/tensor/tensor.hpp

@@ -6,6 +6,7 @@
 #include <vector>
 
 template <typename T, int Dim> class Tensor;
+enum class Function { SIGMOID, RELU, MSE, LINEAR };
 
 template <typename T, int Dim> class ITensor {
 protected:
@@ -73,6 +74,8 @@ public:
 
   Tensor operator*(const Tensor &other) const;
 
+  virtual Tensor apply(Function f, bool derivative = false) const = 0;
+
   // === Utils ===
   virtual std::string toString() const = 0;
 };