Question

The template i have is:

template <class T>
class Shape {
T val,val_new;
public:
Shape(T initval)
{
   val=initval;
}
T get()
{
   return val;
}
void set (T newval)
{
   val_new = newval;
}
void copy()
{
   val= val_new;
}
};

The class to use this template is :

#include <iostream>
#include<math.h>
using namespace std;
class Rectangle
{
 private:
 Shape<TwoPoint> val;
 bool incr_called, decr_called, load_called;
 TwoPoint newval;
 public:
 Rectangle(TwoPoint i)
      : val (Shape<TwoPoint> (i)) {}     
 Shape<TwoPoint> read()
 {
   return val;
 }
 void load(TwoPoint n)
 {
   load_called=1;
   newval=n;
 }
 void increment()
 {
   incr_called=1;
 }
 void decrement()
 {
   decr_called=1;
 }
 void init()
 {
   incr_called=0;
   decr_called=0;
   load_called=0;
 }
 void actions()
 { 
   if (load_called)
       val.set(new TwoPoint(newval));
   if(incr_called && !decr_called)
       val.set((new TwoPoint(val.get())).plus(1));
   if(!incr_called && decr_called)
       val.set((new TwoPoint(val.get())).plus(-1));
 }     
 };

TwoPoint class is defined as:

 class TwoPoint
 {
   int width;
   int value;
   public:
    TwoPoint()
    {
      value=0;
      width=0;
     }
     TwoPoint(int v, int w)
     {
       value=v;
       width=w;
     }
     TwoPoint(const TwoPoint& t)
     {
       value= t.value;
       width= t.width;
     }
     int getWidth()
     {
       return width;
     }
     int getValue()
     {
       return value;
     }
     TwoPoint & plus(int newval)
     {
      value+=newval;
      return *this;
     }
    };

But there are errors:

In member function 'void Rectangle::actions()':
error: request for member 'plus' in '(((TwoPoint*)operator new(8u)), (((*<anonymous>)
<unknown operator> TwoPoint<T>::get() [with T=TwoPoint]()), <anonymous>))', which is of non-class type 'TwoPoint*'

There is another error:

In member function 'void Rectangle::actions()':
error: no pattern matching function for call to 'Shape<TwoPoint>::set(TwoPoint*)'
note: candidates are: void Shape<T>:: set<T> [with T=TwoPoint]

There are two more errors when i am doing similar operations in actions() due to same reason. Can somebody please explain these errors and how to improve them?
Is there any way in which i can make code more efficient?

Answer 1

Shape::set takes its argument by value, but you’re creating a value using new and passing a pointer. You should avoid using new unless you actually need a dynamic object; in which case, you need to make sure it’s deleted when you’ve finished with it.

In this case, you just want to pass an object by value:

 void actions()
 { 
   if (load_called)
       val.set(newval);
   if(incr_called && !decr_called)
       val.set(val.get().plus(1));
   if(!incr_called && decr_called)
       val.set(val.get().plus(-1));
 }     

Is there any way in which i can make code more efficient?

Dynamic allocation is usually less efficient than using automatic objects – fixing the error has also removed a source of inefficiency.

Shape::set, and the constructor, could take their arguments by reference, and Shape::get could return a reference, to avoid unnecessary copying; although in practice the compiler will probably avoid those copies anyway. Also, the constructor could use an initialiser list, to initialise the member(s) directly, rather than default-initialising them and the reassigning them. Code like this might be marginally more efficient in some cases:

Shape(T const & initval)      // pass by reference
  : val(initval)              // use initialiser list
{}
T const & get()               // return by reference
{
   return val;
}
void set (T const & newval)   // pass by reference
{
   val_new = newval;
}

But in general, focus on making the code correct and readable, and on choosing efficient algorithms; only worry about small inefficiencies if they prove to be a bottleneck.