c++ - Constructing result in operator vs operating on default-constructed object -

i have class array labelled contents, , i'd define operators it. i'd in such way changing number of elements in class easy, expect future users change variables tracked, i'd ensure basic arithmetic operations on class efficient possible.

i can see 2 way of implementing operators. taking example of vector2d class:

struct vector2d {      //members       const static int nelem = 2;       double x;       double y;      //constructors       vector2d() {}                 vector2d(double x, double y) : x(x), y(y) {}      //operators       double& operator[] (int index) {         switch(index) {            case 0:             return x;           case 1:             return y;           default:             return std::out_of_range ("oops");         }       }      // option 1: operator+ constructing result     vector2d operator+ (const vector2d & rhs) const {       return vector2d(x + rhs.x, y+rhs.y);     }      // option 2: operator+ using loop , [] operator     vector2d operator+ (const vector2d & rhs) const {       vector2d result;       for(int = 0; < nelem; i++)         result[i] = (*this)[i] + rhs[i];       return result;     } }; 

assuming use -03 optimization, there difference between 2 implementations of operator+? understanding since default vector2d constructor has no code body , class contents default data type, there no overhead in option 2 calling default constructor on result before setting members. expect 2 equivalent, i'm not confident enough in knowledge sure.

your method won't work @ all. if want able change nelem can't use x, y names. why? because changing nelem 3 won't magically add z. , since can't for loop on x , y, definition of nelem meaningless.

finally, textbook case use of number template. create vector templated on how many elements has.

do this:

template<unsigned int len> class vector{   double v[len]; public:   vector operator+(const vector &o){     vector res;     (unsigned int i=0;i<len;++i) // -o3 , small len unrolled       res.v[i]=v[i]+o.v[i];   }   // ... etc. }; 

then use this:

vector<2> my_2d_vec; vector<3> my_3d_vec;