overloaded array subscript [] operator slow

Question

I have written my own Array class in c++ and overloaded the array subscript [] operator, code:

inline dtype &operator[](const size_t i) { return _data[i]; }
inline dtype operator[](const size_t i) const { return _data[i];}

where _data is a pointer to the memory block containing the array. Profiling shows that this overloaded operator alone is taking about 10% of the overall computation time (on a long monte carlo simulation, and I am compiling using g++ with maximum optimization). This seems a lot, any idea why this is?

edited: dtype is a double, and _data is a pointer to an array of double

Answer 1

The const overload of operator[] is actually returning a copy instead of a dtype const &. If dtype is big, the copy might be expensive.

Prototyping it like that should solve that problem :

inline dtype const & operator[] (const size_t i) const { return _data[i]; }