reading from a file

[jacek]

1. I don't know the number of the lines ( I know only its dimesion with instructions below)

If you are going to use merge sort, you don't have to know this.

After that, in your opinion, can I use
[...]
to do all work ( I still understand it'll more speed) ??

The version with std::vector will be easier to implement and you if preallocate memory, it shouldn't be significantly slower.