istream vs memory mapping a file?

Question

I am trying to map a file to memory and then parse line by line- is istream what I should be using?

Is istream the same as mapping a file to memory on Windows? I have h

Answer 1

Abstractly speaking, reading a file sequentially will not be sped up by using memory mapped files or by first reading it into memory. Memory mapped files make sense if reading the file sequentially is not feasible. Pre-caching the file like in the other answer or just by copying the file to a large string which you could then process by other means - again - only makes sense if reading the file once in sequence is not feasible and you have the RAM for it. This is because the slowest part of the operation is actually getting the data off the disk. And this has to be done regardless, whether you copy the file to RAM or you let the operating system map the data before you can access it or when you let std::iostream read it line by line and let it cache from the file just enough to make this work smoothly.

In practice you could potentially eliminate some copying from ram to ram with the mapped or cached versions, by making shallow copies of the buffer ranges. Still this will not change much because this is RAM->RAM and therefore negligible in comparison to disk->RAM.

The best advice in a situation like yours is therefore not to worry too much and just use std::iostream.

[Ths answer is for archival purposes, because the correct answer is buried in the comments]