C++ Converting memcpy to std::copy

Question

I wanted to replace my "memcpy" with "std::copy", but I just can't find the right way for passing the args.

My old memcpy code was

memcpy(&uFeatures.Features[0], &((char*)(m_pData))[iBytePos],iByteCount);

I tried various things with std::copy, but they all did not work.

Could anybody help, please?

What is the type of uFeatures.Features and m_pData? Also, what is the reason why you want to change to std::copy? What "various things" have you tried? — Sven
– Sven, Commented May 11, 2013 at 15:59
I don't see any reason to not use memcpy if your code look like that. — yngccc
– yngccc, Commented May 11, 2013 at 16:07
I would like to let the compiler decide what it prefers (memcopy, memmove or anything else), that is why I want to use std::copy — tmighty
– tmighty, Commented May 11, 2013 at 16:08

Neel Basu · Accepted Answer · 2013-05-11 16:30:13Z

2

from your syntax it seems it will be smething like this. (assuming Features[0] is char*, if not you need to cast (check comments) )

std::copy(&uFeatures.Features[0], &uFeatures.Features[0]+iByteCount, &((char*)(m_pData))[iBytePos])

and as I see in your comments

I would like to let the compiler decide what it prefers (memcopy, memmove or anything else), that is why I want to use std::copy

std::copy will not be translated to memcpy by compiler. they are unrelated. copy will do something like this

while (first != last) {
    *d_first++ = *first++;
}

edited May 11, 2013 at 16:30

answered May 11, 2013 at 15:59

Neel Basu

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15 Comments

tmighty Over a year ago

Thank you, but it crashes for me.

Neel Basu Over a year ago

does m_pData have iBytePos+iByteCount bytes memory allocated on it ?

Tony Delroy Over a year ago

@NeelBasu: that's almost right, but you're mixing "Feature"s (if that's the type of .Features[0]) with byte counts... pointer arithmetic operates in multiples of the structure size so you must either divide iByteCount by that and cast the final char pointer to a Feature*, or covert the Feature*s in the first two arguments to char*. (I can write them both out explicitly if you need).

John5342 Over a year ago

Just a note on the last bit of your answer. I have seen multiple compilers (and standard libraries) where std::copy was eventually lowered to a memcpy or similar instructions and it is in fact perfectly legal for pod types at least (under the as-if rule).

Neel Basu Over a year ago

std::copy is completely different that memcpy memcpy copies a memory chunk, whereas std::copy forwards iterator and assigns. depending on the iterator it may yield the same result. but both are NOT same, and std::copy is NOT a C++ version of memcpy

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