Copy from vector to vector, using C++11 features

Question

I have a std::vector<char*> that I want to copy into a std::vector<MemRef>, in a performance-critical section. I've looked at solutions using std::copy(), boost::make_transform_iterator, std::vector<int> newVec; newVec.reserve(oldVec.size()); .. push_back() and I have no idea what's best.

MemRef is a small object containing a char* and a size_t length.

I'm using GCC 4.4.7 with --std=g++0x so I get some c++ features, including emplace_back() which is nice for avoiding constructors and copying.

I think I'm better off not constructing the vector afresh each time, but rather using the vector clear() method to empty it, which as I understand allows it to refill to capacity without reallocation.

I'd appreciate help getting my head around this!

If you don't have a _specific_ code problem, and just want to know what's best -- _profile it_. It's the only way to know. — Chad, Dec 04 '13 at 02:57
@Chad, Sorry - are you saying this isn't specific enough for the forum? — Chap, Dec 04 '13 at 02:58
FYI, by and large (though not exclusively) `C++11` features add higher-level constructs and functions, not more performant ones. — Paul Draper, Dec 04 '13 at 02:58
@Chad, I'm not sure he necessarily *knows* all the ways it can be done. — Paul Draper, Dec 04 '13 at 02:59
@PaulDraper: I was thinking of emplace_back, and the "move semantics", which seem like performance boosters. — Chap, Dec 04 '13 at 03:01
What are you holding in your `std::vector`? Strings? How do you know the length (`strlen`?) — Chad, Dec 04 '13 at 03:21
@Chad, yes, the char*s are null-terminated 'C'-style strings. — Chap, Dec 04 '13 at 03:27

woolstar · Accepted Answer · 2013-12-04T21:13:03.827

1

I think std::copy would have pretty good performance, especially with the one note I see on cppreference.com

In practice, implementations of std::copy avoid multiple assignments and use bulk copy functions such as std::memmove if the value type is TriviallyCopyable

If the types require conversion then I would do this:

class MemRef
{
  public:
    MemRef(char * astr) : ptr_( astr), size_( strlen( astr)) { }
...
} ;

vecMem.insert(vecMem.end(), cstrVec.begin(), cstrVec.end()) ;

This allows vecMem to figure out how much space it needs to reserve in one go.

edited Dec 04 '13 at 21:13

answered Dec 04 '13 at 03:20

woolstar

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Please correct URL (algorithm/copy). SO won't allow "trivial edits. – Chap Dec 04 '13 at 03:31
This is as good a place to start as any, although I can't imagine bulk copy coming into play when each element must be converted. – Chap Dec 04 '13 at 03:37
I didn't know if you already had an automatic conversion between the types. I have my own type similar to your `MemRef` that has a constructor that takes a `const char *` – woolstar Dec 04 '13 at 04:01

Copy from vector to vector, using C++11 features

1 Answers1