C++11 efficient vector member initialization?

Question

What is the best way to initialize a vector member object in C++11? Will the compiler optimize away the copy in foo or will bar be more efficient?

#include <vector> using namespace std; // C++11 style struct foo { vector<int> vec = vector<int>(256); }; // traditional struct bar { bar() : vec(256) {} vector<int> vec; };

usually it's best to start a vector as empty since push_back is actually very efficient — aaronman
– aaronman, Commented Nov 7, 2013 at 0:16
@aaronman, good point. And if you're determined to give some hint to the system about the expected size, there's alway vector::reserve — Aaron McDaid
– Aaron McDaid, Commented Nov 7, 2013 at 0:23
Default-initialized dynamic arrays are weird in C++. C++ separates memory allocation from object creation, so you should maybe allocate the memory up front (with reserve), but only create the objects when you actually have them. (For example, default-initialization of a vector causes all the elements to be accessed, which may be unnecessary and wasteful.) — Kerrek SB
– Kerrek SB, Commented Nov 7, 2013 at 0:26
@KerrekSB that's why they were supposed to add std::dynarray but it was taken out — aaronman
– aaronman, Commented Nov 7, 2013 at 0:31
@aaronman: Mm, I think even that would have initialized the elements. I'm not sure, though. — Kerrek SB
– Kerrek SB, Commented Nov 7, 2013 at 0:33

Aaron McDaid · Accepted Answer · 2013-11-07 01:05:19Z

In C++11 there probably isn't much difference between them. For example foo does not copy a large vector. The right hand side of the = is an rvalue and will be moved to the left hand side. The only difference is the creation (and quick removal) of the 0-element vector within foo. But that'll take up no time.

But, both C++11 and C++03 allow optimizations ('elision') to skip the assignment in foo. Therefore they can both be very efficient in both standards.

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