CODEWITHSUNDEEP
c++stlstdvector
ddevienne
ddevienne

Reputation: 1882

How many times std::vector::resize should default-construct new elements?

Our team just ran into the same issue described here http://forums.codeguru.com/archive/index.php/t-514404.html, i.e. calling some_vec.resize(new_size), where N = new_size - some_vec.size() with N >= 2, and VC10 default-constructed all N new elements, whereas GCC default constructed a single element, as the prototype element, to copy-construct it N times for the new elements.

Since this is a vector of uuid, where the default-constructor randomly initializes each new instance, we ended up with N times the same uuid with GCC and N different uuids with VC. That was enough to wreak havoc in our test suite on one platform but not another, and was no fun to find.

My question is thus: who is right? VC or GCC? Or is this one of those beloved corners of C++ that's unspecified? TIA, --DD

Upvotes: 5

Views: 209

Answers (1)

Xeo
Xeo

Reputation: 131789

I bet if you compile GCC with -std=c++0x, you'll get the same result as with MSVC, that is, the N default constructions. This has changed in C++11, see here. There are now two overloads, one with just the new size that default constructs the new elements, and another that takes a "prototype" parameter to copy-construct every new element from.

Now, to get consistent results no matter what mode you compile in, just use

v.resize(new_size, T());

Background info: The change was necessary, since there are now types that can be movable, but not copyable (like std::unique_ptr). The old signature demanded copyability. Now the first template parameter to standard container types is only required to be copyable, if you use operations that require a copy.

Upvotes: 6

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