Re: Am I misusing std::vector?
"loose AT astron DOT nl" <loose@astron.nl> schrieb im Newsbeitrag news:1147252936.701110.275280@j73g2000cwa.googlegroups.com...
Hi,
I was surprised by the output of the program below. From what I
understand from the C++ STL documentation, vector<T>::resize(N), will
create N *newly constructed* objects of T. So, I expected *v[0].ip to
be equal to 0, and *v[sz-1].ip equal to -77. However, I discovered,
using a few different compilers (two different versions of gcc, and
icc), that both return -77. It turns out that only *one* object is
being constructed and this one object seems to be bitwise copied to the
other members. See my code below and try it for yourself.
Am I missing something here?
If only one new element is constructed and a bitwise copy would be used, that would really be a serious bug. But I doubt that that really
happens. But there are other ways to construct an object than using its default ctor. Elements of a vector (or any other STL container) must
have a valid copy ctor, and that one could be used by resize. You can simply test that. Just add a copy ctor to your class and verify that it is
called by resize.
struct A
{
A(int i = 0) { ip = new int(i); }
~A() { delete ip; }
int* ip;
};
A class (or struct) that has a non-trivial destructor, should also have a customized copy ctor and an assignment operator (Rule of Three). If
you put instances of that struct in an STL container, it is your fault if the program doesn't work as expected. You used the pointer member and
didn't supply proper assignment and copy construction.
HTH
Heinz
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