Re: static const int problem
On Sun, 28 Jul 2013 03:47:28 -0500
Paavo Helde <myfirstname@osa.pri.ee> wrote:
cathode26@gmail.com wrote in
news:e19651a4-3688-418d-9916-2381b2b5517b@googlegroups.com:
On Tuesday, May 15, 2007 6:17:01 AM UTC-5, mati wrote:
Hi
The following code works:
#include <vector>
class C {
private:
static const int m_static = 2;
public:
void f(const std::vector<int>& v)
{
int a = m_static;
std::vector<int> stripped(v.begin()+a, v.end());
//std::vector<int> s2(v.begin()+m_static,v.end());
}
};
int main()
{
C c;
std::vector<int> pv;
int i;
pv.push_back(i);
pv.push_back(i);
pv.push_back(i);
c.f(pv);
}
But when I erase the comment in the void f(...), then compiler
gives an error:
g++ -ansi -Wall -o test test.cpp
/tmp/cckLnGUY.o: In function `C::f(std::vector<int,
std::allocator<int>
> const&)':
test.cpp:(.text._ZN1C1fERKSt6vectorIiSaIiEE[C::f(std::vector<int,
std::allocator<int> > const&)]+0xdb): undefined reference to
`C::m_static' collect2: ld returned 1 exit status
make: *** [test] Error 1
g++ --version
g++ (GCC) 4.1.2 20060928 (prerelease) (Ubuntu 4.1.1-13ubuntu5)
Can anybody tell me what I'm doing wrong?
So I tried your code in Visual Studio and it works fine, but if you
use http://codepad.org/ your code does not compile. It is true,
this is a declaration and not a definition.
[...]
class C {
private:
static const int m_static = 2;
Yes, this is a declaration which will need a definition in some
source file somewhere. If there is no such definition, the linker may
complain or not, depending on the usage of this symbol and
optimization levels, and obviously depending on which compiler you
use.
Unfortunately, at least some MSVC versions fail to compile the
correct code with the definition added, complaining about multiply
defined symbols. So to avoid all the hassle I would suggest just to
use:
class C {
private:
enum constants {
m_static = 2
};
In most cases where you use a static const member of integral type you
do not need such a definition (nor want one as it forces storage).
It may be worth explaining why this is happening. Presumably
operator+() for std::vector<int>::iterator has an overload which takes
its argument(s) by reference to const. This requires the address of
m_static to be taken, which means that it is "ODR-used" within the
meaning of the standard so requiring storage to be allocated by defining
(as opposed to just declaring) it.
You don't have to define the static member itself: as in the example,
you can initialise a local variable with it (which does not ODR-use the
member). Or you can force the correct behaviour with a cast
(std::vector<int> s2(v.begin()+static_cast<const int>(m_static),v.end());)
Generally I prefer const static integral members (or constexpr static
integral members) to enum, notwithstanding these occasional oddities.
Chris