C++ – vector multiple definition link errors

cmultiple-definition-errorvector

vector is included in only one source file. The only stl include in the header files is string. Yet I cannot get rid of multiple definition errors (example below). Any ideas?

./plugin_dfb.o:mipsel-linux-uclibc/include/c++/4.2.0/bits/stl_bvector.h:182: multiple definition of `std::operator-(std::_Bit_iterator_base const&, std::_Bit_iterator_base const&)'
./painter.o:mipsel-linux-uclibc/include/c++/4.2.0/bits/stl_bvector.h:182: first defined here

Best Answer

This std::operator- is an inline function with external linkage. It appears the linker doesn't support multiple definitions of such inline functions. But the "one definition rule" of C++ (ODR) clearly allows this. Typically such symbols get "weak linkage" (GNU terminology) but I think both, the object file format and the linker, need to support this.

I would try to ask a question in a group / mailing list dedicated to your platform. Something along the lines of "Does platform X support C++ with respect to linking and the C++-related one-definition rule for inline functions and templates?".

You may also want to check the GCC documentation. It's possible that they provide a command line switch as a work-around. If you don't already use g++ as front-end to the linker, you should try it. Maybe that helps, too.

Related Solutions

C++ – How to find out if an item is present in a std::vector

You can use std::find from <algorithm>:

#include <algorithm>
#include <vector>
vector<int> vec; 
//can have other data types instead of int but must same datatype as item 
std::find(vec.begin(), vec.end(), item) != vec.end()

This returns a bool (true if present, false otherwise). With your example:

#include <algorithm>
#include <vector>

if ( std::find(vec.begin(), vec.end(), item) != vec.end() )
   do_this();
else
   do_that();

C++ – How to use std::maps with user-defined types as key

You don't have to define operator< for your class, actually. You can also make a comparator function object class for it, and use that to specialize std::map. To extend your example:

struct Class1Compare
{
   bool operator() (const Class1& lhs, const Class1& rhs) const
   {
       return lhs.id < rhs.id;
   }
};

std::map<Class1, int, Class1Compare> c2int;

It just so happens that the default for the third template parameter of std::map is std::less, which will delegate to operator< defined for your class (and fail if there is none). But sometimes you want objects to be usable as map keys, but you do not actually have any meaningful comparison semantics, and so you don't want to confuse people by providing operator< on your class just for that. If that's the case, you can use the above trick.

Yet another way to achieve the same is to specialize std::less:

namespace std
{
    template<> struct less<Class1>
    {
       bool operator() (const Class1& lhs, const Class1& rhs) const
       {
           return lhs.id < rhs.id;
       }
    };
}

The advantage of this is that it will be picked by std::map "by default", and yet you do not expose operator< to client code otherwise.

Best Answer

Related Solutions

C++ – How to find out if an item is present in a std::vector

C++ – How to use std::maps with user-defined types as key

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