Undefined reference to non-member function - C++

Question

I have the following in header file.

namespace silc{
   class pattern_token_map
   {
      /* Contents */
   };

   pattern_token_map* load_from_file(const char*);
}

In the CPP file (this has got proper includes)

pattern_token_map* load_from_file(const char* filename)
{
   // Implementation goes here
}

In another CPP file. This has got all proper includes.

void some_method()
{
    const char* filename = "sample.xml";
    pattern_token_map* map = load_from_file( filename ); // Linker complains about this.
}

I am getting a linker error saying that undefined reference to load_from_file. I am not able to see what is going wrong here.

Any help would be appreciated.

Compiler : G++ OS : Ubuntu 9.10

Edit

Here is the linker command used.

g++ -L/home/nkn/silc-project/third_party/UnitTest++ -o tests.out  src/phonetic_kit/pattern_token_map.o  tests/pattern_token_map_tests.o  tests/main.o -lUnitTest++

Error is from pattern_token_map_tests.o and the function is available in pattern_token_map.o. So I guess the order of linking is not making the problem. (I have removed some files from the command to simplify it)

Answer 1

In your link phase, are you linking in the object file created by compiling the first cpp file? Linker errors like these occur when one object references a symbol that is not included in the objects being linked.

Edit: I'm still reasonably confident that this is the problem. In the first file, is there a preprocessor symbol redefining load_from_file?

Answer 2

When you implement it, you have to make sure you implement the right function:

namespace silc {
pattern_token_map* load_from_file(const char* filename) {
   // Implementation goes here
}
}

If you instead did this:

using namespace silc; // to get pattern_token_map
pattern_token_map* load_from_file(const char* filename) {
   // Implementation goes here
}

Then you'd be defining a new function rather than silc::load_from_file.

Avoid using directives ("using namespace ...;") outside of function scope, as a general guideline:

using namespace silc; // outside function scope: avoid

silc::pattern_token_map*                      // qualify return type
random_function(silc::pattern_token_map* p) { // and parameters
  using namespace silc; // inside function scope: fine
  pattern_token_map* p2 = 0; // don't have to qualify inside the function
                             // if you want to use the using directive
  silc::pattern_token_map* p3 = 0; // but you can always do this
  return 0;
}