Question

I have a namespace that I am currently using in two classes. When I try to compile my project, I get that error but my namespace is not anonymous!

One of my classes looks like this:

//margin.cpp
#include <math.h>
#include "margin.h"
#include "anotherClass.h"
#include "specificMath.nsp.h" //My namespace

double margin::doSomeMath(double a, double b){
    return specificMath::math_function1(0, 1, 0);
    // Just a simpler, random example
} 

My namespace looks like this:

//specificMath.nsp.h
#ifndef specificMath
#define specificMath
namespace specificMath {
     double math_function1(double, double, double);
     double math_function1(double);
     //more functions
}

---

 //specificMath.nsp.cpp
 #include <stdlib.h>
 #include "constants.h"
 #include "specificMath.nsp.h"

 namespace specificMath{
     double math_function1(double a, double b, double c){
          //some code
     }
     ... more functions
 }

When I try to compile, it seems to compile fine, but when linking (and I’ve been doing “make clean” to make sure it’s using the new files) I get an error saying:

margin.o: In function `margin::doSomeMath(double, double)':
margin.cpp:(.text+0x3d): undefined reference to `(anonymous namespace)::math_function1(double, double, double)'

Why does it think it’s an anonymous namespace? How can I fix this?

I compile doing this:

g++ -I. -c -w *.h *.cpp

And then…

g++ -o myProgram *.o 

Answer 1

You #defined the namespace name away. After the preprocessor sees #define specificMath, then it finds all instances of specificMath after that and replaces them with what you #defined it to, which in this case, is nothing. So it simply eliminates it.

#ifndef specificMath
#define specificMath
namespace specificMath {

After the preprocessor runs

namespace {

Always use all capitals for macros, and never prepend underscores to them.

#ifndef SPECIFIC_MATH_FUNCTIONS

for example.