Function already defined error in C++

Question

I have a file called "SimpleFunctions.h" defined as follow:

#ifndef SIMPLEFUNCTIONS_H
#define SIMPLEFUNCTIONS_H

namespace my_namespace {

double round(double r) { return (r > 0.0) ? floor(r + 0.5) : ceil(r - 0.5); }
float round(float r) { return round((double)r); }

}

#endif // SIMPLEFUNCTIONS_H

This file was previously included in only one file and it was working fine.

Now today I have included it in a second file and it no longer works. At link time, it tells me that the function is already defined in "firstfile.obj".

However, since I am using include guards, I would expect the functions to be defined only once, or am I missing something?

Answer 1

By default, these functions have external linkage. That means each translation unit has functions called double round(double r) and float round(float r), which causes a name collision at link time.

Some possible solutions are:

1. Declare the functions as static, which implies internal linkage
2. Inline the functions
3. Move the implementation out of the header and into a c/c++ file

Read more here: What is external linkage and internal linkage?

By the way, include guards protect a single translation unit from including a header file multiple times. That's a different issue that what you're seeing here.