Can adding default parameters to a library function break the import or useage?

In creating a shared library, I must implement the function void foo(); foo() has dependencies that I would like to mock in my unit tests. My initial thought is to rewrite the signature to include default parameters so that I can inject the dependencies.

One option is to use foo() as a wrapper around bar() and write unit tests for bar() like so:

void bar(SomeClass *someObject=NULL);  // test this   
void foo() { 
    bar(); 
}

However, my question is how safe is it to modify foo() to take default parameters without breaking the library import or usage of foo()?

I can do things with the processor to rewrite the signature depending on build settings(test, debug, release); however, the question still remains. I can't recall how C++ implements default parameters under the hood.