Calling an "external" function from a class method

I have a function that I want to call from within a class method. The function is in a file called mergeSort.cpp. Here is a snippet of the .cpp file that the class is implemented in:

// other includes
#include "mergeSort.cpp"

// other methods
void Servers::sortSites() {
 mergeSort(server_sites.begin(), server_sites.end(), siteCompare);
}
// remaining methods 

When I try to compile I ge开发者_开发技巧t errors saying that mergeSort can't be found. I think this is because it's trying to call Servers::mergeSort. How would I go about calling an external function?

You have to use the "::" external namespace resolutor:

::mergeSort(...);

This tells the compiler to look for the function in the outer namespace. If this particular function is defined in another namespace or class, you have to specify it explicitly:

Namespace::mergeSort(...);

If you don't want to have to resolve the name completely each time you use it, you can import the name into the current namespace by either using:

using namespace Namespace;

or

using Namespace::mergeSort;

(where Namespace is the name in which mergeShort() is defined).

There seem to be a couple of issues here. Firstly, does Servers::mergeSort actually exist? You've speculated that it's looking for that, but you haven't actually said that there is such a thing. If there isn't, that's not the problem. In that case, a likely reason it can't see mergeSort would be that it's not in the global namespace (as other answers have speculated). If Servers::mergeSort does exist, then see Diego's answer.

A separate issue is -- are you including the .cpp file (which is generally a bit odd) because mergeSort is a template? If no, you should probably be including the accompanying .h I guess. If yes, a more usual pattern is to include the file with the template code in the header, like so:

// mergeSort.h
// <Begin include guard

// <Lots of header stuff>

#include "mergeSort.tpp"

// <End include guard>

Then you include mergeSort.h elsewhere, and it's one thing less for clients to remember.

Check if mergeSort() is in a particular namespace.