Tools to find included headers which are unused?

Question

I know PC-Lint can tell you about headers which are included but not used. Are there any other tools that can do this, preferably on linux?

We have a large codebase that through the last 15 years has seen plenty of functionality move around, but rarely do the leftover #include directives get removed when functionality moves from one implementation file to another, leaving us with a pretty good mess by this point. I can obviously do the painstaking thing of removing all the #include directives and letting the compiler tell me which ones to reinclude, but I'd rather solve the problem in reverse - find the unused ones - rather than rebuilding a list of used ones.

Answer 1

DISCLAIMER: My day job is working for a company that develops static analysis tools.

I would be surprised if most (if not all) static analysis tools did not have some form of header usage check. You could use this wikipedia page to get a list of available tools and then email the companies to ask them.

Some points you might consider when you're evaluating a tool:

For function overloads, you want all headers containing overloads to be visible, not just the header that contains the function that was selected by overload resolution:

// f1.h
void foo (char);

// f2.h
void foo (int);


// bar.cc
#include "f1.h"
#include "f2.h"

int main ()
{
  foo (0);  // Calls 'foo(int)' but all functions were in overload set
}

If you take the brute force approach, first remove all headers and then re-add them until it compiles, if 'f1.h' is added first then the code will compile but the semantics of the program have been changed.

A similar rule applies when you have partial and specializations. It doesn't matter if the specialization is selected or not, you need to make sure that all specializations are visible:

// f1.h
template <typename T>
void foo (T);

// f2.h
template <>
void foo (int);

// bar.cc
#include "f1.h"
#include "f2.h"


int main ()
{
  foo (0);  // Calls specialization 'foo<int>(int)'
}

As for the overload example, the brute force approach may result in a program which still compiles but has different behaviour.

Another related type of analysis that you can look out for is checking if types can be forward declared. Consider the following:

// A.h
class A { };

// foo.h
#include "A.h"
void foo (A const &);

// bar.cc
#include "foo.h"

void bar (A const & a)
{
  foo (a);
}

In the above example, the definition of 'A' is not required, and so the header file 'foo.h' can be changed so that it has a forward declaration only for 'A':

// foo.h
class A;
void foo (A const &);

This kind of check also reduces header dependencies.