Are unnecessary include files an overhead?

Question

It does not affect the performance of the binary or even the contents of the binary file, for almost all headers. Declarations generate no code at all, inline/static/anonymous-namespace definitions are optimized away if they aren't used, and no header should include externally visible definitions (that breaks if the header is included by more than one translation unit).

As @T.C. points out, the exception are internally visible static objects with nontrivial constructors. iostream does this, for example. The program must behave as if the constructor is called, and the compiler usually doesn't have enough information to optimize the constructor away.

It does, however, affect how long compilation takes and how many files will be recompiled when a header is changed. For large projects, this is enough incentive to care about unnecessary includes.

Besides the obviously longer compile times, there might be other issues. The most important one IMHO is dependencies to external libraries. You don't want your program to depend on more libraries then necessary.

You also then need to install those libraries in every system you want to the program to build on. This can become a nightmare, especially when the next programmer needs to install some database client library although the program never uses a database.

Also, especially library headers often tend to define macros. Sometimes those macros have very generic names which will break you code or which are incompatible with other library headers you might actually need.

Of course any #include is an overhead. The compiler needs to parse that file.

So avoid them. Use forward declarations where ever possible.

It will speed up compilation. See Scott Myers book on the subject

The simple answer is YES its an overhead as far as the compilation is concerned but for runtime it is merely going to create any difference. Reason being lets say you add #include <iostream> (just for example) and assume that you are not using any of its function then g++ 4.5.2 has some additional 18,560 lines of code to process(compilation). But as far as the runtime overhead is concerned I hardly think that it creates a performance issue.

You can also refer Are unused includes harmful in C/C++? where I really liked this point made by David Young

Any singletons declared as external in a header and defined in a source file will be included in your program. This obviously increases memory usage and possibly contributes to a performance overhead by causing one to access their page file more often (not much of a problem now, as singletons are usually small-to-medium in size and because most people I know have 6+ GB of RAM).