What's the difference between a header file and a library?

Question

Think of both like this (Disclaimer: this is a really high-level analogy ;) ..

• The header is a phone number you can call, while...
• ...the library is the actual person you can reach there!

It's the fundamental difference between "interface" and "implementation"; the interface (header) tells you how to call some functionality (without knowing how it works), while the implementation (library) is the actual functionality.

Note: The concept is so fundamental, because it allows you flexibility: you can have the same header for different libraries (i.e. the functionality is exactly called in the same way), and each library may implement the functionality in a different way. By keeping the same interface, you can replace the libraries without changing your code.

And: you can change the implementation of the library without breaking the calling code!

A header file is generally used to define an interface or set of interfaces within an application. Think of a header file as something which shows the external functionality of a program while omitting the technical implementation details.

For example, if you were optimising a program, you would most likely modify the source (.cpp) file to improve the algorithm, but the header file wouldn't change, because external clients still call the methods using the same set of parameters and return values.

In an object-oriented language like C++, a header file generally includes the following:

• Class description and inheritance hierarchy
• Class data members and types
• Class methods

While there is nothing stopping code from being implemented in a header file, this is generally not favoured as it can introduce extra coupling and dependencies in the code.

In some cases (e.g. templated classes) the implementation must be defined in the header file for technical reasons.

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A library is a collection of code which you want to make available to a program or group of programs. It includes the implementation of a particular interface or set of interfaces.

Code is defined in a library to prevent code duplication and encourage re-use. A library can be statically-linked (.lib) or dynamically-linked (.dll):

• A statically-linked library defines a set of export symbols (which can be thought of as method definitions) which are then linked into the final executable (.exe) during the linking stage of the build process. It has the advantage of faster execution time (as the library doesn't need to be dynamically loaded), at the expense of a larger binary (because the methods are essentially replicated in the executable file).

• A dynamically-linked library is linked during the execution of a program, rather than the linking of a program. It is useful when multiple programs need to re-use the same methods, and is used extensively in technologies such as COM.

One thing that may be confusing you is that the word library can have several meanings in C++. One meaning has been well-discussed here:

A linkable set of functions in a binary file. These can be statically linked or dynamically linked.

But there is another type of library: so-called header-only libraries (including parts of STL, TR1 and Boost). These do not exist in a separate binary form so the word library does not refer to a particular binary file but rather to a set of included header files.

Hope this helps.

Header only contains the declaration, whereas libraries also contains the implementaion.