Static and Dynamic Libraries

Static and Dynamic Libraries

Very little C++ ships as a single huge file. Reusable compiled code is packed into libraries, either static or dynamic.

What a library is

A library is precompiled, reusable code. Instead of rewriting math, strings, or containers each time, you link against a library that already has it.

Static libraries

A static library is archived object code:

• Windows: .lib
• Linux/macOS: .a

At build time, the linker copies the needed code into your executable. The result is self-contained, but every binary duplicates the shared code.

ar rcs libmymath.a mymath.o
g++ main.cpp libmymath.a -o app

Dynamic libraries

A dynamic library is loaded at runtime:

• Windows: .dll
• Linux: .so
• macOS: .dylib

Many program share one copy in memory, so disk and RAM use go down. The catch: that shared copy must exist when the program runs.

g++ -shared -fPIC mymath.cpp -o libmymath.so
g++ main.cpp -lmymath -o app

Linking

Linking joins your object files with the libraries you need. Static linking bakes the code in; dynamic linking leaves a reference the loader resolves at runtime. The standard library is usually freestanding, but your machine's runtime provides its dynamic copy.

Why use libraries

• Reuse: write once, use in many programs.
• Update: fixing one DLL fixes every app using it.
• Smaller executables with dynamic linking.
• Faster builds, because library code rarely recompiles.

TL;DR

• Libraries bundle precompiled reusable code.
• Static (.lib/.a): copied into the executable at link time.
• Dynamic (.dll/.so/.dylib): loaded at runtime, shared.
• Static is self-contained; dynamic saves space but needs the library present.
• Linking is the step that joins your code and the library.