This article describes how to create a simple static library using C++. A static library is a collection of object files that are linked with and copied into a target application at compile time. The resulting executable contains all the object code from the static library, so it can be executed independently of the library.

Static library

Let’s start by creating a simple square class:

// src/square.h
#ifndef SQUARE_H
#define SQUARE_H

class square {
private:
    double length;
public:
    square(double length);
    double area();
};

#endif

and its corresponding implementation:

// src/square.cpp
#include "square.h"

square::square(double length): length(length) {}

double square::area() {
    return length * length;
}

and a simple main.cpp program to use it:

// src/main.cpp
#include "square.h"
#include <iostream>

using namespace std;

int main() {
    square s(5);
    cout << "Area: " << s.area() << endl;

    return 0;
}

The following commands will compile the program in the bin directory:

mkdir -p {obj,lib,bin}
g++ -c --std=c++2a -o obj/square.o src/square.cpp
ar rvs lib/libsquare.a obj/square.o
g++ --std=c++2a -o bin/main src/main.cpp -Llib -lsquare
bin/main

1. g++ -c --std=c++2a -o obj/square.o src/square.cpp

   • This command compiles the square.cpp file using the g++ compiler.
   • -c flag tells the compiler to generate the object file (square.o) but not perform linking.
   • --std=c++2a flag specifies that the C++ language standard to use is C++20 (also known as C++2a).
   • The resulting object file square.o will be used later for linking.

2. ar rvs lib/libsquare.a obj/square.o

   • This command creates a static library named libsquare.a from the square.o object file.
   • ar is the archiver utility used to create or manipulate library files.
   • rvs are the options passed to ar, where:
     • r stands for “replace,” meaning replace or add files to the archive.
     • v stands for “verbose,” displaying a detailed output of the archive’s contents and operations performed.
     • s stands for “create an index,” which adds an index to the archive to speed up symbol lookup during linking.
   • lib/libsquare.a specifies the name and location of the resulting library file. In Unix systems it’s standard to use the lib prefix for libraries.

3. g++ --std=c++2a -o bin/main src/main.cpp -Llib -lsquare

   • This command compiles and links the main.cpp file to create an executable named main.
   • --std=c++2a flag specifies that the C++ language standard to use is C++20.
   • -o main specifies the output file name as main.
   • main.cpp is the source file to be compiled and linked.
   • -Llib flag tells the compiler to search for libraries in the lib directory.
   • -lsquare specifies that the linker should include the libsquare.a library during the linking process.