How to create a static library with g++?

When creating a static library with g++, you need to follow these steps: writing source code, compiling source code into object files, and using the ar command to create the static library. Below are the detailed steps and examples:

Step 1: Write Source Code

First, write your source code. Assume we have two source files: file1.cpp and file2.cpp.

file1.cpp

cpp
// file1.cpp
#include "file1.h"
int add(int a, int b) {
    return a + b;
}

file1.h

cpp
// file1.h
#ifndef FILE1_H
#define FILE1_H
int add(int a, int b);
#endif

file2.cpp

cpp
// file2.cpp
#include "file2.h"
int multiply(int a, int b) {
    return a * b;
}

file2.h

cpp
// file2.h
#ifndef FILE2_H
#define FILE2_H
int multiply(int a, int b);
#endif

Step 2: Compile Source Code to Object Files

Use g++ to compile each source file into object files (.o files):

bash
g++ -c file1.cpp -o file1.o
g++ -c file2.cpp -o file2.o

The -c flag tells g++ to generate only object files without linking.

Step 3: Use the ar Command to Create a Static Library

Use the ar command to create a static library file. ar is a tool used for creating, modifying, and extracting files from archives, which is suitable for static library management.

bash
ar rcs libmystaticlib.a file1.o file2.o

The rcs option for the ar command:

• r: replaces existing files.
• c: creates a new archive file.
• s: creates an index (or symbol table) for the library file, which helps the linker optimize lookup during linking.

The generated libmystaticlib.a is your static library file.

Step 4: Compile a Program Using the Static Library

Now, use this static library to compile and link other programs. Assume you have a main.cpp file:

main.cpp

cpp
#include "file1.h"
#include "file2.h"
#include <iostream>

int main() {
    std::cout << "3 + 4 = " << add(3, 4) << std::endl;
    std::cout << "3 * 4 = " << multiply(3, 4) << std::endl;
    return 0;
}

Compile and link with the following command:

bash
g++ main.cpp -L. -lmystaticlib -o myprogram

The -L. option tells the compiler to search for library files in the current directory, while -lmystaticlib links the created libmystaticlib.a static library.

After performing these steps, you have successfully created a static library using g++ and compiled a program using this static library. This allows you to effectively reuse your code and share the same implementation across multiple programs.