Compiling and Linking in C++

Compiling and linking in C++ are important in the process of transition from source code to an executable program. The compiling takes human-readable C++ code and turns it into a series of machine-readable instructions, while the linking stage brings all the different code modules and external libraries into a single executable program. In this article, we will discuss compiling and linking with the example, their common errors, and best practices.

Table of Contents:

• What is Compiling in C++?
• The Compilation Process
  • Preprocessing
  • Compilation
  • Assembly
• Linking in C++
• Example for Compiling and Linking in C++
  • Step 1: Write the Code
  • Step 2: Compilation
  • Step 3: Linking
  • Step 4: Run the Executable
• Common Errors While Compiling and Linking in C++
• Best Practices for Compiling and Linking in C++
• Conclusion

What is Compiling in C++?

Compiling is the procedure through which high-level code, written in any high-level programming language like C++, is translated into machine language or an intermediate code. This is essential because computers can understand binary language only, while a person can elaborate the codes in an easy human-readable language. It doesn’t directly convert the high-level language into machine code, it has a compilation process which includes the three stages of compilation, such as preprocessing, compilation, and assembling.

The Compilation Process

The compilation process in C++ has typically three main processes. Below, you will find brief details about these processes.

1. Preprocessing

Preprocessing is the first stage of the C++ compilation process. In this stage, the preprocessor handles directives that begin with #. This stage forms the source code for compilation by performing the following tasks:

• File Inclusion: It replaces the #include directives with the contents of the particular header files for the function declarations and definitions.
• Macro Expansion: It processes the #define directives by replacing macros with their defined values in the code.
• Conditional Compilation: It manages the code inclusion or exclusion based on the given conditions using directives such as #ifdef, #ifndef, and #endif.

The output of the preprocessing stage is a modified source code file that is passed to the compilation stage for further processing.

2. Compilation

Compilation is the second stage of the C++ compilation process. In this stage, the preprocessed source code is translated into assembly language. This stage has a few important tasks:

• Lexical Analysis: It is a process of breaking the source code into tokens (keywords, identifiers, literals).
• Syntax Analysis: It is the process of checking the structure of the code against language grammar and constructing a parse tree.
• Semantic Analysis: It is the process of verifying the meaning of the code for type compatibility and variable declarations.
• Optimization: It is the process of improving the efficiency of the generated assembly code by using the optimization techniques.

The output of this stage is assembly code, which is passed to the assembly stage for further processing.

3. Assembly

Assembly is the sub-step in the compilation process. In this stage, the assembly code generated in the compilation stage is translated into machine code. This stage has a few important tasks:

• Translation: It is a process in which an assembler converts assembly language into binary machine code.
• Output: It finally gives the result as an object file, which contains the machine code along with metadata such as symbol tables and relocation information.

The object file is then ready for the linking stage, where it will be combined with other object files to create an executable program.

Linking in C++

Linking happens after the compilation, Linking is the process by which one or more object files are turned into a single executable program. It involves resolving references between code modules. Linking may be static or dynamic.

• Static linking combines all object files and needed libraries into one large executable program at compile time, the executed program is large because no libraries are needed at run time.
• Dynamic linking takes place when a program links to shared libraries at run time. Such linking is allowed for a small executable size, whereby several programs may share the same library, and also allows the independent updating of the library from the executable programs.

Example for Compiling and Linking in C++

Here is a C++ example to understand the practical working of Compiling and Linking.

Consider a program with two files:

1. main.cpp (the main program)
2. math_functions.cpp (a separate source file containing a function)

Step 1: Write the Code

math_functions.h (Header File)

#ifndef MATH_FUNCTIONS_H
#define MATH_FUNCTIONS_H

int add(int a, int b); // Function declaration

#endif

math_functions.cpp (Implementation File)

#include "math_functions.h"

int add(int a, int b) {
    return a + b;
}

main.cpp (Main Program)

#include <iostream>
#include "math_functions.h"

int main() {
    int result = add(3, 5);
    std::cout << "Sum: " << result << std::endl;
    return 0;
}

Step 2: Compilation

Instead of compiling everything in one step, compile each .cpp file separately into object files.

g++ -c main.cpp -o main.o
g++ -c math_functions.cpp -o math_functions.o

Step 3: Linking

Now, link the object files together to create an executable

g++ main.o math_functions.o -o program

Step 4: Run the Executable

 ./program

Output:

The above example explains to you how compiling and linking work in C++.

Common Errors While Compiling and Linking in C++

• In C++, compiling and linking errors could arise due to various reasons like syntax errors, missing semicolons, bracket problems, and statements that are invalid in general.
• In the case of an undefined reference error, if a function or variable has been declared but not defined in the program, the error may occur.
• If the header files are missing or not included properly, then the compilation process fails.
• If the definition of the same function or variable is lying in multiple files without proper header guards, the error will show up, indicating multiple definitions exist.
• An unresolved error occurs when the Linker cannot locate the function or variable that was placed because some object file or library is missing.

Best Practices for Compiling and Linking in C++

• You can use header guards to avoid redefinition errors.
• You must test and compile small modules separately to catch errors easily and early.
• Always turn on the warnings in the compiler to detect the arising errors and potential issues.
• Always link only necessary libraries to reduce the binary size and compilation time.
• You must use the static or inline functions to reduce the linking issues and to improve efficiency.
• Always use namespace carefully to avoid the conflicting errors and ambiguities in large projects.

Conclusion

The compilation and linking process refers to changing the source code of C++ into an executable program. In addition to this, it also involves preprocessing, compilation, assembly, and linking all the modules. If you understand these steps with best practices and common mistakes that must be avoided, then you can write C++ programs more efficiently and error-free.

Compiling and linking in C++ – FAQs