C++ Debugging

🐞 C++ Debugging

Debugging is the process of finding, understanding, and fixing errors (bugs) in a C++ program.
Good debugging skills save time, effort, and frustration.

🔹 1. Types of Bugs You Debug

  1. Compile-time bugs – syntax & type errors

  2. Run-time bugs – crashes, segmentation faults

  3. Logical bugs – wrong output, no crash

🔹 2. Read Compiler Errors Carefully (Most Important)

Always fix the first error first.

int main() {
int x = 10
cout << x;
}

Error:

expected ';'

✔ Fix the missing semicolon first.

🔹 3. Enable Compiler Warnings

Warnings help catch hidden bugs.

g++ -Wall -Wextra -pedantic program.cpp

🔹 4. Use cout for Quick Debugging

cout << "Value of x: " << x << endl;

✔ Simple
✔ Effective for small programs

🔹 5. Debug with a Debugger (GDB)

Compile with Debug Symbols

g++ -g program.cpp

Common GDB Commands

Command Purpose
break main Set breakpoint
run Start program
next Next line
step Step into function
print x Print variable
backtrace Show call stack
continue Resume execution

🔹 6. Debugging Runtime Errors

🔸 Segmentation Fault

Common causes:

  • Dereferencing null pointer

  • Accessing out-of-bounds array

  • Using deleted memory

int *p = nullptr;
cout << *p; // ❌ segfault

✔ Fix:

if (p != nullptr)
cout << *p;

🔹 7. Debugging Logical Errors

Program runs but output is wrong.

int sum = 0;
for (int i = 1; i <= 5; i++)
sum += i;

cout << sum; // expected 15

✔ Trace values step by step

🔹 8. Use Assertions (assert)

#include <cassert>

int divide(int a, int b) {
assert(b != 0);
return a / b;
}

✔ Stops program if condition fails
✔ Useful during development

🔹 9. Check Memory Issues (Advanced)

Common Memory Bugs

  • Memory leaks

  • Dangling pointers

  • Double delete

Tools

  • Valgrind (Linux)

  • AddressSanitizer

g++ -fsanitize=address program.cpp

🔹 10. Debugging with IDEs

Most IDEs provide:

  • Breakpoints

  • Step execution

  • Variable watch

  • Call stack view

Popular IDEs:

  • Visual Studio

  • Code::Blocks

  • CLion

  • VS Code

🔹 11. Rubber Duck Debugging 🦆

Explain your code line by line (even to an object).
Often, the bug reveals itself.

🔹 12. Best Debugging Practices

✔ Write small, testable functions
✔ Test frequently
✔ Use meaningful variable names
✔ Avoid large functions
✔ Use version control (Git)

🔁 Debugging vs Testing

Debugging Testing
Find bugs Detect bugs
After failure Before release
Manual Automated

📌 Summary

  • Debugging is a core programming skill

  • Use compiler warnings & debuggers

  • Print values and trace logic

  • Fix memory errors carefully

  • Practice makes debugging faster

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