Debugging

Debugging

Debugging is the process of finding and fixing errors in your program. A systematic approach beats staring at the code for hours, and the tools in your IDE can show you exactly what the program is doing.

The debugging process

A simple process that works: find the bug, narrow the scope, and reproduce the circumstances around it.

int main() {
    double x = 10.0;
    double y = 0.0;
    cout << "x / y = " << x / y;
}

Enter a zero for y and you get a weird value. The first debugging step is to make the problem happen again, then inspect the values that flow through the calculation.

Printing values as a quick check

The oldest and simplest trick: print intermediate values, then remove them later.

int main() {
    int sum = 0;
    for (int i = 1; i < 5; i++) {
        sum += i;
        cout << "sum after " << i << " = " << sum;   // debug print
    }
}

A quick cout next to a statement lets you see whether each step produces the number you expected. Remove those lines before you ship.

Breakpoints: pausing at a line

A breakpoint tells the debugger "stop right here". When the run reaches the marked line, the program pauses and waits for you.

Stepping: run one step at a time

Once paused, you control the execution:

• Step into dives into a function call and runs it line by line.
• Step over runs the whole function call without going inside.
• Continue keeps running until the next breakpoint.

Watch window and call stack

While paused, a watch window shows the value of a named variable. The call stack is the list of active functions at that moment, in order. Together they answer "what is this variable right now?" and "how did the program get here?"

TL;DR

• Debugging is the process of finding and fixing errors.
• Use printing to inspect intermediate values quickly.
• A breakpoint stops the program at a chosen line.
• Step into dives in, step over hops a function, continue resumes.
• A watch shows a value; the call stack shows how you arrived.