Pretty printer – Own type

Example code

struct List
{
    struct Node
    {
        char val{};
        Node* next{};
    };
    Node* first{};
    Node* last{};

    void add(char val)
    {
        Node* node = new Node;
        node->val = val;

        if (!first) { first = node; }
        if (last) { last->next = node; }
        last = node;
    }
};

int main()
{
    List list;
    list.add('A');
    list.add('B');
    list.add('C');

    return 0;
}

---

Without pretty printer

(gdb) b 29
(gdb) r
Breakpoint 1, main () at main.cpp:29
29		return 0;
(gdb) p list
$1 = {first = 0x55555556b2b0, last = 0x55555556b2f0}

---

With pretty printer

import gdb

class ListPrinter:
    def __init__(self, val):
        self.val = val

    def to_string(self):
        elements = []
        node = self.val['first']
        while node:
            char_val = chr(int(node['val']))
            elements.append(f"{char_val}")
            node = node['next']
        return "[ " + ", ".join(elements) + " ]"

def register_printers(obj):
    if obj is None:
        obj = gdb
    obj.pretty_printers.append(lookup_printer)

def lookup_printer(val):
    if str(val.type) == 'List':
        return ListPrinter(val)
    return None

register_printers(None)

(gdb) source list_printer.py
(gdb) b 29
(gdb) r
Breakpoint 1, main () at main.cpp:29
29	    return 0;
(gdb) p list
$1 = [ A, B, C ]