I am working on a project that uses a linkedList
class that I have written. Each node in the list contains a string which seems to be overwritten when there is any function call outside of the object. The variable is private and is only ever set within the constructor. Here is a stripped down version of the code which reproduces the error:
linkedList.h:
#include <cstring>
#include <iostream>
class Node{
private:
std::string value;
Node* prev_node;
public:
Node* next_node = nullptr;
Node(std::string value_in, Node* prev){
this->value = value_in;
this->prev_node = prev;
this->next_node = nullptr;
}
void print_value(){
std::cout << "value = " << this->value << '\n';
}
void print_prev(){
std::cout << "prev = " << this->prev_node << '\n';
}
Node* get_prev(){
return this->prev_node;
}
};
class linkedList{
public:
Node start_node = Node((std::string) "", (Node*) nullptr);
Node* current_node = &start_node;
void prepend_node(std::string value){
Node new_node = Node(value, &start_node);
new_node.print_value();
new_node.next_node = start_node.next_node;
start_node.next_node = &new_node;
}
void return_to_start(){
current_node = &start_node;
}
void scroll_up(){
if(current_node->next_node){
current_node = current_node->next_node;
}
}
void scroll_down(){
if(current_node->get_prev()){
current_node = current_node->get_prev();
}
}
};
(One thing of note here is that the prepend_node
function will insert a node at position 2 of the list. This is intentional)
main.cpp:
#include <iostream>
#include "linkedList.h"
int main(){
linkedList foo;
foo.prepend_node("value1");
foo.prepend_node("value2");
foo.current_node->next_node->print_value();
foo.scroll_up();
foo.current_node->print_value();
foo.scroll_up();
foo.current_node->print_value();
return 0;
}
output:
value = value1
value = value2
value =
value =
Segmentation fault
Some gdb output:
(gdb) b linkedList.h:20
Breakpoint 1 at 0x101e: file linkedList.h, line 20.
(gdb) run
Starting program: /path/to/main
Breakpoint 1, Node::print_value (this=0x7fffffffdd50) at linkedList.h:20
20 std::cout << "value = " << this->value << '\n';
(gdb) p this->value
$1 = "value1"
(gdb) c
Continuing.
value = value1
Breakpoint 1, Node::print_value (this=0x7fffffffdd50) at linkedList.h:20
20 std::cout << "value = " << this->value << '\n';
(gdb) c
Continuing.
value = value2
Breakpoint 1, Node::print_value (this=0x7fffffffdd50) at linkedList.h:20
20 std::cout << "value = " << this->value << '\n';
(gdb) c
Continuing.
value =
Breakpoint 1, Node::print_value (this=0x7fffffffdd50) at linkedList.h:20
20 std::cout << "value = " << this->value << '\n';
(gdb) this->value
Undefined command: "this->value". Try "help".
(gdb) c
Continuing.
value =
Breakpoint 1, Node::print_value (this=0xa00000000) at linkedList.h:20
20 std::cout << "value = " << this->value << '\n';
(gdb) c
Continuing.
Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7b735b0 in std::basic_ostream<char, std::char_traits<char> >& std::operator<< <char, std::char_traits<char>, std::allocator<char> >(std::basic_ostream<char, std::char_traits<char> >&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) ()
from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
The issue is pretty stupid, I forgot to use new
. Below is a fixed version of prepend_node
that works:
void prepend_node(std::string value){
Node* new_node = new Node(value, &start_node);
new_node->print_value();
new_node->next_node = start_node.next_node;
start_node.next_node = new_node;
}
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