Deleting the first element in a structure - c++

Question

I have a code where I'm trying to delete the first id in a structure. It used to work, but now it just returns "ЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭ" (repeated russian letter). I tried reinstalling VS, didn't help. I know it works because I tried it on an online compiler.
These two warnings also point to the delete function, which I'm guessing is the problem.
Warning C4156 deletion of an array expression without using the array form of 'delete'; array form substituted
Warning C4154 deletion of an array expression; conversion to pointer supplied
Here is the code itslef:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <conio.h>
#include <locale.h>
#include <iostream>
#pragma warning (disable: 4703) //disables warning of uninitialized variable j
using namespace std;
#define MAXDL 9
struct el_sp
{
char id[MAXDL];
struct el_sp* sled;
};
void vkl(struct el_sp** p, char t_id[]) //enters the entered ID's from keyboard into the struct
{
struct el_sp* pt,
* k, * j;
pt = (struct el_sp*)malloc(sizeof(struct el_sp));
strcpy_s(pt->id, t_id);
if (*p == NULL || strcmp(pt->id, (*p)->id) < 0)
{
pt->sled = *p; *p = pt;
}
else
{
k = *p;
while (k != NULL && strcmp(pt->id, k->id) >= 0)
{
j = k; k = k->sled;
}
j->sled = pt; pt->sled = k;
}
}
void pech_sp(struct el_sp* p) //prints the struct
{
struct el_sp* i;
char* o;
printf("\Result:\n");
for (i = p; i != NULL; i = i->sled)
puts(i->id);
}
int main() {
setlocale(LC_ALL, "RUS");
struct el_sp* p;
unsigned n;
unsigned i;
char t_id[MAXDL];
printf("\nEnter the amount of identificators\n n=");
scanf_s("%u", &n);
getchar();
p = NULL;
printf("Enter the identificators (press enter after each one)\n");
for (i = 1; i <= n; i++)
{
gets_s(t_id);
vkl(&p, t_id);
}
delete p->id;
pech_sp(p);
return 0;
}
P.S. Delete does the same thing whatever i try, in any code
P.S.S. Sorry for bad formatting, it's the way our prof needs it

Answer 1

Don't call delete p->id. It serves no purpose except to crash your program: neither it nor the struct it is in was allocated by new.
Also, never use delete to try to free malloced memory, or free to free newed memory.
Did you want to delete the entire first node? Then do something like
el_sp *oldp = p;
p = p->next;
free(oldp);
Deleting a node in the middle is actually a bit easier. Use "chasing pointers" that point to the previous element (prev) and the current element (cur). If cur is the node you want to delete, simply do
prev->next->next = cur->next;
delete cur;
(Assuming you allocate nodes with new, which you should!)

Answer 2

I understand that sometimes professors want things their way, but it's just as important to know what would be the most straightforward way to achieve your goals.
That's what I came up with. Note that there's no manual memory management, the sorting is automated, and you can easily replace the container type. In most cases the vector will be perfectly adequate - the list should only be used if the benchmarks show that you get better performance. In most cases - you won't (contrary to what professors may tell you: trust reality over teachings).
#include <algorithm>
#include <clocale>
#include <iostream>
#include <list>
#include <vector>
struct El_Sp
{
std::string id;
};
#if 1
using El_Spy = std::vector<El_Sp>;
#else
using El_Spy = std::list<El_Sp>;
#endif
template <> struct std::less<El_Sp>
{
bool operator()(const El_Sp &l, const El_Sp &r) const
{ return l.id < r.id; }
};
std::istream &operator>>(std::istream &in, El_Sp &el_sp)
{
return in >> el_sp.id;
}
std::ostream &operator<<(std::ostream &out, const El_Sp &el_sp)
{
return out << el_sp.id;
}
std::ostream &operator<<(std::ostream &out, const El_Spy &el_spy)
{
for (auto &el : el_spy)
out << el << '\n';
return out;
}
template <typename Container, typename T, typename Pred = std::less<typename Container::value_type>>
auto insert_sorted(Container &cont, T &&item, Pred pred = {})
{
return cont.insert(
std::upper_bound(std::begin(cont), std::end(cont), std::as_const(item), pred),
std::forward<T>(item));
}
void enter_id(El_Spy &el_spy)
{
El_Sp el;
std::cin >> el;
insert_sorted(el_spy, el);
}
int main()
{
El_Spy el_spy;
std::setlocale(0, "");
size_t n = 0;
std::cout << "Введите количество идентификаторов n=";
std::cin >> n;
std::cout << "Введите идентификаторы. Нажмите Enter после каждого.\n";
while (n--)
enter_id(el_spy);
std::cout << "Идентификаторы:\n" << el_spy;
}
Example session:
Введите количество идентификаторов n=3
Введите идентификаторы. Нажмите Enter после каждого.
Алла
Дарья
Вера
Идентификаторы:
Алла
Вера
Дарья
When using std::vector, the insertion sort isn't really necessary - you could use std::sort after all identifiers have been entered:
int main()
{
std::setlocale(0, "");
size_t n = 0;
std::cout << "Введите количество идентификаторов n=";
std::cin >> n;
El_Spy el_spy(n);
std::cout << "Введите идентификаторы. Нажмите Enter после каждого.\n";
for (auto &el_sp : el_spy)
std::cin >> el_sp;
std::sort(std::begin(el_spy), std::end(el_spy), std::less<El_Sp>());
std::cout << "Идентификаторы:\n" << el_spy;
}

Answer 3

struct el_sp
{
char id[MAXDL];
struct el_sp* sled;
};
id is a statically allocated array. It cannot be deleted.
delete p->id;
The above is wrong. Deleting a pointer whose memory is not allocated by new leads to undefined behaviour.
You should have
delete p;
And after deletion, you should not use p. So pech_sp should be called before rather than after.
pech_sp(p);
delete p;