I wrote the code trying to serching a word in a string, for I was practicing pointer in C++.
first time when I tried this code is fine, but I wanted it to become decoupling and to practice passing pointer into function.
I then wrote some global function trying to perform the same things, but the concole kept saying things like the title.

I put two versions below, first is the version that cannot be perform.
please give me some advice or corrections about this porblem or programming style.

I post a question for the first time, if the highlightint doesn’t work please pardon me.

#include <iostream>
#include <cstring>
using namespace std;

int serching(char*a , const char* b, const int n); // repeating serching desginated word in the ariticle.

int target(char*& a, int& b); // Let users enter a word.

int main()
{
    int timesA = 1; //recording the numbers of total words in article.
    int timesD = 0; //recording the numbers of designated words.
    const int maxlength = 10000;
    char a[maxlength];
    
    cout << "Please enter the paragraph less than " << maxlength <<  " characters. " << endl;
    
    cin.getline(a, maxlength);
    size_t l = strlen(a);
    
    cout << "Total numbers of characters: " << l << endl;
    
    char* aptr = new char [l]; 
    for(int i = 0; i < l; i++)
    {
        aptr[i] = a[i];
    }
    
        int n;
        char* desg;
        int num = target(desg, n);
        serching(aptr, desg, num);
        
        delete[] desg;
        desg = 0;
        
        cout << "There are " << timesD << " designated words found in this paeagraph. " << endl;
    
        
    delete[] aptr;
    aptr = 0;
    
    return 0;
} 

int serching(char*a , const char* b, const int n) 
{
    int num = 0;
    char* i  = strstr(a, b);
    while(i != 0)
    {
        num++;
        i = strstr((i + n), b);
    }
    delete[] i;
    i = 0;
    return num;
}

int target(char*& a, int& b)
{
    a = new char[b];
    cout << "Please enter a word: " << endl;
    cin.getline(a, 20);
    b = strlen(a);
    delete[] a;
    a = 0;
    return b;
}

Here is the original version which can be execute without problem.

#include <iostream>
#include <cstring>
using namespace std;

int main()
{
    int timesA = 1; //recording the numbers of total words in article.
    int timesD = 0; //recording the numbers of designated words.

    const int maxlength = 10000;    
    char a[maxlength];
    
    cout << "Please enter the paragraph less than " << maxlength <<  " characters. " << endl;
    
    cin.getline(a, maxlength);
    size_t l = strlen(a);
    
    cout << "Total numbers of characters: " << l << endl;
    
    char* aptr = new char [l]; //pass into a dynamic array, for I'm practcing this.
    for(int i = 0; i < l; i++)
    {
        aptr[i] = a[i];
    }

        int n;
        char* desg = new char[n]; 
        cout << "Please enter a word: " << endl;
        cin.getline(desg, 20); 
        n = strlen(desg);
        
        
        char* i  = strstr(aptr, desg); // repeating serching the word in the string.
        while(i != 0)
        {
            timesD++;
            i = strstr((i + n), desg);
        }
        delete[] i;
        i = 0;
        delete[] desg;
        desg = 0;
        
        cout << "There are " << timesD << " designated words found in this paeagraph. " << endl;
    
        delete[] aptr;
        aptr = 0;

return 0;
} 

I thought the problem might be leakinig memory or some value is too big, I have seen some posts that have same console error context as me, but still cannot figure it out.