Deleting the contents of the file not working - c++

Question

I'm writing, reading and deleting the content of a file. Eeverything works fine except the delete part, as when I press y it says deleted but doesn't display any records.
typedef struct ch
{
char str[10];
};
void disp(ch d)
{
cout<<"\n"<<d.str<<"\n";
}
//delete part
cout<<"\nwant to delete??";
char c;
cin>>c;
if(c=='y')
{
char s[10];
cout<<"nter - ";
cin>>s;
file.seekg(0);
int found=0;
fstream temp("temp.dat",ios::in|ios::out|ios::app);
while(file.read((char *)&dta,sizeof(dta)))
{
if(strcmp(dta.str,s)==0)
{
found=1;
cout<<"deleted";
}
else
temp.write((char *)&dta,sizeof(dta));
}
if(!found)
cout<<"not found";
remove("new.dat");
rename("temp.dat","new.dat");
temp.close();
file.open("new.dat",ios::in|ios::out|ios::app);
}

Answer 1

EDIT: Looking over your code again, I see the problem is that you are using the ios::app although you have also passed ios::in.
ios::app -- All output operations are performed at the end of the file, appending
the content to the current content of the file. This flag can only be
used in streams open for output-only operations.
http://www.cplusplus.com/doc/tutorial/files/
Old Post:
Take a look at the following code example:
#include <iostream>
#include <fstream>
#include <cstdio>
#include <cstring>
using namespace std;
const char g_szTestData[] = "This is some test da$$$$######ta and some more"
" tes$$$$######ting";
int main(int argc, char** argv)
{
fstream file("new.dat", ios::in|ios::out);
file << g_szTestData << flush;
cout << "Do you want to delete all $$$$######?";
if (cin.get() == 'y')
{
char szBuffer[10]; //< File read buffer
char szString[11] = "$$$$######"; //< 10 characters + '\0'
bool bFound = false;
fstream temp("temp.dat", ios::out);
file.seekg(0, file.beg);
while (file.read(szBuffer, sizeof(szBuffer)))
{
if (strncmp(szBuffer,szString,10) == 0) bFound = true;
else temp.write(szBuffer, sizeof(szBuffer));
}
temp.flush();
temp.close();
if (bFound)
{
file.close();
remove("new.dat");
rename("temp.dat", "new.dat");
file.open("new.dat",ios::in|ios::out);
}
else cout << "Pattern Not Found!" << endl;
}
/* Do something with the contents of file. */
// Lets clean up at the end
file.close();
return 0;
}
In this example, a file is created and the contents g_szTestData are added, you can verify this by opening the file (before pressing 'y').
The user is then asked if they would like to delete a string of 10 characters $$$$###### from the file. If the user wishes to proceed, a new file is opened temp.dat. The program gradually walks through the existing new.dat file (10 characters at a time). If the string the program reads from new.dat is not the target string, the string is written to the temp file.
If the target string is found, both files are closed, the old file is deleted and the new file is renamed to the name of the old file. The new file is then opened so the program can do additional work on its contents.
Instead of having a fixed 10 character string, it is possible to ask the user for the string they wish to remove using cin >> szString but the string would need to be 10 characters long.

Answer 2

Your code work as expected, it only display deleted because disp() isn't called anywhere.
#include <iostream>
#include <string.h>
#include <fstream>
#include <stdio.h>
using namespace std;
typedef struct {
char str[16];
} ch;
static void disp(ch d) {
cout<<d.str<<"\n";
}
int main(int argc, char **argv) {
fstream file;
ch dta;
file.open("new.dat",ios::in|ios::out|ios::trunc);
for (int i=0; i<3; i++) {
snprintf((char *)&dta.str, sizeof(dta.str)-1, "rec%d", i);
file.write((char *)&dta,sizeof(dta));
}
//delete part
{
char s[16] = "rec1";
file.seekg(0);
int found=0;
fstream temp("temp.dat",ios::out);
while(file.read((char *)&dta,sizeof(dta))) {
if(strcmp(dta.str, s)==0) {
found=1;
cout<<"deleted * ";
disp(dta);
} else {
temp.write((char *)&dta,sizeof(dta));
disp(dta);
}
}
if(!found)
cout<<"not found";
file.close();
remove("new.dat");
rename("temp.dat","new.dat");
temp.close();
}
}