C++ program help

racerday182 · 12-06-2008, 07:25 PM

Hey guys,
I was working on a program where I had to convert an old C program into a C++ program.
I am supposed to create a ciphertext class and use it to output a ciphertext that I get from a file. So far, I have written:

Code:

class Ciphertext{
private:
	char file_name[20];
	char *ciphertext[10];
public:
	Ciphertext(char file_input[20])
	{
	FILE *fp;
	char input[MAX_LENGTH+1];
	int m=0;
	strcpy(file_name,file_input);
	fp=fopen(file_name, "r");
	while((fgets(input,80,fp))!=NULL) 
		{ 	
			ciphertext[m]=malloc(strlen(input)+1);
			strcpy(ciphertext[m],input);  /**** ERROR HERE*****/
			m++;
		}
	~Ciphertext(){}

int main(void)
{
	printf("Please enter file name where the encoded text is found:");
	scanf("%s",file_name);
	while(getchar()!='\n');
	Ciphertext my_ciphertext(file_name);
}

where the member function ciphertext is my constructor function. When I try to compile my code, I am getting an error: "c2440: cannot conver from void* to char*" and it says this error occurs at the line above marked error here. This is very confusing to me because the code above worked perfectly when I was to write the program in regular C. Please help me understand where I am going wrong.
Thanks,
Andy

tabstop · 12-06-2008, 07:30 PM

In C++, calls to malloc are required to be casted to the appropriate type. One idea presumably is to encourage use of the C++ memory acquiring thing, "new", as opposed to the C memory acquiring thing. So maybe

Code:

ciphertext[m] = new char[strlen(input)+1];

This is assuming, of course, that converting C "strings" to C++ std::strings is also not part of the deal.

racerday182 · 12-06-2008, 07:57 PM

Quote:

Originally Posted by tabstop

This is assuming, of course, that converting C "strings" to C++ std::strings is also not part of the deal.

Yeah we don't have to do that

Yay! now the program compiles but it only runs for the first instance of the loop. After the program executes through the while loop the first time it gives me a 'System.NullReferenceException' error. How do I make it work for all instances of the loop?
Thanks,
Andy

rags_to_riches · 12-06-2008, 10:55 PM

You didn't check that your fopen worked (fp would be NULL if it failed); that could be it.

tabstop · 12-06-2008, 11:13 PM

NullReferenceException smells a little like .net, somehow. Anyway, the only way I can see that happening is if new fails. Check to see whether ciphertext[m] == 0 before trying to strcpy into it.

racerday182 · 12-07-2008, 02:38 PM

Quote:

Originally Posted by rags_to_riches

You didn't check that your fopen worked (fp would be NULL if it failed); that could be it.

I am doing that in my code, I just didn't show it here.

Tabstop, I have now changed my code to:

Code:

while((fgets(input,80,fp))!=NULL) 
		{ 	
			ciphertext[m]=new char(strlen(input)+1);
			if(ciphertext[m]==0)
			{
			strcpy(ciphertext[m],input);
			m++;
			}
		}

but now the if statement is never true so nothing gets stored into ciphertext[m]. For some reason my ciphertext[m] never equals 0 to start off with, why, I do not know?

rags_to_riches · 12-07-2008, 03:25 PM

You want to make sure it's != to 0.

racerday182 · 12-07-2008, 03:41 PM

Quote:

Originally Posted by rags_to_riches

You want to make sure it's != to 0.

LOL oh thanks good idea.
Okay, well that gets rid of the System.NullReferenceException but it's still only reading in the first line of the ciphertext because when it loops through, every other line has ciphertext[m]=0 so it skips over the if statement. I'm not quite sure but I feel like the loop is causing problems regarding allocating space for the ciphertext array of pointers.
Also, would any of you happen to have a screen name or gmail account so I could just ask you a few questions quickly and get out of your hair instead of posting on here repeatedly. Please let me know.
Thanks,
Andy

12-06-2008, 07:25 PM	#1
racerday182 Registered User Join Date: Nov 2008 Posts: 11	C++ program help Hey guys, I was working on a program where I had to convert an old C program into a C++ program. I am supposed to create a ciphertext class and use it to output a ciphertext that I get from a file. So far, I have written: Code: class Ciphertext{ private: char file_name[20]; char ciphertext[10]; public: Ciphertext(char file_input[20]) { FILE fp; char input[MAX_LENGTH+1]; int m=0; strcpy(file_name,file_input); fp=fopen(file_name, "r"); while((fgets(input,80,fp))!=NULL) { ciphertext[m]=malloc(strlen(input)+1); strcpy(ciphertext[m],input); /** ERROR HERE**/ m++; } ~Ciphertext(){} int main(void) { printf("Please enter file name where the encoded text is found:"); scanf("%s",file_name); while(getchar()!='\n'); Ciphertext my_ciphertext(file_name); } where the member function ciphertext is my constructor function. When I try to compile my code, I am getting an error: "c2440: cannot conver from void to char*" and it says this error occurs at the line above marked error here. This is very confusing to me because the code above worked perfectly when I was to write the program in regular C. Please help me understand where I am going wrong. Thanks, Andy
racerday182 is offline

12-06-2008, 07:30 PM	#2
tabstop and the Hat of Guessing Join Date: Nov 2007 Posts: 8,740	In C++, calls to malloc are required to be casted to the appropriate type. One idea presumably is to encourage use of the C++ memory acquiring thing, "new", as opposed to the C memory acquiring thing. So maybe Code: ciphertext[m] = new char[strlen(input)+1]; This is assuming, of course, that converting C "strings" to C++ std::strings is also not part of the deal.
tabstop is offline

12-06-2008, 10:55 PM	#4
rags_to_riches and the Hat of Ass Join Date: Dec 2007 Posts: 731	You didn't check that your fopen worked (fp would be NULL if it failed); that could be it.
rags_to_riches is offline

12-06-2008, 11:13 PM	#5
tabstop and the Hat of Guessing Join Date: Nov 2007 Posts: 8,740	NullReferenceException smells a little like .net, somehow. Anyway, the only way I can see that happening is if new fails. Check to see whether ciphertext[m] == 0 before trying to strcpy into it.
tabstop is offline

12-07-2008, 03:25 PM	#7
rags_to_riches and the Hat of Ass Join Date: Dec 2007 Posts: 731	You want to make sure it's != to 0.
rags_to_riches is offline

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