problem with strcat

Hello im kinda new with C++ and im trying to have a program that reads in a keyboard press while the programs continues to run. i seem to have that part of it ok but im trying to create a string with each new key pressed but i get a error that i cant seem to get around. Any help would be good

Code:

#include <iostream> #include <conio.h> #include <ctype.h> //please do not worrry about all these headers #include <cstring> // i have been trying different commands #include <stdio.h> using namespace std; int main(void) { char string[256]; int kb; while(kb!= 27) { if (_kbhit()) { kb =_getch(); _putch(kb); if (kb != 13) { string[0] = '\0'; char c = char(kb); strcat(string, c); // problem is here. i get a error which is //" cannot convert parameter 2 from 'char' to 'const char *'" } } } }

You use strcat like this:

strcat(string1, string2)

you are trying to code

strcat(string1, char1)

I know you made a comment about the headers, but I will say something anyways. This is the C++ forum, so I imagine you're intending to write a "C++" program, but your headers are both C++ and C. Also, if you're using C++, working with the "string" type is generally easier than working with "char*" or "char[]".

As stated, "strcat" requires a "char*". When concatenating strings its good to make sure not to go out of bounds. The function "strncat" is used for this, and

Code:

strncat(strDest, strSrc, 5);

will append up to 5 characters from strDest to the end of strSrc. In your case you only want to append one character, so you'd pass 1. But you still have a "char" and you need it to be a "char*", so you could pass "&c" to get its address. Alternatively, you could just create a "char*" and assign it the address of the char. Using C++ "string"s, it'd be as easy as "myString += myChar;"

Next, to do more than one thing at the "same" time, you'd need to make your program multi-threaded. One thread to read the input, and the other thread to do your other task. This concept requires reading a tutorial, and cant be easily explained in a single post here.

ok i have strcat with 2 strings but i cant convert the keypress into a string, i changed

Code:

. char c = char(kb);

to

Code:

char c[1] = char(kb);

and that one gives an error now.
i guess what im asking is can it be changed into the proper format or do i have to change how i get my keypress and if so what command should i use.

thanks

Instead of

char c

try

char c[2]

and init c[1] to '\0'.

Then, everywhere you are using c, change it to use c[0]. You can then strcat(string,c).

thanks

Thanks i seem to have it working how i want now, the strcat(string, &c) seems to work well

I think that strcat is the wrong approach here: you are trying to append a character, not concatenate a string, and besides, when used in a loop like this it becomes slow. Rather, keep track of the current character (e.g., by storing the current string index), and assign to it accordingly. This also makes it easier for you to check that you are within the bounds of the array.

Quote:

Originally Posted by morngrym

Thanks i seem to have it working how i want now, the strcat(string, &c) seems to work well

That will not work since strcat required a NULL terminated string. The address of c does not contain the NULL terminated string. Most likely, your code will crash.

Code:

#include <iostream> #include <conio.h> #include <ctype.h> //please do not worrry about all these headers #include <cstring> // i have been trying different commands #include <stdio.h> using namespace std; int main(void) { char *p, *endp; char string[256]; char kb; memset(string, 0, sizeof(string)); p = string; pend = p + (sizeof(string) - 1); kb = 0; while(kb!= 27 && p != endp) { if (_kbhit()) { kb =_getch(); _putch(kb); if (kb != 13) { *p++ = kb; } } } }

morngrym, what NightCode said about using "strcat" with a char is correct. However, if you notice in my post above, you'll see I suggested "strncat", so that you can specify (at most) how many characters are copied. If you're using the function with a char, then you specify to copy one character.

since this is the C++ forum, and not the C forum, why not just use std::string? is this a class assignment where you are not allowed to use std::string?

Quote:

Originally Posted by nadroj

However, if you notice in my post above, you'll see I suggested "strncat", so that you can specify (at most) how many characters are copied. If you're using the function with a char, then you specify to copy one character.

Yeah, but frankly that sounds like a hack, is still needlessly inefficient, and does not solve the existing buffer overflow problem (i.e., the loop still needs to check that the number of characters entered does not exceed the maximum length of the string).

I really do not see why there is a fixation with strcat and strncat when the aim is to append character by character.

Quote:

Originally Posted by Elkvis

since this is the C++ forum, and not the C forum, why not just use std::string?

Yes, this will avoid the buffer overflow problem, as well as the need to append the null character after you are done with my suggestion (if you have not initialised the string buffer with null characters).

I agree that its a sneaky workaround, and it comes with potential caveats. I also agree, as I stated earlier, that he/she should be probably be using C++ "string"s instead of managing "char[]"s manually. A key principle of programming is reuse, and in OOP that is object reuse: use "string"s because they were written so you don't have to!

Using strncat() with an allowed length of 1 to eliminate the need for a trailing zero byte is an exceedingly ugly hack. While it will technically work, such things are also an open invitation for some future programmer - including yourself - to misunderstand what's going on, and make dangerous changes.

On other things in the code provided in the OP, this

Code:

int kb; while(kb!= 27)

formally gives undefined behaviour (accessing the value of an uninitialised int). In practice, unless you are unlucky enough that an uninitialised int gets the value 27, this will work. But it is better to explicitly initialise kb to be some value other than 27. You're less likely to get in trouble if you make simple changes to eliminate a potential problem rather than assuming you will always get lucky.

Also, be aware that <conio.h>, _kbhit(), _getch(), and _putch() are generally specific to a particular compiler - and not guaranteed to exist with other compilers.