Question

So my assignment is:

Using the strncpy and strncat functions in #include<cstring>,
implement a function

void concat(const char a[ ], const char b[ ], char result[ ],
                 int result_maxlength)

that concatenates the strings a and b to the buffer result. Be sure
not to overrun the result. It can hold result_maxlength characters,
not counting the \0 terminator. (That is, the buffer has
buffer_maxlength + 1 bytes available.) Be sure to provide a ‘\0’
terminator.

My solution (thus far) is below but I don’t know what I’m doing wrong. Not only do I get a run-time check failure 2 error when I actually run the program, but I’m unsure where I should be adding the \0 terminator or even if I should be using strncat rather than strncpy. Hopefully someone can lead me in the right direction. And yes this is hw. That’s why I said just lead me in the right direction so that I can try to figure it out :p

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

void concat(const char a[ ], const char b[ ], char result[ ], int result_maxlength);

int main()
{
    char a[] = "Woozle";
    char b[] = "Heffalump";
    char c[5];
    char d[10];
    char e[20];

    concat(a, b, c, 5);
    concat(a, b, d, 10);
    concat(a, b, e, 20);
    cout << c << "\n";
    cout << d << "\n";
    cout << e << "\n";

    return 0;
}

void concat(const char a[ ], const char b[ ], char result[ ], int result_maxlength)
{    
    strncat(result, a, result_maxlength);
    strncat(result, b, result_maxlength);
}

Answer 1

1. strncpy from a to result (whatever is smaller, lenght of a or result_maxlength)

2. strncat from b to remaining of result (whatever is smaller, lenght of b or result_maxlength- lenght of a)

3. before every return just put a \0 at last position result[result_maxlength-1] ='\0';

It’s actually not specified WHAT to do if result is too short, should you add trailing 0 or not. I guess you’d better terminate that string.

tip : remaining of result is result+strlen(a)