Hi: I want to write a function of strcat. the code as fellow. but i have a question.if the capacity of firsttr is smaller than the capacity between firststr and secondstr.how to deal with it clearly?

char* StrContact(char *firststr, const char *secondstr)
{
	char *tempstr;

	int i, j;
	i = strlen(firststr);
	j = strlen(secondstr);
	tempstr = firststr;
	while(*tempstr != '\0')
	{
		tempstr++;
	}

	while(*secondstr != '\0')
	{
		*tempstr = *secondstr;
		secondstr++;
		tempstr++;
	}

	return firststr;
}

for example: char a[6] = "fd34"; StrContact(a, "world!"); cout << a << endl; how to deal with the it automatically? thanks!

If you're using cout, then you're using C++, in which case you need to ask yourself why you're not using std::string.

Quote:Original post by huaner
Hi:
I want to write a function of strcat. the code as fellow. but i have a question.if the capacity of firsttr is smaller than the capacity between firststr and secondstr.how to deal with it clearly?

You can't, really. The real strcat() doesn't either. The problem is that there is actually no way, inside the function, to find out what the "capacity" actually *is*.

The library also provides strncat(), which takes another parameter that says what the capacity is for the firststr. But then the caller has to get it right; strncat() will believe whatever it is told, because it can't check.

For these reasons and many more, C++ programmers will use the standard library type 'std::string' to represent text data. It provides many of its own functions for handling text, which are safe (because the data structure holds a pointer to some allocated memory, keeps track of the capacity, and automatically manages the memory - e.g. if the appended text will not fit, it re-allocates, puts stuff into the new memory block, and releases the old memory block).

Doing these things by yourself is a lot of work; but in C, it's probably the best way to make sure you don't have any memory problems. (You will also still need to clean up your "strings" manually when they go out of scope, because C does not have 'destructors'.) In C++, you should not re-invent the wheel; use std::string.