Thread: Functions and pointers.

I'm not sure if the following piece of code should work ok in all conditions so if someone can check the code and see if its right I would appreciate it very much.

Code:

float* returnFloatValue(int block, char *key)
{
	char *tmp = block_value(block, key);
	float *tmpFloat;

	if (strlen(tmp))
	{
		tmpFloat = atof(tmp);
		return &tmpFloat;
	}

	return NULL;
}

PS: If you wonder why I ask, its because the only rule I remember for that kind of function is that you should never return a pointer to a local variable.

Originally Posted by Micko

1. atof returns double and that means that variable on the left side of = must be double, for instance

Code:

double x = atof(...);

I don't need the extra presition given by atof so thats why I do the cast to float in my lastest attempt.

Originally Posted by Micko

2. You shouldn't return pointer to a local object, it's a disaster. Return variable by value.
In that case function prototype should look like this:

Code:

double foo(...);

That's why I created tmpFloat, I allocate the momory for that variable and I will get rid of it at the end of the program to avoid a memory leak.

Can you see any error in my lastest attempt?

It's global variables that you shouldn't return pointers to, not local...

Originally Posted by samGwilliam

It's global variables that you shouldn't return pointers to, not local...

From Bjarne Stroustrup's The C++ Programming Lenguage book:

Each time a function is called, a new copy of its arguments and local (automatic) variables is created. The store is reused after the function returns, so a pointer to a local variable should never be returned. The contents of the location pointer to will change unpredictably:

Code:

int* fp() { int local = 1; /* ... */ return &local; } // bad

This error is less common than the equivalent error using references:

Code:

int& fr() { int local = 1; /* ... */ return local; } // bad

Fortunately, a compiler can easily warn about returning references to local variables.

Oh yeah, I got it wrong. I read somewhere though that you shouldn't return global variables....

well you don't have to return global variables. they're global!