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.

Quote:

---

Originally Posted by Hulag

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.

---

function atof() returns double, not a pointer to float!!!

:eek: :eek:
So how should my function look like to work right?

Here is my attempt:

Code:

---

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

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

        return NULL;
}

---

Is it correct?

No,
I'm not sure what are you trying to do but you must know this:
1. atof returns double and that means that variable on the left side of = must be double, for instance

Code:

---

double x = atof(...);

---

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(...);

---

Quote:

---

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.

Quote:

---

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?

Sorry, I didn't see that you allocated memory on the heap. It's seems OK now. You allocate memory store value and then transfer ownership to caller. Now caller responsibility is to deallocate memory properly.
I' d seriously consider other alternatives, but it's up to you!

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

Quote:

---

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:

Quote:

---

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.

---

No... Returning pointers to global variables is fine, returning pointers to variables with local scope is a disaster because oncce the variable goes out of scope, you lose the memory for it, so your pointer points to memory you no longer have a right to access.

As to the new code, note what happens in strlen(tmp) is 0. You never go into that block of code, and just return NULL. But, you have not deallocated the memory for the float you just created. Try initializing the pointer to NULL, and then creating a new float if the condition evaluates true.

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!