How can I make a function that remembers its returned result?
For example:
char * foo(char * str1, char * str2){
...
return bar;
}
int main(){
...
foo(a,b); //returns a string
foo(NULL,c);//will use the returned above string in the NULL parameter
}
This is similar to how strtok works.
I suppose a global variable could work, but as far as I know it isn't good practice, is there another way? Does strtok works with a global variable?
That's what the static keyword in C is for. Kind of! Be wary, see edits below.
A variable with static storage class will exist for the lifetime of the program, not only to the end of the declaring scope:
int foo()
{
static int counter = 0;
counter ++;
return counter;
}
will return 1,2,3... on successive calls.
EDIT: The commentors seem to agree that it'd be wise to warn you that static is dangerous if you're doing concurring access (ie. multi-threading), which I agree. But then again, you're already aware that global variables can be harmful, so this might not be new, and considering the nature of your question, you might not be at the point in your programming carreer where you're doing multi-threading; if you're doing, please refer to whatever introduces you to multi-threading for safe ways of modifying shared states (semaphores/mutexes...).
EDIT2: It's of course not only hazardous in multi-threading environments; for example, when two different functions call foo internally, and expect to see their own sequence of 1,2,3..., things will go wrong. Use with extreme care.
EDIT3: On third though, @sashoalm 's answer is correct and fits the title of your question best. static was more meant for immutable, one-time initializable states, not for modifiable things that can burst into flames in almost any environment. So here I quote the most important line of the best answer here, since you've decided to accept my answer, which could mislead later readers:
So the most correct answer is NO, you cannot do that in C, and no, static is not the answer.
I'm answering your title, which is "How can a function “remember” it's result for its next use?". Your title implies the general case, and I'm not interested in the particular function in your question body.
First, I'd like to say kudos to Python for the yield keyword, unfortunately, in C there is no such syntactic sugar.
So the most correct answer is NO, you cannot do that in C, and no, static is not the answer.
Static is not reentrant. Sometimes a simple no, you cannot do that, not for real, not in C, is better than telling you half-baked solutions. C just doesn't have this feature, and that's it.
Python has yield, C doesn't. The end.
You'll have to simulate it by using a struct to hold the state.
Some pseudo-code:
struct State { ... };
Output foo(struct State *state, ...) { ... }
...
foo(&state);
foo(&state);
If you don't want to do it the right way, then use static and spend 2-3 days figuring out bugs when you try to nest a call to the function. strtok has bitten people that way.
Note that using static might make the function not reentrant. Be careful:
Output foo(...) { static struct State state; ... }
Functions in the mathematical sense are by definition stateless. If it has state, then it's an object, not a function. A function with static state is semantically really a singleton object.
State implies memory, and memory + methods = object.
