How to create functions within a function and return them?

Question

I want to create a function that returns a function that checks whether a given int is within certain bounds. Therefore the returned function should only take one parameter, an int, and return a bool. This is necessary as the returned function is passed on as a function pointer to another function. So far I would only be able to do it like this:

bool valueIsInBounds(int value) { return value >= 0 && value <= 100; }

int main() {
    functionThatTakesAFunction(&valueIsInBounds);
}

0 and 100 are obviously fixed values, and I would like to change that.
What I would like to be able to do is something like this:

??? functionGenerator(int min, int max) { ??? }

int main() {
    functionThatTakesAFunction(&(functionGenerator(0, 100)));
}

I know this is doable in other languages, though I don't know how this would be achieved in C.

It isn't doable in C. Functions can't be dynamically created at runtime. — John Coleman
– John Coleman, Commented Dec 19, 2016 at 12:55
Barring some elaborate machinery to implement first-class functors before writing this on top of it, you can't do that in C. — Quentin
– Quentin, Commented Dec 19, 2016 at 12:55
@VittorioRomeo how is a C++ functionality at all relevant to a C question ? — Quentin
– Quentin, Commented Dec 19, 2016 at 12:58
@Quentin As a comment it is definitely relevant. If OP has a very strong reason for wanting to do this and already has a code base in C, converting it to C++ to take advantage of such things would be one possible strategy. — John Coleman
– John Coleman, Commented Dec 19, 2016 at 13:05

Paul Ogilvie · Accepted Answer · 2016-12-19 13:21:33Z

4

As dynamically generating a function cannot be done in C, the next best approach will be to parametrize the function.

You can pass (as in your question) the bounds as additional parameters, or you can set global or static variables with these bounds.

bool valueIsInBounds(int value, int min, int max) { return value >= min && value <= max; }

int main() {
    functionThatTakesAFunction(valueIsInBounds);
}

or:

int gMin, gMax;
void setBounds(int min, int max) { gMin= min; gMax= max; }
bool valueIsInBounds(int value)  { return value >= gMin && value <= gMax; }

int main() {
    setBounds(0, 100);
    functionThatTakesAFunction(valueIsInBounds);
}

answered Dec 19, 2016 at 13:21

Paul Ogilvie

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Community · Accepted Answer · 2017-05-23 11:53:41Z

4

Sorry. As the comments says, this can not be implemented in portable C.

You must generate some assembly yourself to bind the parameters (max & min) to the function pointer. This is called a «trampoline» function, and there exists multiple libraries for that. Here is one: https://www.gnu.org/software/libffcall/trampoline.html

Take a look at this SO post for an better explanation and an example of using the trampoline library above.

edited May 23, 2017 at 11:53

CommunityBot

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answered Dec 19, 2016 at 13:09

Torkel Bjørnson-Langen

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