How can I generate multiple function pointers from a parameterized base function?

Question

I have a base function

int base(const int i, const double &x)

In this function, I consider i to be a "parameter" (which can take predefined values from 0 to N-1), while x I consider to be the actual "argument" (which can take any arbitrary value).

What I need is to create an array of function pointers int (*pointers[N])(const double &x). Each member of the array corresponds to a different value for the parameter i and thus each call pointers[n](x) should be equivalent to a call base(n, x).

Is there a way to achieve this functionality?

I have been trying to educate myself about functors (is this a way to go?) and recently also looked into std::bind but my understanding is that it only works with C++11 (which I currently cannot use).

Any advice or code snippet would be really helpful. Thanks!

Kerrek SB · Accepted Answer · 2013-02-20 23:55:24Z

1

You can use non-capturing lambdas for that:

typedef int (*fp)(double);

fp p1 = [](double x) -> int { return base(1, x); };
fp p2 = [](double x) -> int { return base(2, x); };
fp p3 = [](double x) -> int { return base(3, x); };

This doesn't work for capturing lambdas, and so you can't generate those programatically. There's essentially no difference between this and spelling out one function for each value in C:

int f1(double x) { return base(1, x); }
fp p = f1;

answered Feb 20, 2013 at 23:55

Kerrek SB

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4 Comments

Chrys Over a year ago

Thanks for your answer!....the problem is the following: The number of different parameter values is large and, more importantly, not known at compile-time. Thus, I cannot just casually write "p1", "p2", "p3", "f1", etc....I need to be able to do commands in a loop over i....

Kerrek SB Over a year ago

@Chrys: Unfortunately, functions in C++ are not real first-class objects, and you cannot create functions dynamically. If you could use function-like objects, it would be trivial, but not with raw function pointers. They just don't carry enough state.

Chrys Over a year ago

I need to use raw pointers in order to interface with legacy code originally written in Fortran, which I have been calling through C (which has been a mess of its own)....I have been thinking along the lines of using a functor class, the call operator of which will be linked to each different function, and a different object of which functor will be created in the loop over the parameter values (however, I am not too proficient to know how to do this exactly)

Kerrek SB Over a year ago

@Chrys: You can't do that. A raw C function has no state, and you cannot make something that has more state than none at all look like a raw C function...

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