Setting function return type

[SamstaUK]

I want my function to return the type mpz_t, but I'm not sure how?

I've tried:
mpz_t MyFunction(mpz_t A, mpz_t B){}

But it didn't work, here is my code so far, I have bolded the parts of the code which are causing errors and added the errors in the comments:

#include <iostream>
#include <mpir.h>

using namespace std;

???? A(mpz_t m, mpz_t n){
	mpz_t mmo; //used to store the value of m, minus one
	mpz_t nmo; //used to store the value of n, minus one
	mpz_t one; //used to store the value 1
	mpz_init(mmo);
	mpz_init(nmo);
	mpz_init(one);
	mpz_set_ui(one, 1);

	if(mpz_cmp_ui(m, 0) == 0){
		mpz_add_ui(n, n, 1);
		return n; //error: return value type does not match the function type
	}

	if(mpz_cmp_ui(m, 0) > 0 && mpz_cmp_ui(n, 0) == 0){
		mpz_sub_ui(mmo, m, 1);
		return A(mmo, one);
	}

	if(mpz_cmp_ui(m, 0) > 0 && mpz_cmp_ui(n, 0) > 0){
		mpz_sub_ui(mmo, m, 1);
		mpz_sub_ui(nmo, n, 1);
		return A(mmo, A(m, nmo)); 
	}
}

int main(){
	mpz_t param1;
	mpz_init(param1);
	mpz_t param2;
	mpz_init(param2);

	mpz_set_ui(param1, 4);
	mpz_set_ui(param2, 2);

	mpz_out_str(stdout, 10, A(param1, param2)); 
	return 0;
}

Is there a way around this?

Thank you

[GCDEF]

There's a disconnect between what you say you tried and what you implemented. Your implementation shows a void function trying to return something.

[SamstaUK]

Sorry, I was trying something else before I posted this and forgot to change it

[GCDEF]

What is A?

[Paul McKenzie]

I want my function to return the type mpz_t, but I'm not sure how?

Why are you returning a variable name?

mpz_t MyFunction(mpz_t A, mpz_t B){}
//...
return A(mmo, one);   // ?

The A is the name of the variable, not the type. Aren't you supposed to do something like this?

return mpz_t(mmo, one);

?
Also, what is mpz_t? If it's a class or struct, you should be passing them by reference or const reference, not by value, unless there is a compelling and logical reason to be passing them by value.

mpz_t MyFunction(mpz_t& A, mpz_t& B)

Regards,

Paul McKenzie

[laserlight]

I want my function to return the type mpz_t, but I'm not sure how?

Read the GMP manual on parameter conventions:

When a function is going to return a GMP result, it should designate a parameter that it sets, like the library functions do. More than one value can be returned by having more than one output parameter, again like the library functions. A return of an mpz_t etc doesn't return the object, only a pointer, and this is almost certainly not what's wanted.

Also, note that what you mpz_init, you must mpz_clear, similiar to the rule that what you malloc, you must free. Alternatively, you can use the C++ wrapper and let RAII do the work for you.

Also, what is mpz_t?

An opaque pointer type.