C++ GMP, why does this throw an error?

Question

mpz_t* myArr= new mpz_t[M+1];
cout << myArr[0] << endl;
cin.get(); //so I know the program pauses here if everything's OK so far

M is a long long datatype.

I've also tried

mpz_t* myArr= new mpz_t[M+1];
mpz_set_si(myArr[0],0);
cout << myArr[0] << endl;
cin.get(); //so I know the program pauses here if everything's OK so far

Just to give it a value but it does not work still.

Runtime crash

There are many ways in which this could cause a crash: M could be less than 0, or the constructor or `operator<<()` for `mpz_t` could be broken, or the heap could be corrupted from some previous code. You're going to have to do some more work to try to narrow it down. — Ernest Friedman-Hill, Jun 10 '12 at 20:32
All I am doing is outputting the first element of the array after I initialize it. Do they not default to 0? — MyNameIsKhan, Jun 10 '12 at 20:35
If `mpz_t` has a constructor, it will be invoked. If it has an `operator<<()` for printing, that will be invoked. I don't know what an `mpz_t` is, so I can't say whether any of these things matter. — Ernest Friedman-Hill, Jun 10 '12 at 20:36
http://chat.stackoverflow.com/transcript/message/4062981#4062981 — Mooing Duck, Jun 10 '12 at 21:38

wjl · Accepted Answer · 2012-06-10T20:54:58.033

1

You have to initialize mpz_t values, which are just plain C structures with the GMP C API. If you want to use a class with constructors, use mpz_class, which is a C++ class.

Example:

mpz_class x;
x = 3;
mpz_class y;
y = x * 7;

edited Jun 10 '12 at 20:54

answered Jun 10 '12 at 20:38

wjl

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But if I use mpz_class how can I initialize variables/perform arithmetic/etc? For example, if I want to set an mpz_class variable to x*x-1 where x is a long long – MyNameIsKhan Jun 10 '12 at 20:48
@Against you just assign it' e.g. `mpz_class z; z = x*x-1;`. With GMP this won't work easily with long longs and unsigned long longs because of library limitations, but will work with other built in integer types, like int, long, unsigned long, etc. – wjl Jun 10 '12 at 20:55
I try this in my program and it gives me more of the same "invalid operator" problems I've had when trying to force mpz's to play with non mpz's. I can't use long longs? – MyNameIsKhan Jun 10 '12 at 21:16
@Against Short answer: no, you cannot use long longs with GMP. Long answer: you can use long longs, but you will have to encapsulate their interaction with GMP by writing a bunch of your own code, and you will not be able to use mpz_class's operator overloading directly. – wjl Jun 10 '12 at 21:17

C++ GMP, why does this throw an error?

1 Answers1