Can't seem to change the values of variables in a class, keep getting the old values back

Question

I've created a class with a couple of variables in it. I can read these variables from my functions just fine. I seem to be able to update them to new values, however when I try to get the new values in the main() function it gives me the original values.

I'm sure it's something simple I'm missing, I just can't seem to figure it out.

header.h

#pragma once
#include <string>
#include "gmp.h"

class my_class {
public:
    const static int  CPU_GRP_SIZE = 16;
    mpz_t tempx, tempy;

    my_class()
    {
    mpz_init_set_str(tempx, "79BE667EF9DCBBAC55A06295CE870B07029BFCDB2DCE28D959F2815B16F81798", 16);
    mpz_init_set_str(tempy, "483ADA7726A3C4655DA4FBFC0E1108A8FD17B448A68554199C47D08FFB10D4B8", 16);
    }

};

void AddDirect(mpz_t p2x, mpz_t p2y);

main.cpp

#include <string>
#include <iostream>
#include <gmp.h>
#include "header.h"

void main()
{
    my_class secp;
    int hLength = (secp.CPU_GRP_SIZE / 2 - 1);

    mpz_t point2x, point2y;

    mpz_init_set_str(point2x, "c6047f9441ed7d6d3045406e95c07cd85c778e4b8cef3ca7abac09b95c709ee5", 16);
    mpz_init_set_str(point2y, "1ae168fea63dc339a3c58419466ceaeef7f632653266d0e1236431a950cfe52a", 16);

    AddDirect(point2x, point2y);

    std::cout << "+++++++++++++ main \n";
    mpz_out_str(stdout, 16, secp.tempx);
    std::cout << " \n";
}

void AddDirect(mpz_t p2x, mpz_t p2y)
{
    my_class secp;

    mpz_t point2x, point2y;

    mpz_init_set(point2x, p2x);
    mpz_init_set(point2y, p2y);
    // ---------------------------

    mpz_set(secp.tempx, point2x);
    mpz_set(secp.tempy, point2y);
    //------------------
    std::cout << "adddirect secp.tempx\n";
    mpz_out_str(stdout, 16, secp.tempx);
    std::cout << " \n";
}

It shouldn't make any difference in this case, but I'm using visual studio 2022 on Windows 10.

Expected/wanted result:

adddirect secp.tempx 
c6047f9441ed7d6d3045406e95c07cd85c778e4b8cef3ca7abac09b95c709ee5
 +++++++++++++ main 
c6047f9441ed7d6d3045406e95c07cd85c778e4b8cef3ca7abac09b95c709ee5

Real result:

adddirect secp.tempx
c6047f9441ed7d6d3045406e95c07cd85c778e4b8cef3ca7abac09b95c709ee5 
+++++++++++++ main 
79be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798

This question is similar to: Are vectors passed to functions by value or by reference in C++. If you believe it’s different, please edit the question, make it clear how it’s different and/or how the answers on that question are not helpful for your problem. — Stephen Newell
– Stephen Newell, Commented Mar 19 at 18:31
@StephenNewell I don't think that Q&A is related to OPs problem. At least my answer to this question doesn't touch upon anything they mention in that other question or answers. — Ted Lyngmo
– Ted Lyngmo, Commented Mar 19 at 18:34

Ted Lyngmo · Accepted Answer · 2025-03-19 18:26:52Z

Your AddDirect creates a new my_class instance called secp and sets the values in that instance. When the function returns, that instance is destroyed. The instance you've defined in main is unaffected by any of that.

What you probably want to do is to make AddDirect a member function in my_class. Calling that (on an instance of my_class type) will make it possible to set the member variables.

The class definition could then look like this:

#include <gmp.h>
//
#include <iostream>

class my_class {
public:
    const static int CPU_GRP_SIZE = 16;
    mpz_t tempx, tempy;

    my_class() {
        mpz_init_set_str(
            tempx,
            "79BE667EF9DCBBAC55A06295CE870B07029BFCDB2DCE28D959F2815B16F81798",
            16);
        mpz_init_set_str(
            tempy,
            "483ADA7726A3C4655DA4FBFC0E1108A8FD17B448A68554199C47D08FFB10D4B8",
            16);
    }
    my_class(const my_class&) = delete;
    my_class(my_class&&) = delete;
    my_class& operator=(const my_class&) = delete;
    my_class& operator=(my_class&&) = delete;

    ~my_class() {
        // free memory when the instance is destroyed:
        mpz_clear(tempx);
        mpz_clear(tempy);
    }

    // make this a member function:
    void AddDirect(mpz_t p2x, mpz_t p2y);
};

And the implementation of AddDirect specifically:

void my_class::AddDirect(mpz_t p2x, mpz_t p2y) {
    /* unnecessary:
    mpz_t point2x, point2y;
    mpz_init_set(point2x, p2x);
    mpz_init_set(point2y, p2y);
    */
    // ---------------------------
    // set the member variables in the my_class instance:
    mpz_set(tempx, p2x);
    mpz_set(tempy, p2y);
    //------------------
    std::cout << "adddirect tempx\n";
    mpz_out_str(stdout, 16, tempx);
    std::cout << " \n";
}

In main you'd then call AddDirect on the my_class instance that you've defined in main:

int main() { // not void main()
    my_class secp;

    mpz_t point2x, point2y;

    mpz_init_set_str(
        point2x,
        "c6047f9441ed7d6d3045406e95c07cd85c778e4b8cef3ca7abac09b95c709ee5", 16);
    mpz_init_set_str(
        point2y,
        "1ae168fea63dc339a3c58419466ceaeef7f632653266d0e1236431a950cfe52a", 16);

    secp.AddDirect(point2x, point2y); // call the member function to set these
//  ^^^^^

    // manually free these that aren't needed anymore:
    mpz_clear(point2x);
    mpz_clear(point2y);

    std::cout << "+++++++++++++ main \n";
    mpz_out_str(stdout, 16, secp.tempx);
    std::cout << " \n";
}

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