How to change all struct members parameters using function?

Question

In my program (this is only a short version), I have a struct that has 9 members (airplanes) which all have the same variables.

struct FlightP {string FLID; int altitude; int heading; flightMode; } ArrP_1, ArrP_2;//1-9 etc

If I want to change for example the altitude of members ArrP_1 and ArrP_2 I did it like so:

int main(){
    ArrP_1.altitude = 10000;
    ArrP_2.altitude = 10000;
}

But is there a way to use a function like that:

void ALtChange(FlightP flight_p){
    flight_p.altitude = 12000;
}

int main(){
    ALtChange(ArrP_1);
    ALtChange(ArrP_2);
    //If I have to change multiple parameters in multiple members at the same time this function would make things easier. 
}

Somewhy this function doesn't do anything. Is the function wrong or what should I do differently?

Answer 1

Somewhy this function doesn't do anything.

Your function modifies the function argument that is local to the function.

In order to modify a variable from another scope, you can use indirection: Use a reference argument.

Instead of modiying the object in the function, another approach is to write a function that returns a ALtChange object, and you can assign the result to a variable to modify it.

Answer 2

It's like @Jonnhy Mopp said, pass it by reference like so:

#include <iostream>
#include <string>

using namespace std;

struct FlightP {string FLID; int altitude; int heading; int flightMode; } ArrP_1, ArrP_2;//1

void ALtChange(FlightP& flight_p) { flight_p.altitude = 12000; }
void print_altitude(const FlightP& flight) { std::cout << flight.altitude << std::endl; }

int main(){
    ArrP_1.altitude = 10000;
    ArrP_2.altitude = 10000;

    ALtChange(ArrP_1);
    ALtChange(ArrP_2);

    print_altitude(ArrP_1);
    print_altitude(ArrP_2);
}