Array keeps returning only the last element. [C/Arduino] - c++

Question

i've a problem with an array (called "Inputs" of type "GeneralInput") on Arduino,basically,no matter which element i try to have access to,the code always returns me the last element of that array.
Here's part of the code:
//...include statements
//other initializations
GeneralInput *Inputs[19];
void setup()
{
//...
//...
InitializeInputs();
}
void InitializeInputs()
{
//type 0 = pedal switch; 1 = volume pedal
//type 2 = potentiometer; 3= switch;
//pedal switches
Inputs[0] = &GeneralInput(0,0,true,false,NULL,10);
Inputs[1] = &GeneralInput(1,0,true,false,NULL,9);
Inputs[2] = &GeneralInput(2,0,true,false,NULL,6);
Inputs[3] = &GeneralInput(3,0,true,false,NULL,5);
//volume pedal
Inputs[4] = &GeneralInput(4,1,false,false,NULL,A2);
//potentiometer
Inputs[5] = &GeneralInput(5,2,false,true,mux2,5);
Inputs[6] = &GeneralInput(6,2,false,true,mux2,6);
Inputs[7] = &GeneralInput(7,2,false,true,mux2,7);
Inputs[8] = &GeneralInput(8,2,false,true,mux2,8);
Inputs[9] = &GeneralInput(9,2,false,true,mux2,9);
Inputs[10] = &GeneralInput(10,2,false,true,mux2,10);
Inputs[11] = &GeneralInput(11,2,false,true,mux2,11);
//switch
Inputs[12] = &GeneralInput(12,3,true,true,mux2,15);
Inputs[13] = &GeneralInput(13,3,true,true,mux2,14);
Inputs[14] = &GeneralInput(14,3,true,true,mux2,13);
Inputs[15] = &GeneralInput(15,3,true,true,mux2,12);
//joystick
Inputs[16] = &GeneralInput(16,3,true,true,mux1,2); //switch
Inputs[17] = &GeneralInput(17,2,false,true,mux1,1); //x axis
Inputs[18] = &GeneralInput(18,2,false,true,mux1,3); //y axis
}
void loop()
{
int length=0;
//cycle through different inputs
int startIndex=0,endIndex=0;
//temp arrays
byte toSendTmp[30];
for(int i=0;i<30;i++)
toSendTmp[i]=0;
//...
//..
int packetIndex=0;
for(int i=startIndex;i<endIndex;i++)
{
//if the input is updated,fill the array with the new data
/*
* When i try to have access to the i-element i always get
* the last one instead.
*/
if(Inputs[i]->Update())
{
toSendTmp[(packetIndex*3)] = Inputs[i]->GetID();
toSendTmp[(packetIndex*3)+1] = Inputs[i]->GetType();
toSendTmp[(packetIndex*3)+2] = Inputs[i]->GetValue();
packetIndex++;
}
}
//....
//...
}
And if needed here's the GeneralInput.h and GeneralInput.cpp code.
NOTE: I can't tell if the array is always returning the last item or if every slot of the array is filled with a pointer to the same object (the last created).
Any idea on what i'm doing wrong?
Thanks in advance.

Answer 1

Your &GeneralInput are incorrect, in fact you create temporary objects and store their adresses in an array, but as soon as your GeneralInput object get destroy (same line as creation), a new object takes place at the same address:
// Create GeneralInput at address #
Inputs[0] = &GeneralInput(0,0,true,false,NULL,10);
// End of your temporary object, the `GeneralInput` object is destroyed but you still
// points to its address...
/* etc. */
You're getting the last value because the compiler always create the GeneralInput at the same address, so all Inputs[] point to the same address.
You need to dynamically create your GeneralInput:
Inputs[0] = new GeneralInput(0,0,true,false,NULL,10);

Answer 2

Every slot in the array has a pointer to the same memory location which is the occupied by the last element you create. By doing &GeneralInput(...) you are creating a GeneralInput object on the stack and retrieving its stack address. But since the GeneralInput object itself is never assigned to a variable, the memory it occupies is immediately available for reuse. This means that every GeneralInput object is created at the same address on the stack. The solution, however, isn't to change your code to something like
GeneralInput genInput = GeneralInput(...);
Inputs[...] = &genInput;
Code like that will still be filling your array with pointers to stack addresses. Those pointers will immediately become invalid when the function returns. You should be filling your array with something like
Inputs[...] = (GeneralInput*)malloc(sizeof(GeneralInput));
*Inputs[...] = GeneralInput(...);
Using this method make sure that if your Inputs array ever reaches a point where you don't use it anymore loop over it freeing every element.
Edit: Arduino using C, so doesn't have new. Use malloc and free instead.

Answer 3

As others have said, the problem is with the address of the temporary variables. You can get around the "new" problem by having default parameters.
class GeneralInput
{
public:
GeneralInput(int a = 0, int b = 0, bool c = true, bool d = true, int* e = NULL, int f = 0);
...
};
Then declare your array - this takes GeneralInput with the default parameters
GeneralInput inputs[20];
Then in Initialize - then you won't have the new problem or the problem of temporaries disappearing at the end of the routine.
void InitializeInputs()
{
inputs[0] = GeneralInput(0,0,true,false,NULL,10);
...
}
I don't know what the NULL points to but you might want to put in a copy operator for this if it is anything else other than copying the value. Not very efficient because it calls the constructor twice but that only happens at initialization.