How to iterate over a template class? - c++

Question

I'm trying to create a generic menu class that will be used with a 4 line LCD.
I have a specific (non template) version working, but want to extend it to allow the menu to modify a variety of data types (int, float, unsigned...).
Here's the non template version that's working as expected...
/*
* ideally this design allows for defining an arbitrary menu as shown below
* example...
* root1
* sub1-1
* sub1-2
* root 2
* root 3
* sub3-1
* sub3-2
* sub3-2-1
* sub3-2-2
*
* each node in the menu can be executed, and allow for moving to the next/prev sibling or child/parent
* this flexibility requires that each node contains pointers to parent, child, and sibling nodes.
*/
class MenuNode
{
private:
char *prompt;
int value;
public:
MenuNode *parent=NULL;
MenuNode *child=NULL;
MenuNode *prevSibling=NULL;
MenuNode *nextSibling=NULL;
void SetValue(int value)
{
this->value = value;
}
int GetValue()
{
return value;
}
char *Prompt()
{
return prompt;
}
MenuNode(char *prompt, int initialValue, MenuNode *parent, MenuNode *prevSibling)
{
Serial.print(prompt);Serial.println(F(" MenuNode"));
this->prompt = prompt;
if (prevSibling != NULL)
{
this->prevSibling = prevSibling;
prevSibling->SetNextSibling(this);
this->parent = prevSibling->parent;
}
// prevSibling if provided sets the parent
if (prevSibling==NULL && parent != NULL)
{
this->parent = parent;
this->parent->SetChild(this);
}
value = initialValue;
}
void SetChild(MenuNode *child)
{
Serial.print(prompt);Serial.println(F(" SetChild"));
this->child = child;
}
void SetNextSibling(MenuNode *nextSibling)
{
Serial.print(prompt);Serial.println(F(" SetNextSibling"));
this->nextSibling = nextSibling;
}
};
Here's some test code that creates the menu structure...
// Test menu...
MenuNode r1("R1",10,NULL,NULL);
MenuNode r2("R2",20,NULL,&r1);
MenuNode r21("R21",30,&r2,NULL);
MenuNode r22("R22",40,&r2,&r21); // setting parent is optional, the parent will be set by the prev sibling parent
MenuNode r221("R221",50,&r22,NULL);
MenuNode r2211("R2211",60,&r221,NULL);
MenuNode r2212("R2212",70,NULL,&r2211);
MenuNode r3("R3",30,NULL,&r2);
This code iterates over each element printing out the structure
void PrintMenuStructure(MenuNode *node,int offset)
{
while(node != NULL)
{
for (int i=0;i<offset;i++)
Serial.print("-");
Serial.print(node->Prompt());
Serial.print(" = ");
Serial.print(node->Value());
if (node->parent != NULL)
{
Serial.print(" parent=");
Serial.print(node->parent->Prompt());
}
if (node->prevSibling != NULL)
{
Serial.print(" prevSib=");
Serial.print(node->prevSibling->Prompt());
}
if (node->nextSibling != NULL)
{
Serial.print(" nextSib=");
Serial.print(node->nextSibling->Prompt());
}
if (node->child != NULL)
{
Serial.print(" child=");
Serial.print(node->child->Prompt());
}
Serial.println();
if (node->child != NULL)
PrintMenuStructure(node->child,++offset);
node = node->nextSibling;
}
}
This is the output of the previous function demonstrating the structure of the menu...
R1 = 10 nextSib=R2
R2 = 20 prevSib=R1 nextSib=R3 child=R21
-R21 = 30 parent=R2 nextSib=R22
-R22 = 40 parent=R2 prevSib=R21 child=R221
--R221 = 50 parent=R22 child=R2211
---R2211 = 60 parent=R221 nextSib=R2212
---R2212 = 70 parent=R221 prevSib=R2211
-R3 = 30 prevSib=R2
It all works the way I want, but GetValue/SetValue only operate on int data.
I can create a template version of the class, with the data types of GetValue and SetValue defined by the template parameter, but I don't know now to iterate over the nodes once I do that.
Seems like a simple enough task, but I've been beating my head against the wall for a while, and haven't come up with anything that works. Any help pointing me in the right direction would be appreciated.
I'm trying to figure out how to iterate over a linked list of classes, but can't figure out how to get a pointer to start iterating.
Sorry, I couldn't get the code formatting to work... :(

Answer 1

The way I interpret your requirement: it seems your should make your
int value;
a std::variant.
That's the lowest cost path.
If you templatize the MenuNode class with its value type. Then a MenuNode<int>* cannot be the parent of a MenuNode<float*>, etc. Not without some effort. You'd probably better off make it polymorphic by derivate each type of value your want to support from a common abstract virtual base, and depend on how you want to use the value, design your interface.