Store the template that the class was initialized with - c++

Question

Im trying to figure out how to store my template, ill write a example:
class cDebugInfo
{
private:
DWORD * address;
string name;
template Type;
public:
Type GetFormattedValue()
{
return *(Type*)address;
}
cDebugInfo(){}
template <class T>
cDebugInfo(DWORD Address, string Name){
address = Address;
name = Name;
Type = T;
}
};
My goal, is to be able to add a item to my array:
std::vector<cDebugInfo>DebugItems;
template <class T>
void AddItem(std::string name, DWORD Address)
{
DebugItems.push_back(cDebugInfo(Address, name));
}
cDebugInfo* GetItemByNameP(std::string name)
{
for (int i = 0; i < DebugItems.size(); i++)
{
if (DebugItems[i].name == name)
{
return &DebugItems[i];
}
}
}
So i add the items to my array like this:
AddItem<int>(0x1337, "Test");
AddItem<string>(0x1337, "Test2");
And Therefore being able to call:
GetItemByName("Test")->GetFormattedValue();
And that should return the value in INT read form the given address since the template passed when i add the item, is a int. And ofcourse, the following should return the value stored in the address pointed by my pointer as a string:
GetItemByName("Test2")->GetFormattedValue();
I need it to "remember" what template was passed on to the class.
NOTE: Everything else works fine when i use GetItemByName with a template, but the thing is i dont know what template it is when i get them, only when i add them.
Thank you.

Answer 1

What you are asking for is not possible because every expression in C++ must have a known type at compile time. Consider this:
auto value = GetItemByName("BestItem")->GetFormattedValue();
GetItemByName(...) gives me a cDebugInfo*, but what does GetFormattedValue() give me? This must be the same type for every cDebugInfo* so that the above expression can be valid, so the type cannot withheld until runtime. So the general solution is impossible.
However, you can add specific solutions based on what it is you want to do. Let's say we just want to be able to print the formatted value. We can do that:
class cDebugInfo {
std::function<void()> printer; // type-erased functor
...
public:
template <class T>
cDebugInfo(DWORD Address, string Name){
address = Address;
name = Name;
printer = [this]{
std::cout << "Value as " << typeid(T).name() << ": "
<< *reinterpret_cast<T*>(address) << '\n';
};
}
};
This approach is called "type erasure". In this case, the lambda "stores" the type T, but the class itself just needs to know that it's a nullary function that returns void. We can expose that via:
void printValue() { printer(); }
So that:
GetItemByValue("BestItem")->printValue();
will print the value correctly based on the type it was constructed with.