How to implement generically typed member objects in C++? - c++

Question

I have an application which creates simple music visualization animations. These animations are driven by nodes, and each node has a bunch of parameters that could have one of several types: int, float, color, etc. The parameters can either have a user-set value, or can be connected to the output of another node.
I'm currently using a templated type, along with std::function<>, like this:
#include <functional>
template <class PT>
class Param
{
public:
Param(PT value=PT()) : _value(value), _provider(nullptr) {}
void setValue(const PT &value) {_value = value;}
void setProvider(std::function<void(PT&)> provider) {_provider = provider;}
void getCurrentValue(PT &value) {
// update current member value
if (_provider)
_provider(_value);
value = _value;
}
private:
PT _value;
std::function<void(PT &value)> _provider;
};
I then instantiate parameters for an animated nodes like this:
class AnimationNode
{
public:
AnimationNode(Model *model = nullptr);
void evaluate();
private:
Param<int> _xoffset;
Param<int> _yoffset;
Param<float> _scale;
Param<ColorType> _color;
};
These parameters could be connected to a driver node, such as this one:
class SublevelMeter {
public:
SublevelMeter();
void setRange(Subrange &_range);
...
std::function<void(float&)> createProviderClosure();
private:
float _level;
...
}
std::function<void(float&)> SublevelMeter::createProviderClosure() {
return [this] (float &out) {out = _level;};
}
And connect one node to another by doing something like this:
AnimationNode::connectScaleToSublevel(SublevelMeter *slm) {
_scale->setProvider(slm->createProviderClosure());
}
The problem is, I'd like there to be an abstract Param type that I can pass to objects, so rather than the code above, I could pass a param to my SublevelMeter:
SublevelMeter::connectToParam(Param *param) {
param->setProvider(slm->createProviderClosure());
}
This would also help when writing the routines that create my GUI editor widgets: the editor could figure out the correct type by introspection of the Param.
But I'm not sure how to do this from a templated class, nor how the best way to implement the introspection in C++. (I'm coming at this from a python design background, which is perhaps encouraging me to think about this in a pythonic rather than C++ way; if there's a better way to approach this, I'd love to hear about it!)
I'm using Qt, so I've considered using QVariant, or other Qt Meta-Object stuff, but I'm not sure how to make that work, or if it would even be appropriate. (I'm not using Boost, and while I know it has certain type erasure facilities, I'm wary of wading into those waters...)
I'm interested in what the cleanest/"best" way to do this. Although efficiency is a consideration (getCurrentValue() is called many times per frame while the animation is playing) I can still probably afford run-time overhead of dynamic type stuff.

Answer 1

At least the first part of your question is solvable without abstract Param:
class SublevelMeter {
...
template<class PT>
void connectToParam(Param<PT> *param) {
param->setProvider(createProviderClosure<PT>());
}
// specialize this for different PTs
template<class PT>
std::function<void(PT&)> createProviderClosure();
}
If you really need to manipulate dynamic lists of Param-s, and you don't want to use any kind of RTTI, consider using Visitor pattern:
class Visitor;
class ParamBase
{
public:
virtual ~ParamBase() = default;
virtual void acceptVisitor(Visitor* v) = 0;
};
template <class PT>
class Param : public ParamBase
{
public:
...
void acceptVisitor(Visitor* v) override;
};
class Visitor {
public:
virtual ~Visitor() = default;
void visit(ParamBase* p) {
p->acceptVisitor(this);
}
virtual void visitParam(Param<float>* p) = 0;
// add more functions for other Params
};
class PrintVisitor : public Visitor {
public:
void visitParam(Param<float>* p) override {
std::cout << "visited Param<float>, value = " << p->getValue() << std::endl;
}
};
template<class PT>
void Param<PT>::acceptVisitor(Visitor* v) {
v->visitParam(this);
}
int main() {
std::unique_ptr<ParamBase> p(new Param<float>(123.4f));
std::unique_ptr<Visitor> v(new PrintVisitor());
v->visit(p.get());
return 0;
}

Answer 2

I implemented for you a simple class for the generic type management. This class is implemented without using template, so you can declare your variables and assign a value and a type directly at runtime. This implementation is very simple you should use it as reference to develop your own solution. In the following example I implemented the support for only 3 types: int, double and char* (C string). The main function shows you as to use the generic type class for both LVALUE and RVALUE assignment:
#include <stdio.h>
#include <stdlib.h>
enum Types {tInteger, tDouble, tString};
class TGenericType
{
private:
char m_Value[100];
Types m_Type;
protected:
public:
void operator=(int AValue)
{
m_Type = tInteger;
sprintf(m_Value, "%d", AValue);
}
operator int()
{
// try to convert the m_Value in integer
return atoi(m_Value); // the result depend by atoi() function
}
void operator=(double AValue)
{
m_Type = tDouble;
sprintf(m_Value, "%f", AValue);
}
operator double()
{
// try to convert the m_Value in double
return atof(m_Value); // the result depends by atof() function
}
void operator=(char* AValue)
{
m_Type = tString;
strcpy(m_Value, AValue);
}
operator char*()
{
return m_Value;
}
};
int _tmain(int argc, _TCHAR* argv[])
{
TGenericType LVar;
// int assignment LVar used as LVALUE
LVar = 10;
// int assignment LVar used as RVALUE
int i = LVar;
// Double assignment LVar used as LValue
LVar = 10.1;
// double assignment LVar used as RVALUE
double d = LVar;
// costant string assignment LVar used as RVALUE
LVar = "Ciao Mondo";
// string copying LVar used as const string RVALUE
char Buffer[100];
strcpy(Buffer, LVar);
return 0;
}
I tested above code on c++builder 32bit and c++builder (CLang) 64bit
If my solution answer your question, please check it as answered.
Ciao from Italy!
Angelo