The crux of the issue is I want to create a vector of base pointers to reference children objects. However I'm having issues accessing the methods of the children. I've seen examples of downcasting online but I don't feel it's the best thing for me since I want to keep my code generic. Please look below for a sample of what I'm trying to accomplish.
class Base
{
public:
stuffx;
private:
stuffy;
}
template<typename U>
class Child : public Base
{
public:
Child(
std::function<U()> getterFunc,
std::function<void(U)> setterFunc
):
mgetter(getterFunc),
msetter(setterFunc)
{
}
U getFunction() const {return m_getter();}
void setFunction(U input) const {return m_setter(input);}
private:
observableValues() {}
std::function<U()> m_getter;
std::function<void(U)> m_setter;
}
int main()
{
std::vector<std::shared_ptr<Base>> Dummy = {std::make_shared<Child<int>> (std::bind(..), std::bind(...)),
std::make_shared<Child<string>> (std::bind(..), std::bind(...)) };
Dummy.at(0)->getGFunction(); // this throws an error as out of scope.
(dynamic_cast<Child<int>>(Dummy.at(0))->getGFunction(); // this is ok
}
In this example above my vector is of size 2 which is manageable but my goal is to serialize c++ classes to a psql server and may have to handle vectors of size 30+. My next question is is there a way to automate this in a for loop taking into the account the type deduction that may need to be performed for typename U.
int main()
{
std::vector<std::shared_ptr<Base>> Dummy = {std::make_shared<Child<int>> (std::bind(..), std::bind(...)),
std::make_shared<Child<string>> (std::bind(..), std::bind(...)) };
std::vector<std::shared_ptr<Base>>::const_iterator it_base = Dummy.begin();
for (; it_base != Dummy.end(); ++it_base)
{
//insert method here for downcasting
}
}
Related
I have a struct A that inherits from other classes (which I'm not allowed to change). Inside A and it's methods I can call inherited methods (lets say A_method(int i), for example) without problem but when I tried to write a nested struct (lets say In) and call A_method(int i) and there is were I'm stuck.
The initial code looks like this, and I can't change it, is some kind of college assigment.
#include "Player.hh"
struct A : public Player {
static Player* factory () {
return new A;
}
virtual void play () {
}
};
RegisterPlayer(PLAYER_NAME);
Then I tried this:
#include "Player.hh"
struct A : public Player {
static Player* factory () {
return new A;
}
//My code
struct In {
int x;
void do_smthing() {
A_method(x);
}
}
virtual void play () {
}
};
RegisterPlayer(PLAYER_NAME);
Ok, from a beginning I knew I could't do this, for In to see it's parent class it should have a pointer to it but In is a often instantiated object in my code and I wanted to avoid passing this constantly to a constructor so I tried this aproach:
#include "Player.hh"
struct A : public Player {
static Player* factory () {
return new A;
}
//My code
static struct Aux
A* ptr;
Aux(A* _p) { ptr = _p; }
} aux;
struct In {
int x;
void do_smthing() {
aux.ptr->A_method(x);
}
}
virtual void play () {
//the idea is to call do_smthing() here.
}
};
RegisterPlayer(PLAYER_NAME);
What I want to avoid (if possible) is something like this:
struct In {
int x;
A* ptr;
In (A* _p) : ptr(_p) {}
void do_smthing() {
ptr->A_method(x);
}
}
The main reason for this: I have more struct definitions and they they are instantiated multiple times through the rest of the (omitted) code, and I don't like the idea of seeing In(this) so many times.
I don't know if I'm completly missing something or what I want to do it's just not possible... Please ask for clarifications if necessary.
(Also, performance is kind of critical, my code will be tested with limited CPU time so I kinda have to avoid expensive approachs if possible. Using C++11)
There is no way you can skip passing the this pointer. Instead, you could create a helper function in A:
template <typename InnerType, typename ...Params>
InnerType makeInner(Params&&... params)
{
return InnerType(this, std::forward<Params>(params)...);
}
Then you can use
auto * a = A::factory();
auto inner = a->makeInner<A::In>();
I have some suggestions which are not directly related to you question but may help:
A::facotry() returns a std::unique_ptr<A> instead of raw pointer
Try to describe what problem you are trying to solve. I have a strong feeling that there can be a better design other than creating many nested structs.
I don't see passing a this pointer could have any impact on the performance. The more important thing is to identify the path that is latency-sensitive and move expensive operations out of those paths.
There is a custom defined map, with an element std::function()>.
The lambda code is working, but I don't know how to expand it to a normal formation. The code is following.
class TestA{
public:
TestA() {}
~TestA() {}
TestA(const TestA &) {}
static void print()
{
cout << __FUNCTION__ << endl;
return;
}
};
void testComplexMap1()
{
typedef map<string, std::function<std::unique_ptr<TestA>()>> TempMap;
TempMap m;
// the lambda format code, it works
//m.insert({ "TestA", []() {return std::unique_ptr<TestA>(new TestA());}});
// I want to expand it, but failed.
TestA *t = new TestA();
//function<unique_ptr<TestA>()> fp(unique_ptr<TestA>(t));
function<unique_ptr<TestA>()> fp(unique_ptr<TestA>(t)()); //warning here
//m.emplace("TestA", fp); // compile error here
}
Any help will be greatly appreciated.
fp is not initialized with a function so compilation fails.
You can expand it like this:
TestA *t = new TestA();
std::unique_ptr<TestA> UT(t);
auto func = [&]() { return move(UT);};
std::function<std::unique_ptr<TestA>()> fp(func);
m.emplace("TestA", fp);
See DEMO.
In C++ everything that looks like it could be a declaration is treated as such.
This means the line
function<unique_ptr<TestA>()> fp(unique_ptr<TestA>(t)());
is interpreted as:
fp is the declaration of a function returning an std::function<unique_ptr<TestA>()> and expecting a parameter called t which is a function pointer to a function returning a std::unique_ptr<TestA> and getting no parameter. (Which is not what you intended.)
This also means that the t in this line is not the same t as in the previous line.
You have to pass fp something that is actually callable like this:
std::unique_ptr<TestA> f() {
return std::make_unique<TestA>();
}
void testComplexMap1() {
// ...
function<unique_ptr<TestA>()> fp(f);
m.emplace("TestA1", fp);
}
If you want to add a function to the map that wraps an existing pointer into a unique_ptr you would need either a functor:
class Functor {
public:
Functor(TestA * a) : m_a(a) {}
~Functor() { delete m_a; }
std::unique_ptr<TestA> operator()(){
auto x = std::unique_ptr<TestA>(m_a);
m_a = nullptr;
return std::move(x);
}
private:
TestA * m_a;
};
void testComplexMap1() {
//...
TestA * t = new TestA();
m.emplace("TestA", Functor(t));
}
Or a lambda with capture:
void testComplexMap1() {
//...
TestA * t = new TestA();
m.emplace("TestA", [t](){ return std::unique_ptr<TestA>(t); });
}
The lamda is translated more or less to something like the Functor class. However in each case you have to be really careful: The functions in the map that encapsulate an existing pointer into a std::unique_ptr can and should only be called once.
If you don't call them, memory allocated for t won't be freed. If you call them more than once you get either a std::unique_ptr to nullptr (in my Functor class variant) or a more than one std::unique_ptr tries to manage the same memory region (in the lambda with capture variant), which will crash as soon as the second std::unique_ptr is deleted.
In short: I would advice against writing code like this and only put functions in the map that are callable multiple times.
The PyBind11 documentation talks about using enum here.
The example shown assumes that the enum is embedded within a class, like so:
struct Pet {
enum Kind {
Dog = 0,
Cat
};
Pet(const std::string &name, Kind type) : name(name), type(type) { }
std::string name;
Kind type;
};
py::class_<Pet> pet(m, "Pet");
pet.def(py::init<const std::string &, Pet::Kind>())
.def_readwrite("name", &Pet::name)
.def_readwrite("type", &Pet::type);
py::enum_<Pet::Kind>(pet, "Kind")
.value("Dog", Pet::Kind::Dog)
.value("Cat", Pet::Kind::Cat)
.export_values();
My situation is different. I have a global enum the value of which is used to alter the behaviour of several functions.
enum ModeType {
COMPLETE,
PARTIAL,
SPECIAL
};
std::vector<int> Munger(
std::vector<int> &data,
ModeType mode
){
//...
}
I have tried to register it like so:
PYBIND11_MODULE(_mlib, m) {
py::enum_<ModeType>(m, "ModeType")
.value("COMPLETE", ModeType::COMPLETE )
.value("PARTIAL", ModeType::PARTIAL )
.value("SPECIAL", ModeType::SPECIAL )
.export_values();
m.def("Munger", &Munger, "TODO");
}
Compilation is successful and the module loads in Python, but I do not see ModeType in the module's names.
What can I do?
the sample below works for me. Like in my comment I used an "unscoped enum" (github.com/pybind/pybind11/blob/master/tests/test_enum.cpp).
I can use it like this
import pybind11_example as ep
ep.mood(ep.Happy)
code:
#include <pybind11/pybind11.h>
enum Sentiment {
Angry = 0,
Happy,
Confused
};
void mood(Sentiment s) {
};
namespace py = pybind11;
PYBIND11_MODULE(pybind11_example, m) {
m.doc() = "pybind11 example";
py::enum_<Sentiment>(m, "Sentiment")
.value("Angry", Angry)
.value("Happy", Happy)
.value("Confused", Confused)
.export_values();
m.def("mood", &mood, "Demonstrate using an enum");
}
I'm writing a simple, lightweight engine in D. For the input calls I use GLFW3. The library in question uses callbacks to send input events to the program.
What I would like is to use a method from a class as the callback function, rather than a function. This is proving difficult (just as it is in C++). I believe there is an elegant way to do it, but this is how I got it right now.
public void initialise(string logPath) {
[...]
m_Window = new RenderWindow();
m_Window.create();
// Lets set up the input loop.
GLFWkeyfun keyCB = function(GLFWwindow* win, int key, int scancode, int action, int mods) {
printf("Got key event: %d:%d:%d:%d\n");
RenderWindow rw = Root().getRenderWindow();
switch (key) {
case KeyboardKeyID.Q:
glfwSetWindowShouldClose(win, true);
break;
case KeyboardKeyID.H:
if (rw.hidden) {
rw.show();
} else {
rw.hide();
}
break;
default:
break;
}
};
glfwSetKeyCallback(m_Window.window, keyCB);
}
Here is the definition of the callback setting function and type:
extern (C) {
alias GLFWkeyfun = void function(GLFWwindow*, int, int, int, int);
GLFWkeyfun glfwSetKeyCallback(GLFWwindow*, GLFWkeyfun);
}
What I would like to do instead, is create a method that is part of the class. Is there any way to do this?
A solution I tried was a static method wrapped around in extern (C), this worked for calling it, but then I could (obviously) not access this or any other methods, which defeats the point of the exercise.
Thanks in advance.
The way I'd do it is to have a static map of the pointers to the class, so like:
static YourWindowClass[GLFWwindow*] mappings;
Then, in the constructor, once you get a GLFWwindow pointer, add it right in:
mappings[m_Window.window] = this;
Now, make the static extern(C) function to use as the callback. When it gets a pointer from C, look up your class reference in that mappings array and then go ahead and call the member function through that, forwarding the arguments.
So a bit of an extra step, but since it doesn't look like the callback lets you pass user-defined data to it (BTW, attention all lib writers: user-defined void* to the callbacks is sooooo useful, you should do it whenever possible!), but since it doesn't do that the associative array is the next best thing.
Well, I have figured it out my own. The solution I went with was a Singleton class InputManager. Instances of RenderWindow attach themselves to it with the following function. The InputManager then creates an anonymous function() for the RenderWindow that receives events, which then calls a function that handles the actual event.
The idea is then that listeners attach themselves to the InputManager and receive keyboard events for the RenderWindow they requested.
class InputManager {
private static InputManager m_Instance;
private RenderWindow[] m_Watched;
private KeyboardListener[][RenderWindow] m_KeyListeners;
public void recvKeyEvent(GLFWwindow* w, int k, int c, int a, int m) {
writeln("Received key: ", k);
}
public void watch(RenderWindow win) {
if (!isWatched(win)) {
// Relay the key callbacks onto the InputManager.
GLFWkeyfun keyCB = function(GLFWwindow* w, int k, int c, int a, int m) {
InputManager().recvKeyEvent(w, k, c, a, m);
};
glfwSetKeyCallback(win.window, keyCB);
}
}
private bool isWatched(RenderWindow win) {
foreach(RenderWindow w; m_Watched) {
if (win == w) {
return true;
}
}
return false;
}
public static InputManager opCall() {
if (m_Instance is null) {
m_Instance = new InputManager();
}
return m_Instance;
}
private this() {
// nothing
}
}
Works like a charm, now to figure out how to properly attach listeners elegantly.
For those curious, the full source code with how this is set up can be found at https://github.com/Adel92/Mage2D. I hope it helps someone else in a similar position with callbacks.
I need helping trying to retrieve data held in a std::list<boost::shared_ptr<boost::any>>
I working on a Singleton Controller class with a private std::list. Client class(es) will be able to add/remove/edit concrete class objects to be used by the program through this Controller class.
The reason for using boost::shared_ptr is because I assign a unique objID to each concrete class created. Once instance objs are added to controller, user will be able to search and remove objs later. The Add(....) and Remove(...) overloaded methods for each concrete class work fine.
I am now trying to create getObject(int index) & setObject(int index) methods but can't seem to figure out how to cast the returned pointer to a Concrete class.
Please advise.
My current code:
//===============================================================
//Singleton.h controller class
private:
static Singleton *mgr;
typedef boost::shared_ptr<boost::any> Shapes_Ptr;
//private static list
static std::list<Shapes_Ptr> shapes;
public:
const Shapes_Ptr getObject( int index) const; //Return Shape
Shapes_Ptr EditObject( const int index ); //Edit Shape
Info(); //Prints contents of instance to console screen
//===============================================================
//Singleton.cpp
//Return Shape
const Shapes_Ptr getObject( int index) const
{
int cc = 0;
if ( (int)shapes.size() > ZERO && index < (int)shapes.size() )
{
list<Shapes_Ptr>::const_iterator i;
for ( i = shapes.begin(); i != shapes.end(); ++i )
{
if ( cc == index )
{
return (*i);
break;
}
else { ++cc; }
}//for-loop
}
}
//Edit Shape
Shapes_Ptr EditObject( const int index )
{
//same code as getObject()...
}
//===============================================================
//main.cpp
Singleton *contrl= Singleton::Instance();
int main()
{
for ( int i=0; i< 2; ++i )
{
contrl->CreateObject(Box2D() );
}
for ( int i = contrl->Begin(); i< contrl->End(); ++i )
{
if ( boost::any_cast<boost::any> (contrl->getObject(i)).type() == typeid(Physics::Box2D) )
{
//Code compiles but crashes on launch....
any_cast<Box2D> (contrl->getObject(i) ).Info(); // <== ERROR CODE
}
//More if checks for other Concrete classes....
}
}
Putting aside whatever the particular issue with your current code is, I think there is an issue with your design.
You have this Singleton manager class that acts as a sort of pool and also as you say assigns unique IDs to each object so they can be found later. But you know what lets code find objects? Pointers! If you use a normal pool, one per type hierarchy (so no more Boost Any), you may find it just as useful, and there will be less nasty if/else typeid-checking code (which everyone would agree is not a good use of RTTI, aside from being poor OOP).
So, what do you say? Chuck this, and use Boost Pool if you want something to allocate your objects from a central place, and use pointers as your unique IDs, thus avoiding lookups along the way.