How can I use QtConcurrent::mapped with a member function as operator? Currently I'm using a callable object (which is ugly solution):
struct Vectorizer {
Vectorizer(DialogCreateSamples* cs)
: m_createSamples(cs) { }
typedef QString result_type;
QString operator()(const QString &input)
{
return m_createSamples->generateVector(input);
}
DialogCreateSamples* m_createSamples;
};
QFuture<QString> future = QtConcurrent::mapped(m_inputs,Vectorizer(this));
Also tried to pass lambda expressions but compiler says there is no result_type defined in lambda. This works with QtConcurrent::map because map do not need result_type. So if I can add a typedef in lambda it should work...
Maybe bind? Either std::bind if you're using C++11 or std::tr1::bind or boost::bind otherwise.
Something like:
QtConcurrent::mapped(m_inputs, std::bind(&Class::member_function, pointerToObjectOfTypeClass /* this? */, _1 /* placeholder for argument of your function filled by data from m_inputs */));
We use this in our code; example at: https://github.com/clementine-player/Clementine/blob/d03c1aa2419a0ceefd7f65114c1ac8991790b716/src/playlist/playlistbackend.cpp#L187
Hope it helps.
Related
I need to design a map, which save all function i may use in the futures.
all the functions will have double as its return value.
and all function share a common parameters const std::vector<float>&
so i define the map as:
typedef std::function<double(const std::vector<float>&)> func;
std::unordered_map<std::string, func> f_map;
for example. if i have a function looks like:
double func1(const std::vector<float>& v) {
return 0.; // just a demo
}
i can put it into map like this:
f_map.emplace("2015", static_cast<double(*)(const std::vector<float>&)>(func1));
it's ok since i tested it.
but the problem is:
i also have some function like this:
double func2(int a, const std::vector<float>& v) {
return 0.; // just for demo
}
how can i input this kind of function in my map?
std::bind doesnt work if i do like this:
f_map.emplace("2000", static_cast<double(*)(const std::vector<float>&)>(std::bind(func2, 1)));
can you help on this?
and can you give me your advice on how to save function into map better, thanks very much
What you're asking is essentially the same as:
How can I store both int and float in a map and use them equally?
Functions with different parameters are simply different. A map can only hold a single type. Suppose you get function "2019", how will you or the compiler ever know what kind of function it is?
There are a couple of ways to solve this problem. The essence is to have them be the same type. With std::bind you're on the right track. The correct way to use bind with parameters is to use the std::placeholders.
Say we have this function, double DoThing(int num, std::vector<float>& vec), and wish to fill in the first parameter, but leave the second one open for later. We can do:
mymap.emplace("2000", std::bind(&DoThing, 123, std::placeholders::_1));
You are free to shift the placeholder around however you like and add other ones too.
If you store the object bind returns and later get it back you can call it like so:
mymap["2000"](vec); //will call DoThing(123, vec)
Also. std::bind returns an object that holds both the function pointer to DoThing and any value you prefilled as well. Casting this to a function pointer like you did is not possible. That object is not a problem here since std::function, the map's contained type, has functionality to also store and, later, be able to call this object correctly.
Simillarly you can use lambda's to achieve the same effect as bind:
mymap.emplace("2000", [](std::vector<float>& vec){ return DoThing(123, vec); });
Other possibilities include unions/variants or other possibly tricky stuff. Though these methods will only make your code more complex if you don't know how to use them, so I don't recommend these.
Avoiding type casts is better and more safe. The first emplace [1] is valid without static_cast.
You should inform std::bind where to place the unbound argument with help of placeholders [2]. _1 of the namespace std::placeholders sets the unbound argument v to be the first argument of the resulting functional object.
If you use not capturing lamba expression, you can store pointers to functions. Pointers take less bytes than std::function objects. See [3].
#include <functional>
#include <string>
#include <unordered_map>
double func1(const std::vector<float>&) {
return 0.; // just a demo
}
double func2(int, const std::vector<float>&) {
return 0.; // just for demo
}
int main() {
using namespace std::placeholders;
typedef std::function<double(const std::vector<float>&)> func;
std::unordered_map<std::string, func> f_map;
f_map.emplace("2015", func1); // [1]
f_map.emplace("2000", std::bind(func2, 1, _1)); // [2]
// [3]
typedef double(*pfunc)(const std::vector<float>&);
std::unordered_map<std::string, pfunc> f_map2;
f_map2.emplace("2015", func1);
f_map2.emplace("2000", [](const std::vector<float>&v) { return func2(1, v); });
}
I have a legacy C code base, which I am migrating to C++ in a piecemeal fashion. It includes an interpreter, so there is a need to wrap static functions and arguments for use by the interpreter. So a typical function for export to the interpreter may have the following signature:
static void do_strstr(struct value * p)
and be exposed to the interpreter like so:
using vptr = void (*) ();
template <typename Func>
constexpr vptr to_vptr(Func && func)
{ return reinterpret_cast<vptr>(func); }
struct function string_funs[] = {
...
{ C_FN3, X_A3, "SSI", to_vptr(do_strstr), "find" },
...
};
This has been proven to work. The drawback with the method so far is that the called function must allocate memory onto a temporary stack. An improvement would be where the called function just returns a string, for example. This function is then wrapped, where the wrapper does the memory magic behind the scenes. This allows functions to created in a more vanilla way.
Here is an implementation which concatenates two strings using my improved method:
static std::string do_concata(struct value* p)
{
std::string s1 = (p)->gString();
std::string s2 = (p+1)->gString();
return s1+s2;
}
I create a helper function:
static void do_concata_1(struct value* p)
{
wrapfunc(do_concata)(p);
}
where the somewhat generic wrapper is defined as:
std::function<void(struct value*)>
wrapfunc(std::function<std::string(struct value*)> func)
{
auto fn = [=](struct value* p) {
std::string s = func(p);
char* ret = alloc_tmp_mem(s.size()+1);
strcpy(ret, s.c_str());
p->sString(ret);
return;
};
return fn;
}
which is exposed to the interpreter as follows:
struct function string_funs[] = {
...
{ C_FN2, X_A2, "SS", to_vptr(do_concata_1), "concata" },
...
};
I am not satisfied with this solution, though, as it requires a helper function for each function I define. It would be better if I could eliminate do_concata_1 and write another function that wraps the wrapfunc.
And this is where the problem is. If I write:
vptr to_vptr_1(std::function<void(struct value*)> func)
{
return to_vptr(wrapfunc(func));
}
then the compiler complains:
stringo.cc: In function ‘void (* to_vptr_1(std::function<void(value*)>))()’:
stringo.cc:373:30: error: could not convert ‘func’ from ‘std::function<void(value*)>’ to ‘std::function<std::__cxx11::basic_string<char>(value*)>’
return to_vptr(wrapfunc(func));
which is bizarre in my mind, because where did the std::__cxx11::basic_string<char> come from? It should be void, surely?
I'm at a loss to see what the fix should be. I am also a bit confused as to whether I should be passing copies of functions, references to functions, or the enigmatic && r-vale references.
In to_vptr_1(), func is established as a function that returns void. But func is passed to wrapfunc(), which expects a function that returns std::string. The compiler does not have a way to convert func from std::function<void(struct value*)> to std::function<std::string(struct value*)>, so it emits the error message.
reinterpret_cast from std::function to raw function pointer is not going to work. This question has some good discussion on the topic, and this one has a solution that could perhaps be reworked for this situation.
I want to call the following function and pass it a function with a parameter. The purpose of that is that it should call the function with my specified parameter so I know what triggered the function (in that case a gpio pin on the Raspberry Pi).
int wiringPiISR( int pin, int edgeType, void (*function)( void ) );
Currently I have:
for ( int i = 0; i < myValues.size(); ++i )
{
int myValue = myValues[ i ];
wiringPiISR( myValue, INT_EDGE_RISING, &myCallback( myValue ) );
}
Though this is giving me the following error:
error: lvalue required as unary ‘&’ operand
Which I can't really understand as to my understanding, myValue is an lvalue or is it not?
Is it what I want do even possible? If so how?
The function wiringPiISR is from a library called wiringPi and I would like to avoid modifying it as much as possible.
You could combine the answers from imreal and Ryan Haining something like this.
std::function<void()> cbfunc;
void myCallback()
{
cbfunc();
}
void myWiringPiISR(int val, int mask, std::function<void()> callback)
{
cbfunc = callback;
wiringPiISR(val, mask, &myCallback);
}
... and then use it...
void myActualCallback(int v)
{
... do something...
}
myWiringPiISR(myValue, INT_EDGE_RISING, std::bind(myActualCallback, myValue));
No need to patch library, and you can use all the bind/function goodness. I'll leave you to find a way around the thread safety issues...
How does it work? Put simply 'std::bind' is binding together a function and it's parameters into a single std:function object which can then be 'called' from the myCallback function which acts as a shim around the callback that you pass. I'd given the callback function a confusing name before, but this edit has hopefully fixed that.
You can "vomit" the function. This doesn't require a user-defined mutable global variable and is thread-safe, unless you have a compiler that supports multiple threads but not per-thread exceptions which would be basically unusable.
myWiringPiISRWrapper(Value value, int edge, std::function<void()> func) {
try {
throw &func;
} catch(...) {
myWiringPiISR(value, edge, [] {
try {
throw;
} catch(std::function<void()>* func) {
(*func)();
}
});
}
}
It's disgusting and slow, but it's totally encapsulated which I think is a worthwhile upside. Note that this only works if the callback is never executed after the call to myWiringPiISR returns. In this case you can of course have a callback with whatever bound state you desire.
If myValue is something you can decide at compile time, you could set it statically and use an intermediate function to pass in.
void myCallbackHelper() {
static constexpr int myValue = 3;
myCallback(myValue);
}
wiringPiISR(myValue, INT_EDGE_RISING, &myCallbackHelper);
If you need to determine myValue at run time, you could still accomplish this, but not really thread-safely.
int& getMyValue() {
static int myValue;
return myValue;
}
void setMyValue(int i) {
getMyValue() = i;
}
void myCallbackHelper() {
myCallback(getMyValue());
}
Then set it and call
setMyValue(3);
wiringPiISR(myValue, INT_EDGE_RISING, &myCallbackHelper);
I looked up wiringPiISR and found that it is some sort of api call, so i am assuming you cannot change it.
Having said that, there is a reason most api-calls with a function-pointer-callback look sort of like this
void setCallback( void (*function)(void* data), void* userdata);
This allows people to cast their struct {blabla} data; to add some userdata, and when the function is called, it is passed along.
So basically, apart from hacking stuff with static variables, you can't pass any arguments.
You need to use std::function and std::bind.
Change your function signature to
int wiringPiISR (int pin, int edgeType, std::function<void()> func);
Inside you can call the callback simply using func()
And your call to:
int myValue = 3;
wiringPiISR(myValue, INT_EDGE_RISING, std::bind(myCallback, myValue));
What this does is create a std::function object (i.e. a callable) that wraps your function and keeps your desired value in its state.
This will only work on C++11 and newer.
If you have c++11, I suggest using std::function - it's quite a bit cleaner.
If not, your function signature is wrong. You want a callback with the type void(int) but your function takes a void()
Say that you define a callback function as such:
typedef std::function<void(float)> Callback;
And you have a function as such:
void ImAFunction(float a)
{
//Do something with a
}
Is there a way to be able to store a function without an argument then pass one to it at a later time?
Such as this:
//Define the Callback storage
Callback storage;
storage = std::bind(ImAFunction, this);
//Do some things
storage(5);
This wont work which I explain with some of my real code below.
I can get close to what I wan't if I bind the value in with the std::bind function. Such as:
//Change
//storage = std::bind(ImAFunction, this);
storage = std::bind(ImAFunction, this, 5.0); //5.0 is a float passed
This works but when I go to pass a value through the function the outcome is whatever I set it to before:
storage(100); //Output is still 5
I am basing the fact that I think this is possible on this article.
http://www.cprogramming.com/tutorial/function-pointers.html
It doesn't use the function or bind functions but it does pass pointer arguments and performs exactly what I need. The reason I don't just skip the bind function is because I am trying to store the function in a class (private) and I can't store it if it's a template because it's created with the class.
The error produced above comes from this code:
struct BindInfo {
Callback keyCallback;
int bindType;
bool isDown;
bool held;
std::string name;
};
template <class T1>
void bindEvent(int bindType, T1* keydownObj, void(T1::*keydownF)(float), std::string name)
{
BindInfo newKeyInfo = { std::bind(keydownF, keydownObj), bindType, false, false, name };
inputBindings.insert(std::pair<int, BindInfo>(BIND_NULL, newKeyInfo));
};
The error is:
No viable conversion from '__bind<void(Main::*&)(float), Main *&>' to 'Callback' (aka 'function<void (float)>'
Is this possible? Thanks in advance.
You can include a placeholder for an unbound argument:
std::bind(&Main::ImAFunction, this, std::placeholders::_1);
If you find that a bit of a mouthful, a lambda might be more readable:
[this](float a){ImAFunction(a);}
It sounds like what you're looking for is a function pointer. While I don't have a lot of experience using them in C++ I have used them in C so: Yes, it is possible. Perhaps something like this:
void (*IAmAFunctionPointer)(float) = &IAmAFunction;
The best way to think about that line is, that IAmAFunctionPointer is a pointer (hence the *), it returns a void, and takes a float. Then later:
float a = 5;
IAmAFunctionPointer(a);
You could even design it so that the callback function is passed into the method (I assume this is what you're looking for).
void DoStuffThenCallback(float a, void (*callback)(float))
{
//DoStuff
callback(a);
}
further reading: http://www.cprogramming.com/tutorial/function-pointers.html
I've recently become enamored with the simplicity of Erlang's actor-based concurrency model, and am playing around with ideas for implementing some parts of it in C++. Along these lines, I also like the idea of implementing a finite state machine as a collection of functions representing states, where transitions are made by tail-calling from one function to the next.
I'd like to try something similar in C++. But a naive implementation of this is likely to run into the fact that tail calling in my compiler (GCC 4.1 with -O0) will eventually cause a stack overflow. So instead, what I'd like to do is have each state/function return a functor (the next state to enter), and have an underlying loop which just sequentially calls a functor, then calls the functor thus returned, then calls the functor thus returned, etc. Something like:
typedef ... context_t;
// A statefunctor is a functor which takes a context_t and
// returns a statefunctor
//
// FIXME: of course, this typedef won't compile.
typedef boost::function<statefunctor (context_t& )> statefunctor;
// NULL boost::function<> represents the exit condition.
static const statefunctor EXIT_FSM;
// primary loop which runs the FSM
void run_fsm(context_t& ctx, statefunctor initial_state)
{
while (initial_state)
{
initial_state=initial_state(boost::ref(ctx));
}
}
// states 'foo', 'bar', and 'baz';
statefunctor foo(context_t& ctx);
statefunctor bar(context_t& ctx, int inval);
statefunctor baz(context_t& ctx);
// State 'foo'
statefunctor foo(context_t& ctx)
{
// act somehow on the external context
int magic_number_1=ctx.get_magic_number();
int magic_number_2=ctx.get_magic_number();
// Always transition to 'bar'
return boost::bind(&bar, _1, magic_number_1-magic_number_2);
}
// State 'bar'
statefunctor bar(context_t& ctx, int inval)
{
inval+=ctx.get_magic_number(); // Act on external context somehow
// transition to foo or baz
if (inval>0) { return &foo; }
else { return &baz; }
}
// State 'baz'
statefunctor baz(context_t& ctx)
{
// Transition to foo or exit
if (ctx.get_magic_number()==5) {return EXIT_FSM;}
else {return &foo;}
}
int main()
{
context_t ctx;
// start the state machine in state 'foo'
run_fsm(ctx, &foo);
}
So, my question is, how do I define statefunctor? In particular, I want it to be capable of holding arbitrary functors (like boost::bind(...) might create), and not just function pointers.
NOTE: I'm using boost::bind, boost::function, boost::ref instead of their std:: counterparts because I'm stuck using GCC 4.1, which has no support for C++11. Solutions valid in C++03 are appreciated ;-).
You can't directly do this through a typedef, but you can wrap the boost::function in a struct / class (thanks to #R. Martinho Fernandes for making me have this insight):
#include <boost/function.hpp>
typedef int context_t;
struct statefunctor
: boost::function<statefunctor(context_t&)>
{
typedef boost::function<statefunctor(context_t&)> base_type;
statefunctor() : base_type(){}
template<class F>
statefunctor(F f) : base_type(f){}
};
Live example.
This is impossibru. The type would be infinite, and the problem is identical to the one you would encounter defining a function pointer that returns itself. The only way to do this is to manually write your own function object with an operator(), this can return *this, and chain() calls. You can also use operator chaining in other ways, like you can see in std::cout.
You cannot. The problem is that the definition of the returned type would have to be recursive and that is not possible.