I'm using pthreads to pass multiple parameters to a function by way of a struct. I'm attempting to convert the struct I pass in using a conversion from void * to struct thread_args *. This is giving me an error. Can someone please explain how to manage this in C++.
Here is my code:
struct thread_args
{
int customer_num;
int res_vector[NUMBER_OF_RESOURCES];
};
typedef struct thread_args theArgs;
int request_resources(int customer_num, int request[]);
int release_resources(int customer_num, int release[]);
void req_converter( void * x );
void rel_converter( void * x );
void req_converter( void * x )
{
theArgs * args = dynamic_cast<theArgs *>(x); //(struct arg_struct *) x;
request_resources( x.customer_num, x.res_vector);
}
The error message I'm getting is:
error: cannot dynamic_cast ‘x’ (of type ‘void*’) to type ‘theArgs* {aka struct thread_args*}’ (source is not a pointer to class)
Can someone please explain the best way to pass a struct using pthreads to a function that will use the struct parameters to call a second function with multiple parameters in C++?
Related
I coded an Arduino project for my son and learned about C in the process. All works fine but after dividing up the code into ten files and grouping the variables into structs in each file I'm not able to solve one wish for clarity. We need to empirically determine the best size of an array for storing and averaging port reads so this is what I want:
struct Alarms {
// Configurable parameters
const unsigned int number_of_reads = 24;
// State variables
int reads[number_of_reads]; // Error: invalid use of non-static data member 'Alarms::num_of_reads'
};
It’s simple but doesn't work. I tried flexible array members until I found that that feature is not supported in C++. Arduino compiles with C++. I tried many examples of the 'struct hack' but they all returned errors like this one:
struct Alarms {
// Configurable parameters
int number_of_reads = 24;
// State variables
int reads[];
} ar;
void setup_alarm() {
ar.reads = malloc(sizeof(int) * ar.number_of_reads); // Error: incompatible types in assignment of 'void*' to 'int [0]'
}
That looked promising but I suspect my ignorance is glowing brightly. Most struct hack examples call for declaring the struct and later initializing the struct variables. I’m hoping to not duplicate the struct.
I considered splitting the struct but that would be error prone and, well, another compile error:
struct Alarms2 {
int reads[ar.num_of_reads]; // Error: array bound is not an integer constant before ']' token
} ar2;
An alternative is to size the array and get the size later but it needs an explanation:
struct Alarms {
// Configurable parameters
int reads[ 24 ]; // Put number of reads to average between brackets
// State variables
int number_of_reads;
};
void setup_alarm() {
ar.number_of_reads = sizeof(ar.reads) / sizeof(ar.reads[0]); // this works
}
Is there a way to work the struct hack or some similar solution in Arduino to like achieve the first example?
The size of the struct must be known at compilation time. Const data types in structs can change per instance of the structure, that is why you are getting the invalid use of non-static data member 'Alarms::num_of_reads' when you try to initialize your array. The best way to solve this is to have an init_alarm and destroy_alarm functions. Like so ...
#include <stdio.h>
#include <stdlib.h>
#define DEFAULT_NUM_OF_READS (24)
struct alarm {
// Configurable parameters
const int number_of_reads;
// State variables
int *reads;
};
void init_alarm(struct alarm *alarm)
{
alarm->reads = (int *) malloc(alarm->number_of_reads * sizeof(int));
}
void destroy_alarm(struct alarm *alarm)
{
free(alarm->reads);
}
int main(int argc, char **argv)
{
// When we create our struct, set number_of_reads to default
struct alarm alarm = {.number_of_reads = DEFAULT_NUM_OF_READS, .reads = NULL};
init_alarm(&alarm);
alarm.reads[0] = 13;
alarm.reads[23] = 100;
printf("alarm.reads[0] = %d, alarm.reads[23] = %d\n", alarm.reads[0], alarm.reads[23]);
destroy_alarm(&alarm);
return 0;
}
Note: Inorder to use the designated initializer to initialize a structure you must compile with ANSI (C99) like so ...
gcc --std=c99 test.c -o test
I have some homework, and I have troubles understanding, (probably) how passing parameters to std::thread constructor works.
Assume following code (I deleted unneeded parts)
template<typename T, typename Task>
class Scheduler
{
private:
typedef std::unordered_map<std::size_t, T> Results;
class Solver
{
public:
Solver(Task&& task) : m_thread(&Solver::thread_function, std::move(task))
{
m_thread.detach();
}
Solver(Solver&& solver) = default; // required for vector::emplace_back
~Solver() = default;
private:
void thread_function(Task&& task)
{
task();
}
std::thread m_thread;
};
public:
Scheduler() = default;
~Scheduler() = default;
void add_task(Task&& task)
{
m_solvers.emplace_back(std::move(task));
}
private:
std::vector<Solver> m_solvers;
};
template<typename T>
struct Ftor
{
explicit Ftor(const T& t) : data(t) { }
T operator()() { std::cout << "Computed" << std::endl; return data; }
T data;
};
int main()
{
Scheduler<int, Ftor<int>> scheduler_ftor;
Scheduler<int, std::function<int(void)>> scheduler_lambda;
Ftor<int> s(5);
scheduler_ftor.add_task(std::move(s));
scheduler_lambda.add_task([](){ std::cout << "Computed" << std::endl; return 1; });
}
Why it doesn't compile?
MVS2015 is complaining about
functional(1195): error C2064: term does not evaluate to a function taking 1 arguments
functional(1195): note: class does not define an 'operator()' or a user defined conversion operator to a pointer-to-function or reference-to-function that takes appropriate number of arguments
note: while compiling class template member function 'Scheduler<int,Ftor<int> >::Solver::Solver(Task &&)'
While G++ 4.9.2
functional: In instantiation of ‘struct std::_Bind_simple<std::_Mem_fn<void (Scheduler<int, Ftor<int> >::Solver::*)(Ftor<int>&&)>(Ftor<int>)>’:
required from ‘void Scheduler<T, Task>::add_task(Task&&) [with T = int; Task = Ftor<int>]’
functional:1665:61: error: no type named ‘type’ in ‘class std::result_of<std::_Mem_fn<void (Scheduler<int, Ftor<int> >::Solver::*)(Ftor<int>&&)>(Ftor<int>)>’
typedef typename result_of<_Callable(_Args...)>::type result_type;
I suppose there are some problems with std::moving to std::thread.
If you use member function as first thread argument, second argument supposed to be this pointer, pointing to the object to which member function could be called to
UPDATE
Good discussion here
Start thread with member function
I don't follow your code, but addressing the question, a extrapolated answer will be( most of the code is psuedocode)
lets assume that there is a function int test(int name).
thread t0;
t0 = thread(test,32);
thread t1(test,43);
Passing a argument to function.
int temp = 0;
int testfunc(int& q)
{
cout<<q;
}
thread t1;
t1 = thread(testfunc,ref(temp));
In short, you just pass the name of the function that must be run in the thread as the first argument, and the functions parameters follow it in same order as they are in the function definition, for passing by reference you can use the ref() wrapper.See the below example.
#include <iostream>
#include <thread>
#include <string>
using namespace std;
void test(int a,int &a,string test)
{
\\do something
}
int main()
{
int test1 = 0;
string tt = "hello";
thread t1;
t1 = thread(23,&test1,tt);
t1.detach();
return 0;
}
if you are wondering about the use of join() and detach(), refer to this thread: When should I use std::thread::detach?, refer to my answer post in that thread.
//Block.h
#pragma once
class Block
{
public:
CRect pos;
int num;
public:
Block(void);
~Block(void);
};
//view class
public:
Block currentState[5]; // stores the current state of the blocks
void CpuzzleView::OnDraw(CDC* pDC)
{
CpuzzleDoc* pDoc = GetDocument();
ASSERT_VALID(pDoc);
if (!pDoc)
return;
//draw the 4 blocks and put text into them
for(int i=0;i<4;i++)
{
pDC->Rectangle(currentState[i].pos);
// i'm getting an error for this line:
pDC->TextOut(currentState[i].pos.CenterPoint(), currentState[i].num);
}
pDC->TextOut(currentState[i].pos.CenterPoint(), currentState[i].num);
The error says that no instance of overloaded function CDC::TextOutW() matches the argument list . But the prototype for the function is:
CDC::TextOutW(int x, int y, const CString &str )
all i've done is that instead of the 2 points i've directly given the point object returned by CenterPoint() ... shouldn't it work?
That's because you didn't supplied arguments list correctly. Please read compiler error message carefully, it's usually helps to solve the problem.
TextOut(currentState[i].pos.CenterPoint(), currentState[i].num);
In this call you passed CPoint object and int. This is not correct, you need to pass int, int and CString (or const char* and int length).
To fix this you shall do something like this:
CString strState;
strState.Format("%d", currentState[i].num); // Or use atoi()/wtoi() functions
TextOut(currentState[i].pos.CenterPoint().x, currentState[i].pos.CenterPoint().x, strState);
I have the following in my WinRT component:
public value struct WinRTStruct
{
int x;
int y;
};
public ref class WinRTComponent sealed
{
public:
WinRTComponent();
int TestPointerParam(WinRTStruct * wintRTStruct);
};
int WinRTComponent::TestPointerParam(WinRTStruct * wintRTStruct)
{
wintRTStruct->y = wintRTStruct->y + 100;
return wintRTStruct->x;
}
But, it seems that the value of winRTStruct->y and x are always 0 inside the method, when called from C#:
WinRTComponent comp = new WinRTComponent();
WinRTStruct winRTStruct;
winRTStruct.x = 100;
winRTStruct.y = 200;
comp.TestPointerParam(out winRTStruct);
textBlock8.Text = winRTStruct.y.ToString();
What is the correct way to pass a struct by reference so it an be updated inside the method of a WinRTComponent written in C++/CX?
You cannot pass a struct by reference. All value types (including structs) in winrt are passed by value. Winrt structs are expected to be relatively small - they're intended to be used for holding things like Point and Rect.
In your case, you've indicated that the struct is an "out" parameter - an "out" parameter is write-only, its contents are ignored on input and are copied out on return. If you want a structure to be in and out, split it into two parameters - one "in" parameter and another "out" parameter (in/out parameters are not allowed in WinRT because they don't project to JS the way you expect them to project).
My co-worker helped me solve this.
In WinRT components, it seems that the best way to do this is to define a ref struct instead of a value struct:
public ref struct WinRTStruct2 sealed
{
private: int _x;
public:
property int X
{
int get(){ return _x; }
void set(int value){ _x = value; }
}
private: int _y;
public:
property int Y
{
int get(){ return _y; }
void set(int value){ _y = value; }
}
};
But this creates other problems. Now the VS11 compiler gives INTERNAL COMPILER ERROR when I try to add a method to the ref struct that returns an instance of the struct.
I am trying to store a pointer to a member function in a structure which will be used to call the function later in my program.
Something like this:
// abc.h
namespace XYZ {
typedef void func(const uint8_t *buf, int len);
struct holder
{
// other members
func * storePtr;
}
} // end of namespace
the other file as:
// pqr.h
#include abc.h
namespace XYZ {
class pqr {
// data members and other functions
void func1(const uint8_t *buffer, int length);
void func2(func *section);
void func3();
}
} // end of namespace
Now my cpp file needs to store instance of this func1 in my structure member storePtr
// app.cpp
#include pqr.h
void pqr::funct3()
{
// Do something
func2(func1);
}
void pqr::func2(func * section)
{
holder h;
h.storePtr = section;
}
But I am getting compilation error at line "func2(func1);" as
"error C3867: 'pqr::func1': function call missing argument list; use '&pqr::func1' to create a pointer to member"
I have used &pqr:: to define the scope but it also doesn't solve my problem and I am not able to understand what to do.
Pointers to member function are not the same thing as pointers to normal functions - have a look at the explanation and example here: http://msdn.microsoft.com/en-us/library/k8336763.aspx