Is there a way for a function to append to variable number of arguments it already received? I'm using __builtin_va* of libgcc for ARM to implement variable argument functions (typedef'ed appropriately)
int wrapper(char *name, ...)
{
...
va_list args;
va_start(args, name);
another_function(name, args);
va_end(args);
...
}
...
void another_function(char *name, va_list p)
{
}
Since another_function has no way of detecting the end of arguments, I want to pass NULL as the last argument. Is there anything that can be done in wrapper to have NULL accessible in another_function using va_list pointer p?
Related
I'd like to have a wrapper thread function, i.e. a function executed by a thread which does some extra stuff, and then calls the user function.
template<class F, class... Args>
void wrapper(F&& user_function, Args&&... args) {
// do some extra stuff
user_function(args); // maybe I need to forward args
// do some extra stuff
}
Ok, this could be a nice wrapper, so I need a manager that uses this wrapper function and allows the user to spawn his own threads:
class ThreadManager {
public:
template<class F, class... Args>
std::thread newThread(F&& f, Args&&... args) {
return std::thread(thread_wrapper<F,Args...>, std::forward<F>(f), std::forward<Args>(args)...);
}
};
this way the thread manager SHOULD spawn a thread that uses the wrapper function which, in turn, does its extra work and calls the user function.
But the compiler now says: Attempt to use a deleted function.
The error is in the thread header:
template <class _Fp, class ..._Args, size_t ..._Indices>
inline _LIBCPP_INLINE_VISIBILITY
void
__thread_execute(tuple<_Fp, _Args...>& __t, __tuple_indices<_Indices...>)
{
__invoke(_VSTD::move(_VSTD::get<0>(__t)), _VSTD::move(_VSTD::get<_Indices>(__t))...);
}
What am I missing/doing wrong?
[edit]
Using test:
void foo(int i) {
std::cout << "foo: " << i << std::endl;
}
int main(int argc, const char *argv[]) {
ThreadManager mgr;
auto t = mgr.newThread(foo, 10);
t.detach();
std::this_thread::sleep_for(std::chrono::milliseconds(1000));
return 0;
}
I'm using Xcode 7.1 with LLVM compiler, but fails on FreeBSD clang 3.3 too.
The Xcode error is:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/thread:337:5: error: attempt to use a deleted function
__invoke(_VSTD::move(_VSTD::get<0>(__t)), _VSTD::move(_VSTD::get<_Indices>(__t))...);
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/thread:347:5: note: in instantiation of function template specialization 'std::__1::__thread_execute' requested here
__thread_execute(*__p, _Index());
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/thread:359:42: note: in instantiation of function template specialization 'std::__1::__thread_proxy >' requested here
int __ec = pthread_create(&__t_, 0, &__thread_proxy<_Gp>, __p.get());
I'm not sure what is causing the "Attempt to use a deleted function" in your example, I get other errors related to std::thread's bind mechanism.
It appears the way you are spelling out the template arguments for thread_wrapper is not playing nice with std::thread's constructor - in particular when it uses a simplified std::bind internally. The mix of perfectly forwarded function types and std::decayed function pointers seems to upset std::result_of.
we can make it work by applying some std::decay in newThread ourselves:
return std::thread( thread_wrapper<typename std::decay<F>::type,
typename std::decay<Args>::type...>,
std::forward<F>(f),
std::forward<Args>(args)... );
...but to be honest I'm not entirely sure why that works.
Alternatively, with some indirection and more forwarding, we can avoid having to spell out the template arguments.
We just need a functor that forwards to thread_wrapper (or a polymorphic lambda in C++14):
struct wrapper_helper {
template<class F, class... Args>
void operator()(F&& f, Args&&... args) const {
thread_wrapper(std::forward<F>(f), std::forward<Args>(args)...);
}
};
And use it in newThread:
return std::thread(wrapper_helper{}, std::forward<F>(f), std::forward<Args>(args)...);
Here's the full example showing arguments passed by value, reference and rvalue reference working as intended: http://coliru.stacked-crooked.com/a/b75d5a264f583237
Note: For move-only types like std::unique_ptr, you will definitely want to forward args... in thread_wrapper.
I created a threadpool which captures a function and arguments into tuples and then perfect forwards when the task is dequeued.
However I am unable to pass a vector of unique_ptr's to the thread by rvalue. A simplified project is below:
#include <future>
#include <memory>
#include <vector>
template <typename F, typename... Args>
typename std::result_of<F(Args...)>::type pushTask(F&& f, Args&&... args)
{
using result_type = typename std::result_of<F(Args...)>::type;
// create a functional object of the passed function with the signature std::function<result_type(void)> by creating a
// bound Functor lambda which will bind the arguments to the function call through perfect forwarding and lambda capture
auto boundFunctor = [func = std::move(std::forward<F>(f)),
argsTuple = std::move(std::make_tuple(std::forward<Args>(args)...))](void) mutable->result_type
{
// forward function and turn variadic arguments into a tuple
return result_type();
};
// create a packaged task of the function object
std::packaged_task<result_type(void)> taskFunctor{ std::move(boundFunctor) };
}
int main(int argc, char *argv [])
{
auto testvup = [](std::vector<std::unique_ptr<int>>&& vup)
{
};
std::vector<std::unique_ptr<int>> vup;
pushTask(testvup, std::move(vup));
}
I get the following compiler error with VS2015 rather I use the std::function or std::packaged_task
Severity Description Project File Line
Error error C2280: 'std::unique_ptr<int,std::default_delete<_Ty>>::unique_ptr(const std::unique_ptr<_Ty,std::default_delete<_Ty>> &)': attempting to reference a deleted function Stack xmemory0 659
passing other arguments by rvalue including std::vector works.
Has anyone else run across this or have suggestions.
C++ Standard section ยง20.9.11.2.1 [func.wrap.func]
template<class F> function(F f);
template <class F, class A> function(allocator_arg_t, const A& a, F
f);
Requires: F shall be CopyConstructible. f shall be Callable for
argument types ArgTypes and return type R. The copy constructor and
destructor of A shall not throw exceptions.
Your lambda function boundFunctor is a move only type (because it captures move only types, since std::unique_ptr cannot be copied)
Hence, boundFunctor is not copyable and not suitable as an argument to an std::function
# include "stdafx.h"
# include <iostream>
#include <ctype.h>
using namespace std;
class a
{
protected:
int d;
public:
virtual void assign(int A) = 0;
int get();
};
class b : a
{
char* n;
public:
b()
{
n=NULL;
}
virtual ~b()
{
delete n;
}
void assign(int A)
{
d=A;
}
void assignchar(char *c)
{
n=c;
}
int get()
{
return d;
}
char* getchart()
{
return n;
}
};
class c : b
{
b *pB;
int e;
public:
c()
{
pB=new b();
}
~c()
{
delete pB;
}
void assign(int A)
{
e=A;
pB->assign(A);
}
int get()
{
return e;
}
b* getp()
{
return pB;
}
};
int _tmain(int argc, _TCHAR* argv[])
{
c *pC=new c();
pC->assign(10);
b *p=pC->getp();
p->assignchar("a");
char *abc=p->getchart();
delete pC;
cout<<*abc<<endl;
getchar();
}
i'm a noob at c++ and was experimenting when i got to this point. I don't understand why i keep getting a memory corruption message from VS2010. I am trying to replicate a problem which is at a higher level by breaking it down into smaller bits, any help would be appreciated.
From a cursory glance, you are passing a static char array to AssignChar that cannot be deleted (ie when you type "A" into your code, its a special block of memory the compiler allocates for you).
You need to understand what assignment of a char* does (or any pointer to type). When you call n=c you are just assigning the pointer, the memory that pointer points to remains where it is. So, unless this is exactly what you meant to do, you will have 2 pointers pointing to the same block of memory.. and you need to decide which to delete (you can't delete it twice, that'd be bad).
My advice here is to start using C++, so no more char* types, use std::string instead. Using char* is C programming. Note that if you did use a std::string, and passed one to assignChars, it would copy as you expected (and there is no need to free std::string objects in your destructor, they handle all that for you).
The problem occurs when you're trying to delete pC.
When ~c() destructor calls ~b() destructor - you're trying to delete n;.
The problem is that after assignchar(), n points to a string literal which was given to it as an argument ("a").
That string is not dynamically allocated, and should not be freed, meaning you should either remove the 'delete n;' line, or give a dynamically-allocated string to assignchar() as an argument.
How do I write je_malloc_stats_print() result into a file instead of stderr?
Now what I can do is only je_malloc_stats_print(NULL, NULL, NULL) to stderr;
The first parameter is a callback function pointer, and the second is for passing data to the callback. You can use these parameters to implement writing to a file. From the jemalloc manual page:
void malloc_stats_print(
void (*write_cb) (void *, const char *) ,
void *cbopaque,
const char *opts);
/*Actually The default function, it uses inside print the stat data on STDERR output stream. To print it to a file, It needs to provide a callback function, which it will call to print the buf stream into. */
int g_fd;
void print_my_jemalloc_data(void *opaque, const char *buf);
int main()
{
int fd = open("heap_stats.out,O_CREAT|O_WRONLY,0666);
g_fd = fd;
malloc_stats_print(print_my_jemalloc_data,NULL,NULL);
/*Passing my callback routine which jemalloc will use internally to print data into*/
return 0;
}
void print_my_jemalloc_data(void *opaque,const char *buf)
{
write(g_fd,buf,strlen(buf));`enter code here`
}
You may replace your function, with same signature, and replace at malloc_stat_print API, first parameter call back function. It will be passed on buf, which you may print in your defined file stream, which you have opened before..
//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);