将std :: bind函数指针转换为void *并返回

Question

Let's say I want to bind a function and then pass it to another function as a void *, and cast it back.

I have a global function in which I try to cast the function:

void function_wrapper(void * func){

std::function<void(const std::vector<double>&, const std::vector<double>&)> * function = reinterpret_cast<std::function<void(const std::vector<double>&, const std::vector<double>&)> *>(func);

std::function<void(const std::vector<double>&, const std::vector<double>&)> function_ = *function;
}

func, meanwhile, is created as:

auto func = std::bind(&MyObj<dim>::myfunc, this, _1, _2);

Here dim is a templated integer equal to 2 And function_wrapper is called via

function_wrapper((void *)(&func));

Meanwhile, myfunc is a method of MyObj<dim> with type:

void myfunc(const std::vector<double>& x, std::vector<double>& F) const;

When I try to call function_wrapper as described above, I get the following error when I dereference it:

Exception thrown: read access violation.

std::_Func_class<void,std::vector<double,std::allocator<double> > const & __ptr64,std::vector<double,std::allocator<double> > const & __ptr64>::_Getimpl(...) returned 0xFFFFFFFFFFFFFFFF.

If there is a handler for this exception, the program may be safely continued.

I presume I have the typing wrong of my cast, but I don't know the correct way to declare the type. What is the proper way to do this?

Answer 1

The result of a std::bind is not a std::function , as the shown code assumes.

As such, when the shown code casts the pointer to the return value from std::bind to a void * , then casts it back to a pointer to a std::function , then invokes or accesses the function object, this results in undefined behavior.

[func.bind.isbind] specifies std::bind as follows:

template<class F, class... BoundArgs> unspecified bind(F&& f, BoundArgs&&... bound_args);

With the return value of: "A forwarding call wrapper g with a weak result type (20.9.2). The effect of g(u1, u2, ..., uM) shall be INVOKE (fd, std::forward(v1), std::forward(v2), ..., std::forward(vN), result_of_t)".

That's it. The class returned by std::bind is unspecified, and although it is certainly permitted for a given C++ implementation to, somehow, produce a std::function , it is not required to do so.

But, the solution is simple: explicitly assign the return value from std::bind to an appropriate std::function .