Necromancing.
I think the answers to date are a little unclear.
Let's make an example:
Supposed you have an array of pixels (array of ARGB int8_t values)
// A RGB image
int8_t* pixels = new int8_t[1024*768*4];
Now you want to generate a PNG.
To do so, you call the function toJpeg
bool ok = toJpeg(writeByte, pixels, width, height);
where writeByte is a callback-function
void writeByte(unsigned char oneByte)
{
fputc(oneByte, output);
}
The problem here: FILE* output has to be a global variable.
Very bad if you're in a multithreaded environment (e.g. a http-server).
So you need some way to make output a non-global variable, while retaining the callback signature.
The immediate solution that springs into mind is a closure, which we can emulate using a class with a member function.
class BadIdea {
private:
FILE* m_stream;
public:
BadIdea(FILE* stream) {
this->m_stream = stream;
}
void writeByte(unsigned char oneByte){
fputc(oneByte, this->m_stream);
}
};
And then do
FILE *fp = fopen(filename, "wb");
BadIdea* foobar = new BadIdea(fp);
bool ok = TooJpeg::writeJpeg(foobar->writeByte, image, width, height);
delete foobar;
fflush(fp);
fclose(fp);
However, contrary to expectations, this does not work.
The reason is, C++ member functions are kinda implemented like C# extension functions.
So you have
class/struct BadIdea
{
FILE* m_stream;
}
and
static class BadIdeaExtensions
{
public static writeByte(this BadIdea instance, unsigned char oneByte)
{
fputc(oneByte, instance->m_stream);
}
}
So when you want to call writeByte, you need pass not only the address of writeByte, but also the address of the BadIdea-instance.
So when you have a typedef for the writeByte procedure, and it looks like this
typedef void (*WRITE_ONE_BYTE)(unsigned char);
And you have a writeJpeg signature that looks like this
bool writeJpeg(WRITE_ONE_BYTE output, uint8_t* pixels, uint32_t
width, uint32_t height))
{ ... }
it's fundamentally impossible to pass a two-address member function to a one-address function pointer (without modifying writeJpeg), and there's no way around it.
The next best thing that you can do in C++, is using a lambda-function:
FILE *fp = fopen(filename, "wb");
auto lambda = [fp](unsigned char oneByte) { fputc(oneByte, fp); };
bool ok = TooJpeg::writeJpeg(lambda, image, width, height);
However, because lambda is doing nothing different, than passing an instance to a hidden class (such as the "BadIdea"-class), you need to modify the signature of writeJpeg.
The advantage of lambda over a manual class, is that you just need to change one typedef
typedef void (*WRITE_ONE_BYTE)(unsigned char);
to
using WRITE_ONE_BYTE = std::function<void(unsigned char)>;
And then you can leave everything else untouched.
You could also use std::bind
auto f = std::bind(&BadIdea::writeByte, &foobar);
But this, behind the scene, just creates a lambda function, which then also needs the change in typedef.
So no, there is no way to pass a member function to a method that requires a static function-pointer.
But lambdas are the easy way around, provided that you have control over the source.
Otherwise, you're out of luck.
There's nothing you can do with C++.
Note:
std::function requires #include <functional>
However, since C++ allows you to use C as well, you can do this with libffcall in plain C, if you don't mind linking a dependency.
Download libffcall from GNU (at least on ubuntu, don't use the distro-provided package - it is broken), unzip.
./configure
make
make install
gcc main.c -l:libffcall.a -o ma
main.c:
#include <callback.h>
// this is the closure function to be allocated
void function (void* data, va_alist alist)
{
int abc = va_arg_int(alist);
printf("data: %08p\n", data); // hex 0x14 = 20
printf("abc: %d\n", abc);
// va_start_type(alist[, return_type]);
// arg = va_arg_type(alist[, arg_type]);
// va_return_type(alist[[, return_type], return_value]);
// va_start_int(alist);
// int r = 666;
// va_return_int(alist, r);
}
int main(int argc, char* argv[])
{
int in1 = 10;
void * data = (void*) 20;
void(*incrementer1)(int abc) = (void(*)()) alloc_callback(&function, data);
// void(*incrementer1)() can have unlimited arguments, e.g. incrementer1(123,456);
// void(*incrementer1)(int abc) starts to throw errors...
incrementer1(123);
// free_callback(callback);
return EXIT_SUCCESS;
}
And if you use CMake, add the linker library after add_executable
add_library(libffcall STATIC IMPORTED)
set_target_properties(libffcall PROPERTIES
IMPORTED_LOCATION /usr/local/lib/libffcall.a)
target_link_libraries(BitmapLion libffcall)
or you could just dynamically link libffcall
target_link_libraries(BitmapLion ffcall)
Note:
You might want to include the libffcall headers and libraries, or create a cmake project with the contents of libffcall.
