c++ void* to parameter to a function - c++

Question

I have this function in some library:
class myConsole
{
void addCommand( std::string command, void* fn );
...
}
and in my class I have this function:
void myApp::TestFn( const std::vector<std::string> & args )
{
// do something
}
in the same class I call this:
void myApp::initApp( )
{
myConsole::getSingleton( ).addCommand( "FirstTest", &myApp::TestFn );
}
but this gives me this error:
error c2664 cannot convert parameter 2 from 'void(__thiscall
myApp::*)(const std::vector<_Ty>&)' to 'void *'
how can I solve this?
thanks in advance!

Answer 1

You can't solve this. You can't reliably cast a function pointer to void * and back.
(I suggest you redesign the program and stay clear of void*; there's no real need for it in C++.)

Answer 2

The problem here is that you are trying to pass a class method as it were a void * pointer. This cannot be done.
The right way of doing this is by using templates for the void addCommand (std::string, void *) method. Something like
class myConsole {
template <typename T>
void addCommand( std::string command, T f);
};
struct Callback {
myApp &app;
Callback (myApp &a) : app(a) {
}
void operator() (const std::vector<std::string> &args) {
app.TestFn(args);
}
};
void myApp::initApp( )
{
myConsole::getSingleton( ).addCommand( "FirstTest", Callback(*this) );
}
This gives you the callback principle in C++, but I think you need something more flexible than this solution, since you actually want to choose automatically the command that will be executed by the callback (in this case TestFn).

Answer 3

You should avoid void*, especially when trying to use function pointers. I'm going to assume that you are looking only at member-function pointers in the myApp class, and that you are only interested in member-function pointers which take const std::vector<std::string> &args as an argument. This typedef will create the appropriate type and call it MemFunType
typedef void (myApp :: * MemFunType) (const std::vector<std::string> &);
Here is a complete example (on ideone), where there are two different member-functions you may be interested in, TestFn and TestFnBackwards. This example probably isn't very useful, but it gives some examples of member-function pointers.
#include<iostream>
#include<vector>
using namespace std;
struct myApp;
struct myConsole
{
typedef void (myApp :: * MemFunType) (const std::vector<std::string> &);
void addCommand( std::string command, MemFunType fn );
};
struct myApp {
void TestFn( const std::vector<std::string> & args ) {
cout << " TestFn" << endl;
for(std :: vector<std::string> :: const_iterator i = args.begin(); i!=args.end(); i++) {
cout << *i << endl;
}
}
void TestFnBackwards( const std::vector<std::string> & args ) {
cout << " TestFnBackwards" << endl;
for(std :: vector<std::string> :: const_reverse_iterator i = args.rbegin(); i!=args.rend(); i++) {
cout << *i << endl;
}
}
static myApp & getSingleton();
} ma;
myApp& myApp :: getSingleton() {
return ma;
}
void myConsole :: addCommand( std::string , MemFunType fn ) {
vector<string> words;
words.push_back("hello");
words.push_back("world");
myApp &ma = myApp :: getSingleton();
(ma.*fn)(words); // execute the member on the singleton object, using the words as the argument.
}
int main() {
myConsole m;
m.addCommand( "FirstTest", &myApp::TestFn );
m.addCommand( "FirstTest", &myApp::TestFnBackwards );
}