如何将双参数函数“转换”为单参数函数？

Question

In matlab, one can write:

S = @(x,y) x^2+y^2-1
G = @(x) S(x,1);

If I have a function expecting a one-argument function, I can do the above. How can I do this in c/c++?

I have a library function (from the CGAL library) that expects as an argument a function that itself has only one argument. Ideally, I have a class ( SphericalHarmonics ) and I would like to have a member function which takes the one argument. So I have:

FT SphericalHarmonics::distFunction(Point_3 p)

(note that FT is a type similar to double ) but of course when I try

SphericalHarmonics *sh = new SphericalHarmonics(1);
Surface_3 surface(sh->distFunction, Sphere(ORIGIN,2.));

this is also treated as an argument, my distFunction function is a two-argument function, and an error is thrown.

Note that this can be solved with global variables, ie

SphericalHarmonics *sh;
FT dist_function(Point_3 p) {
    return sh->distFunction(p);
}

main() {
    sh = new SphericalHarmonics(1);
    Surface_3 surface(dist_function);
}

However, this is really non-ideal. I'd like a way to do this without global variables, as it would be far better to be able to have a class function that easily integrates with the CGAL library.

Thanks in advance!

[UPDATED]

@Andy-Prowl: I have tried your std::bind and lambda solutions, but still seem to be running into errors with regards to the number of arguments.

When, in main , I use the code:

SphericalHarmonics *sh = new SphericalHarmonics(cInit, numL, symm);
auto fxn = std::bind(&SphericalHarmonics::distFunction, sh, std::placeholders::_1);
Surface_3 surface(fxn, Sphere_3(ORIGIN,2.));

I get the errors:

~/lib/basisfunctions/SphericalHarmonics2/mesh_an_implicit_function.cpp:62:48: 
error: no matching function for call to     
‘CGAL::Implicit_surface_3<CGAL::Robust_circumcenter_traits_3<CGAL::Epick>, 
double (*)
(CGAL::Point_3<CGAL::Epick>)>::Implicit_surface_3(std::_Bind<std::_Mem_fn
<double (SphericalHarmonics::*)(CGAL::Point_3<CGAL::Epick>)>
(SphericalHarmonics*, std::_Placeholder<1>)>&, Sphere_3)’

and

~/CGAL-4.1/include/CGAL/Implicit_surface_3.h:50:5: note:   no known conversion 
for argument 1 from ‘std::_Bind<std::_Mem_fn<double (SphericalHarmonics::*)
(CGAL::Point_3<CGAL::Epick>)>(SphericalHarmonics*, std::_Placeholder<1>)>’ to 
‘CGAL::Implicit_surface_3<CGAL::Robust_circumcenter_traits_3<CGAL::Epick>, 
double (*)(CGAL::Point_3<CGAL::Epick>)>::Function 
{aka double (*)(CGAL::Point_3<CGAL::Epick>)}’

and

~/CGAL-4.1/include/CGAL/Implicit_surface_3.h:34:9: note:   
candidate expects 1 argument, 2 provided

[UPDATED]

It is now clear to me that I need a function which can be converted to a function pointer (ie surface required a function pointer argument). This rules out the std::bind option. Moreover, it appears that a lambda cannot be converted to a function pointer if it captures variables ( capture-less vs. capturing lambdas ). So I think Andy-Prowl's answer below is in general the correct answer to this question, although I'll need to find a different work-around.

Answer 1

Use std::bind or boost::bind :

#include <functional>   

SphericalHarmonics *sh = new SphericalHarmonics(1);
surface(std::bind(&SphericalHarmonics::distFunction, sh, _1));

Answer 2

OPTION 1:

In case your member function does not to implicitly work on an instance of your class (and therefore does not need to receive a this pointer), you can make it static :

class SphericalHarmonics
{
    ...
    static double distFunction(Point p);
    ...
};

double SphericalHarmonics::distFunction(Point p)
{
    ...
}

Now, your function will effectively have a single argument:

surface(SphericalHarmonics::distFunction);

OPTION 2:

Otherwise, you may use std::bind() to curry the member function distFunction and fix its first, implicit argument (if you are not working with a C++11 compiler, you can use the equivalent boost::bind() from the Boost.Bind library):

#include <functional>

SphericalHarmonics *sh = new SphericalHarmonics(1);
auto fxn = std::bind(&SphericalHarmonics::distFunction, sh, _1);
surface(fxn);

OPTION 3:

Alternatively, in C++11, a lambda could do the job:

SphericalHarmonics *sh = new SphericalHarmonics(1);
auto fxn = [=] (double d) { return sh->distFunction(d); } 

Answer 3

In the specific case of dealing with CGAL's templated Surface_3 class. You're probably using something like this (from their example) to define Surface_3 type:

typedef CGAL::Surface_mesh_default_triangulation_3 Tr;
// c2t3
typedef CGAL::Complex_2_in_triangulation_3<Tr> C2t3;
typedef Tr::Geom_traits GT;
typedef GT::Sphere_3 Sphere_3;
typedef GT::Point_3 Point_3;
typedef GT::FT FT;
typedef FT (*Function)(Point_3);
typedef CGAL::Implicit_surface_3<GT, Function> Surface_3;

This is misleading because it makes it seem like CGAL's isosurface classes can only deal with function pointers (as opposed to std::function etc.). But the problem is only that we've just defined it that way . Simply define Surface_3 to use std::function<FT (Point_3)> as its template argument and the std::bind and lambdas from Andy Prowl's answer will work just fine:

...
typedef std::function<FT (Point_3)> Function;
typedef CGAL::Implicit_surface_3<GT, Function> Surface_3;