Question

I have a pure virtual class CF that denotes any continuous function in 1D it is defined a header file

// tools.h
class CF
{
public:
    virtual double operator()(double x) const=0 ;
};

what I intend to do is to wrap a couple of simple functions using functors and have them defined inside tools.h. If I use anonymous classes, like:

// tools.h
class : public CF{
public:
    virtual double operator()(double x) const { return 0.0; }
} zero;

then I run into trouble sine the class is redefined everywhere tools.h is included. gcc complains:

// tools.h included in main.cpp, foo.cpp, and bar.cpp
foo.o:(.bss+0x0): multiple definition of `zero'
main.o:(.bss+0x0): first defined here
bar.o:(.bss+0x0): multiple definition of `zero'
main.o:(.bss+0x0): first defined here

I figured I could fix this by doing something like:

// tools.h
class zero_: public CF{
public:
    virtual double operator()(double x) const { return 0.0; }
};

const static zero_ zero;

but I don’t like it since zero_ is visible everywhere and I don’t need it to be! How can I fix this issue?

Answer 1

By not doing it. Just have

struct zero : public CF {
  double operator()(double x) { return 0.; }
};

And use it with

void fun(const CF& cf);
fun(zero());

One of the reasons to have functors over functions is their ability to carry and expose state through operations, your approach would throw that away for gaining 2 parenthesis in conciseness.