Question

The basic idea is that I have a “family” of classes that all do the same identical thing, but in a slightly different way. This “family” is used in a “high performance” loop, so speed is of the essence. Further, the specific family is specified by a configuration file…

The problem is that there is tremendous code repetition in the main function here. Is there a better way to structure this so I don’t have to write HP<objx> test and test.loop(bobloblaw) three times? (In practice, this chunk of code is many more lines than 2…)

class obj1 {
public:
    double f(double x) const { return 1.; }
};

class obj2 {
public:
    double f(double x) const { return x; }
};

class obj3 {
public:
    double f(double x) const { return x*x; }
};

template <class O>
class HP {
private:
    O obj;

public:

    double loop(const vector<double>& x) {
        double s = 0.;
        for (auto i : x) s += obj.f(i);

    return s;
    }
};

int main() {
    string config = "bob";
    double result = 0;
    vector<double> bobloblaw;

    /* Read configuration file to determine which object to use. */
    if (config == "obj1") {
        HP<obj1> test;
        result = test.loop(bobloblaw);
    } else if (config == "obj2") {
        HP<obj2> test;
        result = test.loop(bobloblaw);
    } else if (config == "obj3") {
        HP<obj3> test;
        result = test.loop(bobloblaw);
    }

    return result;
}

Answer 1

The following is untested, but should work:

class obj1
{
public:
  double f(double x) const { return 1.; }
};

class obj2
{
public:
  double f(double x) const { return x; }
};

class obj3
{
public:
  double f(double x) const { return x*x; }
};

class HPbase
{
public:
  virtual double loop(const vector<double>&) = 0;
};

template <class O> class HP:
  public HPbase
{
public:
  double loop(const vector<double>& x)
  {
    double s = 0.;
    for (auto i : x)
      s += obj.f(i);
    return s;
  }
private:
  O obj;
};

std::unordered_map<std::string, std::unique_ptr<HPbase>> decode{
  {"obj1"}, new HP<obj1>()},
  {"obj2"}, new HP<obj2>()},
  {"obj3"}, new HP<obj3>()} };

int main()
{
  string config = "bob";
  double result = 0;
  vector<double> bobloblaw;

  /* Read configuration file to determine which object to use. */
  result = decode[config].loop(bobloblaw);
}

Note that the only addition is a base class for HP<> and a map which replaces the if/else logic of your code.