How do to simulate lambda functions and closures in C++11 with classes?

Question

Let's say I have an integration function double integral(double (*f)(double), double a, double b) which computes the integral of f(x) in [a,b]. However, I am in a situation where f is not known in compile time, and may change during runtime. Therefore in my current design, I have a class that tracks and mutates several f1, f2, f3 etc.. However, since f1, f2, f3 etc are now a member of a class, I can't just take its function pointer &my_class::f1 and feed it to integral(f1,a,b). The ideal way of doing this would be to just spit out a lambda function during runtime.

Some pseudo code to be more precise about what I want:

double integral(double (*f)(double), double a, double b);

class secret {
  public:
    // all three of these rely on internal state that this class manages
    // absolutely none of these methods can be static!
    double f1(double x);
    double f2(double x);
    double f3(double x);

    void do_stuff(); // mutates internal state, changing the behavior of f1,f2,f3
    void do_something_else(); // mutates internal state, changing the behavior of f1,f2,f3
    double execute(); // internally, this function calls integrate(f1,a,b), integrate(f2,a,b) and integrate(f3,a,b)
}

// needs to handle functions that are not part of a class
double quadratic(double x) {
  return x*x;
}

int main() {
   secret s1, s2;
   s1.do_stuff() // change the state of s1, changing f1,f2,f3
   s2.do_something_else() // change the state of s2. 
   // We can have multiple instances of the secret class, all with different internal state, 
   // but need the ability to call "integral" with a lot of freedom

   // I want a good design that will let me do this kind of thing
   cout<<integrate(&s1.f1, 0, 1)<<endl;
   cout<<integrate(&quadratic, 0, 1)<<endl;
   cout<<integrate(&s2.f1, 0, 1)<<endl;
   cout<<s1.execute()<<endl;
   cout<<s2.execute()<<endl;
}

I am limited to an old machine that only supports C++11. My question is in two parts, what is the best design to do this in C++11, and what is the best design in g++ 9.2?

You can use lambdas to "simulate lambda functions" in C++11. — eerorika
– eerorika, Commented Jan 14, 2020 at 19:36
"Lambda expressions (since C++11)": en.cppreference.com/w/cpp/language/lambda — Richard Critten
– Richard Critten, Commented Jan 14, 2020 at 19:40
How do closures work in C++11? Can those lambda functions access class members of the instantiated class? — lee
– lee, Commented Jan 14, 2020 at 19:42
@SpentDeath Lambdas can capture variables from its environment, yes. — Guillaume Racicot
– Guillaume Racicot, Commented Jan 14, 2020 at 19:43

NathanOliver · Accepted Answer · 2020-01-14 19:49:35Z

1

You can solve this by making integral a function template and take a generic type as the function type. Using

template <typename Func>
double integral(Func func, double a, double b)
{
    auto result = func(a);
    // do something with result
    return result;
}

Allows you to pass function pointers and functors to the function. If you need to pass a member function to it, then you just wrap that call into a lambda and then pass that lambda to intergral like

secrete s;
auto result = integral([&s](double a){ return s.f1(a); }, 42.0, 3.14);

edited Jan 14, 2020 at 19:49

answered Jan 14, 2020 at 19:44

NathanOliver

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2 Comments

lee Over a year ago

This is exactly what I was looking for. Related question:auto blah = [&s](double a){ return s.f1(a); }; if (some_statement) { blah = [&s](double a){ return s.f2(a);} blah();} How can I make assignments with lambdas like this?

NathanOliver Over a year ago

@SpentDeath Use a std::function instead of auto. Then you can reassign it. It cost some more since it uses type erasure but it basically the only way to do it.

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