C/C++ – Creating Functions Inside Another Function

c++

Could someone please tell me if this is possible in C or C++?

void fun_a();
//int fun_b();
...
main(){
   ...
   fun_a();
   ...
   int fun_b(){
     ...
   }
   ...
}

or something similar, as e.g. a class inside a function?

thanks for your replies,

Best Answer

Wow, I'm surprised nobody has said yes! Free functions cannot be nested, but functors and classes in general can.

void fun_a();
//int fun_b();
...
main(){
   ...
   fun_a();
   ...
   struct { int operator()() {
     ...
   } } fun_b;

   int q = fun_b();
   ...
}

You can give the functor a constructor and pass references to local variables to connect it to the local scope. Otherwise, it can access other local types and static variables. Local classes can't be arguments to templates, though.

Modern C++ - Yes with lambdas!

In current versions of c++ (C++11, C++14, and C++17), you can have functions inside functions in the form of a lambda:

int main() {
    // This declares a lambda, which can be called just like a function
    auto print_message = [](std::string message) 
    { 
        std::cout << message << "\n"; 
    };

    // Prints "Hello!" 10 times
    for(int i = 0; i < 10; i++) {
        print_message("Hello!"); 
    }
}

Lambdas can also modify local variables through **capture-by-reference*. With capture-by-reference, the lambda has access to all local variables declared in the lambda's scope. It can modify and change them normally.

int main() {
    int i = 0;
    // Captures i by reference; increments it by one
    auto addOne = [&] () {
        i++; 
    };

    while(i < 10) {
        addOne(); //Add 1 to i
        std::cout << i << "\n";
    }
}

C++98 and C++03 - Not directly, but yes with static functions inside local classes

C++ doesn't support that directly.

That said, you can have local classes, and they can have functions (non-static or static), so you can get this to some extend, albeit it's a bit of a kludge:

int main() // it's int, dammit!
{
  struct X { // struct's as good as class
    static void a()
    {
    }
  };

  X::a();

  return 0;
}

However, I'd question the praxis. Everyone knows (well, now that you do, anyway :)) C++ doesn't support local functions, so they are used to not having them. They are not used, however, to that kludge. I would spend quite a while on this code to make sure it's really only there to allow local functions. Not good.

Best Answer

Related Solutions

C++ Functions – Can Functions Be Inside Functions

Modern C++ - Yes with lambdas!

C++98 and C++03 - Not directly, but yes with static functions inside local classes

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