Can we have functions inside functions in C++?

Question

I mean something like:

int main() 
{
  void a() 
  {
      // code
  }
  a();

  return 0;
}

Answer 1

No, it's not allowed. Neither C nor C++ support this feature by default, however TonyK points out (in the comments) that there are extensions to the GNU C compiler that enable this behavior in C.

Answer 2

But we can declare a function inside main():

int main()
{
    void a();
}

Although the syntax is correct, sometimes it can lead to the "Most vexing parse":

#include <iostream>


struct U
{
    U() : val(0) {}
    U(int val) : val(val) {}

    int val;
};

struct V
{
    V(U a, U b)
    {
        std::cout << "V(" << a.val << ", " << b.val << ");\n";
    }
    ~V()
    {
        std::cout << "~V();\n";
    }
};

int main()
{
    int five = 5;
    V v(U(five), U());
}

=> no program output.

(Only Clang warning after compilation).

C++'s most vexing parse again

Answer 3

Let me post a solution here for C++03 that I consider the cleanest possible.*

#define DECLARE_LAMBDA(NAME, RETURN_TYPE, FUNCTION) \
    struct { RETURN_TYPE operator () FUNCTION } NAME;

...

int main(){
  DECLARE_LAMBDA(demoLambda, void, (){ cout<<"I'm a lambda!"<<endl; });
  demoLambda();

  DECLARE_LAMBDA(plus, int, (int i, int j){
    return i+j;
  });
  cout << "plus(1,2)=" << plus(1,2) << endl;
  return 0;
}

(*) in the C++ world using macros is never considered clean.

Answer 4

No.

What are you trying to do?

workaround:

int main(void)
{
  struct foo
  {
    void operator()() { int a = 1; }
  };

  foo b;
  b(); // call the operator()

}

Answer 5

Local classes have already been mentioned, but here is a way to let them appear even more as local functions, using an operator() overload and an anonymous class:

int main() {
    struct {
        unsigned int operator() (unsigned int val) const {
            return val<=1 ? 1 : val*(*this)(val-1);
        }
    } fac;

    std::cout << fac(5) << '\n';
}

I don't advise on using this, it's just a funny trick (can do, but imho shouldn't).

---

2014 Update:

With the rise of C++11 a while back, you can now have local functions whose syntax is a little reminiscient of JavaScript:

auto fac = [] (unsigned int val) {
    return val*42;
};

Answer 6

Starting with C++ 11 you can use proper lambdas. See the other answers for more details.

---

Old answer: You can, sort-of, but you have to cheat and use a dummy class:

void moo()
{
    class dummy
    {
    public:
         static void a() { printf("I'm in a!\n"); }
    };

    dummy::a();
    dummy::a();
}