Why are local function definitions illegal in C++?

Question

It comes across as odd to me that local function definitions are illegal. As things stand now, I doubt it would be too hard to implement, and the erasure of what could be a potential feature (like in python for example) for no reason at all seems strange, especially for a language like C++, where you can shoot yourself in the foot if you want to. The illegality of local function definitions seems doubly so strange if things like this are allowed.

int main()
{
    void func(); // Allowed?

    class BypassRules {
    public:
        static void func() { return; }
    };

    BypassRules::func();
    func();

    return 0;
}

void func() { return; }

Oddly enough, local function 'Declarations' are allowed, but definitions are not. And the rule on local function definitions not being allowed can be easily bypassed by writing a class encapsulator. So, could I have some clarification? Why are local class definitions classified as 'illegal' in C++?

Answer 1

Oddly enough, local function 'Declarations' are allowed, but definitions are not. And the rule on local function definitions not being allowed can be easily bypassed by writing a class encapsulator. So, could I have some clarification? Why are local class definitions classified as 'illegal' in C++?

Illegal because they don't make much sense, whether it may be defining a function inside a function or a class inside a function like main().

But then you can always use workarounds, such as:

• For functions you can always use a lambda, which acts as an anonymous temporary function, can be called inside main plus be used within function parameters as well.

It can be invoked whenever necessary and subsequently discarded like a throw-away function, but it does the job - whatever you would expect from a function.

Example:

int main() 
{ 
  // Define within the scope of main()
  auto temp = [](int x)
  { std::cout<< x; 
  }
  temp(10); // call like a function
}

---

• For classes, use static functions, (being local to the translation unit) with public access specifiers (as you did in your question).

Example:

int main()
{
   class temp
   {  public:
      static void a()
      {  std::cout<< 10;
      }
   };
   temp::a();
}

---

Output for both cases: 10

Provided there are alternatives approaches like the ones above to define a function/class/struct inside main(), there isn't really much of a point to make them legal. ¯\_(ツ)_/¯