We can have function declarations inside of function bodies:

void f(double) {
    cout << "f(double) called";
}

void f(int) {
    cout << "f(int) called";
}

void g() {
    void f(double); //Functions declared in non-namespace scopes do not overload
    f(1);           //Therefore, f(1) will call f(double)                  
}

This can be done in order to hide a function such as f(int) in this case, but my question is: Is this really the only reason why it is possible to declare a function inside of another?

Or in other words:
Does the possibility of declaring a function inside of a function exist for the sole purpose of hiding a function?

>Solution :

This can also be used to limit the visibility of a function declaration to only one specific function. For example:

file1.cpp:

#include <iostream>
void f(double d) {
    std::cout << d << '\n';
}

file2.cpp:

int main() {
   void f(double);
   f(1); // prints 1
}

void g() {
    f(2); // error: 'f' was not declared in this scope
}