Question

This is a small program :

#include <iostream>
using namespace std;

int main() {
    f();
    system("pause");
}

void f() {
    static int x = 20 ;
    class tester {
    public :
        tester() {
            cout << x ;
        }
    } x1;
}

The error that i get here is :error C3861: ‘f’: identifier not found

If i place the function f above main I will get the desired output.

Why it is so ?
I was told that program execution begins at main. According to this the code should run in the first case also.

How does the compiler start reading the program?

Answer 1

The beginning of the compilation and the beginning of the execution of the program are two different things.

The execution starts from the main.

The compilation begins from the beginning of the file; the compiler don’t “jump around” the file to find the needed pieces, but it reads the input in a linear fashion (I suspect that this related, among the other things, to the fact that the C++ grammar is really complicated).

When the compiler is at some point in parsing the file, it only knows what has been declared/defined up to that point¹.

Because of this, function prototypes (and non-defining declarations in general) have been invented: the prototypes of all the functions defined in the file are put at the beginning of the file, typically after the #include directives or in a separated include file. The prototypes tell to the compiler that such functions will be defined later, and what is the function signature (i.e. name, parameters, return value).

The prototype is made as a normal function, but without the body, which is replaced by a semicolon². For example, in your code you would write

void f();

before the main.

---

1. IIRC there are some relaxations to this rule that allow the compiler to “wait” for some declarations to make some template magic work, but this is not relevant here.

2. In a prototype is also common not to write the names of the parameters, leaving just their type (this can be done also in function definitions, but it doesn’t make much sense there unless you have a formal parameter you don’t use). Still, I prefer to leave the parameter names there as a form of documentation.