can someone please explain this to me

[ecox76]

why does this code work, but if the main function is before the rest of them, it won't compile? i don't understand why "main" has to be on the bottom! very puzzling.

==============================

#include <iostream.h>

int one()
{
cout <<"you picked one!" <<endl;

return 0;
}

int two()
{
cout <<"you picked two!" <<endl;

return 0;
}

int three()
{
cout <<"you picked three!" <<endl;

return 0;
}


int main()
{
int pick;

cout <<"Pick 1,2, or 3: " <<endl;
cin >> pick;

        if (pick==1)
                {
                one();
                }
        if (pick==2)
                {
                two();
                }
        if (pick==3)
                {
                three();
                }

return 0;
}

===========================================

[abc123]

because otherwise you have no reference to them

to solve the problem try this:

int abc(int x); // declare your func
// obviously, however, this declaration must be above where you call the function ( in this case; main() )
int main(){
    int i;
    i = abc(34);
}

int abc(int x){ //implement your func
  return x + 1;
}

[alpha]

functions must either be prototyped or defined above main. main is a function, the first function called, and returned to once a function that is called is complete. once main is done, the program is done.

functions can only see what is above them.

for example, for the code you posted. if you want function three to call function two, it can. but function two cannot call function three. if you prototype the funtions, i believe you could call function three from function two. (not sure though)

however, functions cannot call main.

hope this helps some.

edit:

#include <iostream>

void test_one();
void test_two();

int main()
{
	test_one();
	
	std::cin.get();
	return 0;
}

void test_one()
{
	std::cout << "Test One";
	std::cout << std::endl;
	test_two();
}

void test_two()
{
	std::cout << "Test Two";
	std::cout << std::endl;
}

this will output:

Test One
Test Two

you can call function two from one if it is prototyped and declared before main.

[abc123]

Originally posted by alpha
however, functions cannot call main.

... what on earth makes you say that?

[saline]

alpha got it completely right but seemed unsure. So I'm here to say, yes.

The functions can be prototyped (what I always do) like so:

(for your program they would look like this)

//include files

#include <iostream.h>

//function prototypes

int one(); //They're just the functions titles copied with 
int two(); // semicolons at the end.
int three();


int main()  //Now main can go up top which makes sense because
{  //its the first function the program goes to.
int pick;

cout <<"Pick 1,2, or 3: " <<endl;
cin >> pick;

        if (pick==1)
                {
                one();
                }
        if (pick==2)
                {
                two();
                }
        if (pick==3)
                {
                three();
                }

return 0;
}


int one()
{
cout <<"you picked one!" <<endl;

return 0;
}

int two()
{
cout <<"you picked two!" <<endl;

return 0;
}

int three()
{
cout <<"you picked three!" <<endl;

return 0;
}

When you do this you can place the main function at the top of the program. I feel this makes more sense as its the first function the program goes into anyway.

The reason the compiler works this way (main on top or prototypes) is because the compiler needs advanced warning about whats going on in the program before it compiles. It needs to allocate (reserve for future use) memory for function calls, variables and other things. It does this so it doesn't run out of memory part way through and crash your computer (among other reasons). When you prototype the top of your program it gives the compiler a concise list of everything your program is going to need before it compiles.

If I'm off on this anyone feel free to correct me.

It also gives you a quick way to look at all your functions. I like prototyping and would recommend it to new C and C++ coders.

Very good question and welcome to code newbie.

[alpha]

Originally posted by abc123
... what on earth makes you say that?

you can call main. but is there a reason to?

[joe_bruin]

Originally posted by saline
The reason the compiler works this way (main on top or prototypes) is because the compiler needs advanced warning about whats going on in the program before it compiles. It needs to allocate (reserve for future use) memory for function calls, variables and other things. It does this so it doesn't run out of memory part way through and crash your computer (among other reasons). When you prototype the top of your program it gives the compiler a concise list of everything your program is going to need before it compiles.

the compiler needs to know what kind of parameters to pass to your function and what kind of parameters to expect in return. memory for function variables is dynamically done on the stack.
if your program runs out of memory (overflows the stack), it will crash itself, but should not crash your computer. modern operating systems use virtual memory systems that keep the program memory space separate from kernel memory. this is not true if you're running an archaeic non-memory protected os like DOS or macos <= 9 (and is partially true for halfassed solutions such as win95/98).

as for calling main(), you can definitely do it. however, there are very few valid reasons for doing so.

[ecox76]

ok, that makes sense. thanks for the replies.