FAQ: Functions: Scope & Flexibility - Multi-File Programs

This community-built FAQ covers the “Multi-File Programs” exercise from the lesson “Functions: Scope & Flexibility”.

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Why do we need to declare the functions above main in the main cpp file in the first place?
I assume the system wouldn’t actually look for the functions in other files unless we declare them above main?

simple enough to find out:

#include <iostream>

int main() {

  eat();

}

// Definition at the bottom:
void eat() {

  std::cout << "nom nom nom\n";

}

as you can see, i removed the declaration at the top, now the program gives an error. This is because you use the function before its declared.

Yeah Well was wondering if we could just call it without actually declaring it at all ( since we declare and define it in the other file) … gave it a few tests and faced fatal errors. see why now.


why i can’t compile although i did same the hint

Because your code contains syntax errors? You declare functions, but the function don’t seem to have a body/purpose.

Did you defined the functions in the other file named “my_functions.cpp” ??

But why do we need to declare them after all ?? Don’t the compiler automatically know that the functions exist in the other file ?

thankfully not, once you have several hundred files or maybe thousand files, you don’t want to load the files/functions you don’t need. Which is why we import them manually.

2 Likes

Really helpful :innocent: Thank you ! @stetim94

Why do we need to declare a function? I thought you can just write a function out with the parameters and it’s declared automatically. For example,

void random_function(){
std::cout << "Hello World!";
}
int main(){
random_function();
}

can’t I just do this??? or do I have to do my code like this

void random_function();
void random_function(){
std::cout << "Hello World!";
}
int main(){
random_function();
}

this is possible, but as your program grows this is less then ideal, we quickly want to see the main function to see what the program does

so we can define the function below the main function. But then we first have to declare the function before main. Otherwise, main doesn’t know what the function is

by declaring the function first ,we promise c++ we will define the function later

but all this is explained in the lesson?

So in other words, Making a function the way I did before (declaring it and having its body/purpose right under it) is possible but declaring functions in a different file makes it easier and more concise correct?

this is good:

void random_function(){
std::cout << "Hello World!";
}
int main(){
random_function();
}

this isn’t:

void random_function();
void random_function(){
std::cout << "Hello World!";
}
int main(){
random_function();
}

the deceleration is now pointless, given you define the function before your main function

yes, one file of several thousand of lines (programs will easily get this big) are hard to read and maintain.