Question

i have two questions that are fairly small and related so i will put them both in the same question.

i have been experimenting with classes and i was attempting to access a class in another file that wasn’t in a class so for example.

//class 1 .cpp
void Class1::function1()//another error 
{
    function()
}


//main.cpp

void function()
{
//stuff happens
}

is there a way to-do this? or would i need to add this function to a class to get it to work. also how would you go about creating a function that receives a function as it parimetre? for example function(function2())

i am simply trying to access a function from a class as it would make my code easier to use later if the function that i am using doesn’t get added to a class. with regards to the seconds question i which to create a function that receives a time and a function as an argument. it will wait for the specified time then execute the program

Answer 1

How to access a function in another file?
Depends on the type of function, there can be to cases:

1. Accessing class member functions in another file(Translation Unit):
Obviously, you need to include the header file, which has the class declaration in your caller translation unit.

Example code:

//MyClass.h

class MyClass
{
    //Note that access specifier
    public:
        void doSomething()
        {
             //Do something meaningful here
        }

};

#include"MyClass.h"    //Include the header here
//Your another cpp file
int main()
{
    MyClass obj;
    obj.doSomething();
    return 0;
}

2. Accessing free functions in another file(Translation Unit):

You do not need to include the function in any class, just include the header file which declares the function and then use it in your translation unit.

Example Code:

//YourInclude.h

inline void doSomething() //See why inline in @Ben Voight's comments
{
    //Something that is interesting hopefully
}

//Your another file

#include"YourInclude.h"

int main()
{
    doSomething()
    return 0;
}

Another case as pointed out by @Ben in comments can be:
A declaration in the header file, followed by a definition in just one translation unit

Example Code:

//Yourinclude File
void doSomething();  //declares the function

//Your another file
include"Yourinclude"
void doSomething()   //Defines the function
{
    //Something interesting
}

int main()
{
    doSomething();
    return 0;
}

Alternately, a messy way to do this can be to just mark the function as extern in your another file and use the function.Not recommended but a possibility so here is how:

Example Code:

extern void doSomething();

int main()
{
    doSomething();
    return 0;
}

How would you go about creating a function that receives a function as it parameter?
By using function pointers
In a nutshell Function pointers are nothing but pointers but ones which hold address of functions.

Example Code:

int someFunction(int i)
{
    //some functionality
}


int (*myfunc)(int) = &someFunction;


void doSomething(myfunc *ptr)
{
    (*ptr)(10); //Calls the pointed function

}