Functions

int myfunc(int x); //function prototype / declaration

int myfunc(int a){
    //do some shit
    return val;
}
int main(){
    //do some shit
    int c = myfunc(b);
    //do some shit
}

Usually it is a good programming practice to have a header file that contains the definitions of functions used along with a comment gitving its function (pun intended), and a separate file containing the function itself (the function is not redefined unless you put a semicolon at its end).

Format of a function: return-value-type function-name(parameter-list);
Parameter list is a comma separated list of values that have to be taken as input.

We can return void from a function that doesn’t have to return a value, but just perform operations.

Arrays cannot be returned by a function unless the array is declared globally or passed as a heap allocated pointer.

To take an array as an input, simply pass the pointer to its first element and keep incrementing its pointer to access further elements. However, C performs no checks for illegal access if the pointer crosses the array length. It is a good practice for us to pass pointers to arrays along with their lengths as functions.

Default arguments for functions

int func(int x = 6){
    return 2*x;
}

int main(){
    int y = func(); //gets value 12
    int z = func(7); //gets value 14
}

Pure and Impure functions

Since C++ supports functions, in theory, it supports functional programming. In functional programming we have the notion of pure and impure functions. Pure functions are those that do not change the value of anything except for the parameters that are passed in them. On the other hand, impure functions are changinge values outside of the function’s scope. Do note that passing a pointer and dereferencing it to change its value is also an example of an impure function.