Parameters and arguments in functions

In the realm of C++ programming, functions play a pivotal role in creating modular and organized code. Put simply, functions are like machines: you put something in (input), they process it, and something comes out (output). This topic will discuss input (passing input data to a function). Understanding how input data are passed to functions is crucial for effective program design. This topic delves into the various ways to pass input data to functions.

Parameter and arguments in functions

To avoid confusion, you first need to understand the basic terms:

• In C++, a parameter is a variable that a function's prototype declares. This parameter signals the type and number of values that a function expects upon its call. It closely resembles a typical variable declaration (like int x) and operates as a regular, localized variable within the function. The main role of this parameter is to facilitate the passage of values from the calling point to the function.
• On the other hand, arguments are the actual values that a function receives when someone invokes it. These arguments match the defined parameters and the function uses them for computation within its body.

Here is an example of using parameters and arguments when writing functions:

#include <iostream>

void byValue(int num) { // Here is our num parameter
    num = 10; // which is further used in our function.
}

int main() {
    int x = 5;
    byValue(x); // And here we are passing an argument to our function
    byValue(3); // we can pass the argument like this
    return 0;
}

You can pass any type of data to a function: variables, pointers, constants, arrays, structures, or any user data.

There are various ways to pass arguments to a function:

• passing by value;
• passing by reference;
• passing by pointer;

Now, let's consider each option in more detail.

Passing by value

Passing by value involves sending a copy of the argument's value to the function. This means that any changes made to the parameter inside the function do not affect the original argument outside of the function. This method is useful when working with the argument's value without altering the original data.

#include <iostream>

void byValue(int num) {
    num = 10; // Changes here won't affect the original value
    std::cout << num;
}

int main() {
    int x = 5;
    byValue(x);
    std::cout << x; // Outputs: 5
    return 0;
}

Output:
10
5

In the example above, the function byValue takes an integer argument num and tries to modify it. However, since it's passed by value, the change is local to the function and doesn't affect the original value of x in the main function.

When passing by value, you "send" a copy of your object to the function. If you need to transfer a large amount of data (for example, an array of 1 million elements of a complex structure can weigh 1 GB), then this can be expensive and not optimal for your computer.

Passing by reference

Passing by reference involves sending a reference to the argument, allowing the function to modify the original value directly. This method is particularly useful when you want to alter the original value and avoid the overhead of copying large data.

#include <iostream>

void byReference(int &num) {
    num = 10; // Changes here will affect the original value
}

int main() {
    int x = 5;
    byReference(x);
    std::cout << x; // Outputs: 10
    return 0;
}

Output:
10

In this example, the function byReference takes an integer reference as its argument. When the reference is modified within the function, it directly affects the original value of x in the main function.

In order to pass values by reference to a function, you need to:

1. Declare the function parameter as a reference (int &num).
2. Call the function, just like you would when passing parameters by value (which is very convenient) - byReference(x).

Passing by pointer

Passing by pointer involves sending the memory address of the argument to the function. This allows the function to access and modify the original value indirectly through the pointer. Pointers provide more control, as they can be reassigned to point to different memory locations. To do this, declare the function parameter as a pointer (int *ptr), call the function by passing the address (byPointer(&x)) of the variable. Or you can pass the value of the pointer.

#include <iostream>

void byPointer(int *ptr) {
    //in order not to work with the address, we dereference the pointer
    *ptr = 10; // Changes here will affect the original value through the pointer
}

int main() {
    int x = 5;
    byPointer(&x); // passing the address of the variable.
    std::cout << x; // Output: 10
    return 0;
}

Output:
10

In this example, the function byPointer takes an integer pointer as its argument. The pointer is dereferenced to access and modify the original value of x in the main function.

Tips for usage

1. Pass by Const Reference: When you want to pass an argument without modifying it, pass by const reference to prevent accidental changes. This is particularly useful for objects that are expensive to copy (big objects);
2. Prefer pass by reference for modification: If a function needs to modify the argument, pass by reference to avoid unnecessary copying. Also, passing by reference is preferable to passing by pointer in most cases;
3. Be cautious with pointers: Passing pointers requires careful memory management. Ensure that the pointer is valid and points to valid memory to avoid issues like null pointer dereferencing.
4. Document your code in detail: Clearly document whether functions modify arguments or not in your code comments. This helps other developers understand how to use the functions correctly. This is important because the call for "pass by value" and "pass by reference" look exactly the same.

Conclusion

In this topic, you've covered three fundamental methods: passing by value, passing by reference, and passing by pointer. Each method offers distinct advantages and considerations.

Remember, passing mechanisms influence memory usage, performance, and the potential for modifying original values. In essence, parameters are the function's expectations, while arguments fulfill those expectations. The choice of passing mechanism should align with the function's purpose and desired behavior.