Passing Arguments by Value/Pointer/Reference

C++ offers a unique advantage by enabling direct manipulation with computer memory, providing greater flexibility in our programs. The following discussion will explore three methods of passing function arguments from a memory perspective.

Pass by value

In the previous section, we discussed variable scopes and established that variables created within a function are only accessible within that function. This concept remains true even when passing arguments to a function. Variables declared in the function signature have local scope; they are passed by value, meaning their values are copied into the function and stored in separate variables (remember our library book example to understand it).

main

#include <iostream>

// Function that takes an argument by value

void modifyValue(int num) {

// Modify the parameter (local copy of the argument)

num = num * 2;

std::cout << "Inside function: " << num << std::endl;

}

int main() {

int number = 5;

std::cout << "Before function call: " << number << std::endl;

// Call the function and pass 'number' by value

modifyValue(number);

// Varibale in the main function remains unchanged

std::cout << "After function call: " << number << std::endl;

}


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#include <iostream>

// Function that takes an argument by value
void modifyValue(int num) {
  // Modify the parameter (local copy of the argument)
  num = num * 2;
  std::cout << "Inside function: " << num << std::endl;
}

int main() {
    int number = 5;
    std::cout << "Before function call: " << number << std::endl;

    // Call the function and pass 'number' by value
    modifyValue(number);
    // Varibale in the main function remains unchanged
    std::cout << "After function call: " << number << std::endl;
}

Pass by pointer

It's not always good to pass arguments by value. Firstly, passing a copy consumes additional memory. Secondly, there are situations where it's necessary to modify the variable inside a function without relying on a return statement. In such cases, using a pointer to pass the variable is more appropriate.

Passing a variable by pointer in C++ as a function argument involves passing the memory address of the variable rather than its actual value.
Passing by pointer is done using pointers (* operator) in function parameters.

main

#include <iostream>

// Function that takes an argument using pointer

void modifyValue(int* numPtr)

{

// Modify the value at the memory address pointed by numPtr

// '*' is used to extract the value from the address (pointer)

*numPtr = (*numPtr) * 2;

std::cout << "Inside function: " << *numPtr << std::endl;

}

int main()

{

int number = 5;

std::cout << "Before function call: " << number << std::endl;

// Call the function and pass the address of 'number' using a pointer

// '&' is used to get an address of the variable

modifyValue(&number);

// 'number' in the main function is modified through the pointer

std::cout << "After function call: " << number << std::endl;

}


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#include <iostream>

// Function that takes an argument using pointer
void modifyValue(int* numPtr)
{
    // Modify the value at the memory address pointed by numPtr
    // '*' is used to extract the value from the address (pointer)
    *numPtr = (*numPtr) * 2;
    std::cout << "Inside function: " << *numPtr << std::endl;
}

int main()
{
    int number = 5;
    std::cout << "Before function call: " << number << std::endl;

    // Call the function and pass the address of 'number' using a pointer
    // '&' is used to get an address of the variable
    modifyValue(&number);
    // 'number' in the main function is modified through the pointer
    std::cout << "After function call: " << number << std::endl;
}

Pass by reference

In C++, passing by reference means passing the memory address of a variable directly to a function, allowing the function to modify the original variable's value without any additional operations (like dereferencing pointer using the * operator).

Passing by reference is done using references (& operator) in function parameters. In general, passing by reference is very similar to passing by pointer, but there are two important differences:

we don't need to use dereferencing (*) inside the function when passing by reference to use the variable (as a result, we have direct access to the corresponding variable).
we don't need to use the "address-of" operator (&) to pass the reference of the variable as an argument of the function when calling it (but we still have to use the & operator instead of * in the function signature).

main

#include <iostream>

// Function that takes an argument by reference

void modifyValue(int& num)

{

// Modify the parameter directly (no need for dereferencing)

num = num * 2;

std::cout << "Inside function: " << num << std::endl;

}

int main()

{

int number = 5;

std::cout << "Before function call: " << number << std::endl;

// Call the function and pass 'number' by reference

modifyValue(number);

// 'number' in the main function is modified directly through the reference

std::cout << "After function call: " << number << std::endl;

}


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#include <iostream>

// Function that takes an argument by reference
void modifyValue(int& num)
{
    // Modify the parameter directly (no need for dereferencing)
    num = num * 2;
    std::cout << "Inside function: " << num << std::endl;
}

int main()
{
    int number = 5;
    std::cout << "Before function call: " << number << std::endl;

    // Call the function and pass 'number' by reference
    modifyValue(number);
    // 'number' in the main function is modified directly through the reference
    std::cout << "After function call: " << number << std::endl;
}

Conclusion

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