void Data Type

The void type

The void type is the simplest data type in C++ - it represents the absence of a type.

Void is an incomplete type, meaning the compiler recognizes it but can't determine how much memory it needs. Since void represents "nothing," you cannot create variables of type void:

void data; // error: cannot create a variable of type void

Despite this limitation, void serves important purposes in C++.

Functions without return values

The most common use of void is to indicate that a function doesn't return any value:

void displayMessage(int count)
{
    std::cout << "You have " << count << " messages\n";
    // no return statement needed
}

If you attempt to return a value from a void function, the compiler will reject your code:

void printTotal(int amount)
{
    std::cout << "Total: " << amount << "\n";
    return amount; // error: cannot return a value from void function
}

Empty parameter lists

In C, void was used to explicitly indicate that a function accepts no parameters:

int readNumber(void) // C style - avoid this
{
    int num{};
    std::cin >> num;
    return num;
}

While C++ compilers accept this syntax for backward compatibility, it's considered outdated. Modern C++ uses an empty parameter list instead:

int readNumber() // preferred C++ style
{
    int num{};
    std::cin >> num;
    return num;
}

Summary

Void has two primary uses:

• Indicating that a function doesn't return a value
• Advanced usage with pointers (covered in a later lesson)