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7.2 — Passing arguments by value

Pass by value

By default, arguments in C++ are passed by value. When arguments are passed by value, a copy of the argument is passed to the function.

Consider the following snippet:

void foo(int y)
{
    using namespace std;
    cout << "y = " << y << endl;
}

int main()
{
    foo(5); // first call

    int x = 6;
    foo(x); // second call
    foo(x+1); // third call

    return 0;
}

In the first call to foo(), the argument is the literal 5. When foo() is called, variable y is created, and the value of 5 is copied into y. Variable y is then destroyed when foo() ends.

In the second call to foo(), the argument is the variable x. x is evaluated to produce the value 6. When foo() is called for the second time, variable y is created again, and the value of 6 is copied into y. Variable y is then destroyed when foo() ends.

In the third call to foo(), the argument is the expression x+1. x+1 is evaluated to produce the value 7, which is passed to variable y. Variable y is once again destroyed when foo() ends.

Thus, this program prints:

y = 5
y = 6
y = 7

Because a copy of the argument is passed to the function, the original argument can not be modified by the function. This is shown in the following example:

void foo(int y)
{
    using namespace std;
    cout << "y = " << y << endl;

    y = 6;

    cout << "y = " << y << endl;
} // y is destroyed here

int main()
{
    using namespace std;
    int x = 5;
    cout << "x = " << x << endl;

    foo(x);

    cout << "x = " << x << endl;
    return 0;
}

This snippet outputs:

x = 5
y = 5
y = 6
x = 5

At first, x is 5. When foo() is called, the value of x (5) is passed to variable y inside foo(). y is assigned the value of 6, and then destroyed. The value of x is unchanged, even though y was changed.

Advantages of passing by value:

  • Arguments passed by value can be variables (eg. x), literals (eg. 6), or expressions (eg. x+1).
  • Arguments are never changed by the function being called, which prevents side effects.

Disadvantages of passing by value:

  • Copying large structs or classes can take a lot of time to copy, and this can cause a performance penalty, especially if the function is called many times.

In most cases, pass by value is the best way to pass arguments to functions — it is flexible and safe.

7.3 — Passing arguments by reference
Index
7.1 — Function parameters and arguments

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