Question

I am reading Bjarne Stroustrup’s Programming : Principles and Practice Using C++

In the drill section for Chapter 2 it talks about various ways to look at typing errors when compiling the hello_world program

#include "std_lib_facilities.h"

int main()  //C++ programs start by executing the function main
{
    cout << "Hello, World!\n",  // output "Hello, World!"
    keep_window_open();         // wait for a character to be entered
    return 0;
}

In particular this section asks:

Think of at least five more errors you
might have made typing in your
program (e.g. forget
keep_window_open(), leave the Caps
Lock key on while typing a word, or
type a comma instead of a semicolon)
and try each to see what happens when
you try to compile and run those
versions.

For the cout line, you can see that there is a comma instead of a semicolon.
This compiles and runs (for me). Is it making an assumption ( like in the javascript question: Why use semicolon? ) that the statement has been terminated ?

Because when I try for keep_terminal_open(); the compiler informs me of the semicolon exclusion.

Answer 1

The comma operator in C++ can be used as follows:

a, b;

It means “do a, disregard the result, then do b.” You can chain it together like this:

a, b, c, (etc.), n;

In general, this isn’t considered good style. The comma operator is rarely used in practice because it’s confusing. The few times it’s legitimately useful usually come up with for loops:

for (int a = 0, b = 0; a < 100; a++, b++) {
    /* ... */
}

Here, we use the comma operator in the last part of the for loop to mean “increment both a and b.”

To answer your question, yes, you should have a semicolon after the cout. Using the comma operator technically works as well, but it’s inelegant and likely to confuse people.