usage - Should I always use a comma before a quote?

When quoting something said by someone else, should I use a comma before the quote?
Suppose I quote the definition given in a dictionary. Am I correctly using the the punctuation marks in the following sentences? (I am using them in the way I think correct in American English.)

One of the meanings of cat is, "a small animal with soft fur that people often keep as a pet."

A cadet is, "a young person who is training to become an officer in the police or armed forces."

Should I use a comma only when I explicitly quote somebody?

Groucho Marx once said, "Although it is generally known, I think it's about time to announce that I was born at a very early age."

Should I use a comma in this case too?

As Groucho Marx said,

No one is completely unhappy at the failure of his best friend.

On Comma Sense, A fun-damental guide to punctuation (Richard Lederer and John Shore), I read the following about using the comma:

Use commas to set off complete quotations:

The great general George S. Patton once said, "No, no—the war is this way, you idiots!"

Answer

I've found a few sources that say to use a comma before the quotations, but they all seem to have different standards.

This one just says to use them all the time.

1. Use a comma to shift between the main discourse and a quotation.

This source has a rule for short quotations.

Rule 16

Use commas to introduce or interrupt direct quotations shorter than three lines.

Examples:

He actually said, "I do not care."
"Why," I asked, "do you always forget to do it?"

And this last source has different rules for different situations.

First, use a comma to separate quoted material from the rest of the sentence that explains or introduces the quotation. If you are splitting the quotation with attributing the person, you will need two commas.

"The question is," said Alice, "whether you can make words mean so many things."

But if you are introducing the quote with the word that or the quote is only a small segment of the sentence, do not use a comma.

Peter Coveney writes that "[t]he purpose and strength of […]."
We often say "Sorry" when we don't really mean it.

The last thing explains when to use a colon instead of a comma.

Use a colon to set off explanatory or introductory language from a quoted element that is either very formal or long (especially if it's longer than one sentence):

Peter Coveney had this to say about the nineteenth-century's use of children in fiction: "The purpose and strength of […]."