Overriding "endl" in C++ by #define preprocessor directive

Question

I used the following code to override and replace endl with something other than "a new line".

#include <iostream>
#include <string>
using namespace std;

#define endl "OOOO"  //replacing "endl"


int main(){

    cout << "start " << endl << " end";
    return 0;

}

Then the result will be:

start OOOO end

instead of:

start
end

But doing the same to cout causes errors.

Why can we do this to endl but we cannot do the same to cout?

Answer 1

It of course depends upon what you define your macro as, and how you use it. After preprocessing, this:

#define endl "OOOO"
cout << "start " << endl << " end";

becomes this:

cout << "start " << "OOOO" << " end";

Which is a perfectly valid statement. However, after preprocessing this:

#define cout "OOOO"
cout << "start " << endl << " end";

becomes this:

"OOOO" << "start " << endl << " end";

Which is not a valid statement. If you do this though:

#define printf "OOOO"
cout << printf;

That becomes this:

cout << "OOOO";

Which is fine. Likewise, if you did this:

#define cout "OOOO"
printf(cout);

it becomes this:

printf("OOOO");

Which is also fine.