About operator << overloading

Question

Here is a simple "hello world" program.

int main(){
  std::cout << "hello world\n";
  return 0;
}

How do I make the program print "hello world"+{some other string} for example: "hello world this is my string" without touching and changing the main ?. I tried to overload the << operator of string but it didn't work. thank you.

Also, whay do you want to do that overloading to add extra information? If it's just plain curiosity then that's okay, but please state so. Otherwise please [edit] your question to ask about the *actual* problem you want to solve, telling us what you think might solve it. Always ask for the actual problem, to avoid your question being [an XY problem](https://meta.stackexchange.com/questions/66377/what-is-the-xy-problem). — Some programmer dude, Jun 28 '21 at 16:21
Do you want this overload only for `"hello world\n"` or any character literal string? — Thomas Matthews, Jun 28 '21 at 17:46

score 3 · Answer 1 · answered Jun 28 '21 at 16:24

3

You can do this, but you really shouldn't.

#include <iostream>

std::ostream& operator<<(std::ostream & os, const char (&)[13]) {
    return os << "hello world this is my string\n";
}

int main(){
  std::cout << "hello world\n";
  return 0;
}

answered Jun 28 '21 at 16:24

Caleth

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This is the solution I was looking for .. so thank you very much I just did not define the char* parameter as const... my bad – Arikost Jun 28 '21 at 17:01
I didn't think this is allowed by the standard. If I'm wrong please let me know. – Bathsheba Jun 28 '21 at 17:02
@Bathsheba it's not adding to `std`, so I don't know why it would be prohibited. Interestingly `const char *` in `::` is unambiguously worse than the one in `std`, and `const char (&)[N]` is ambiguous – Caleth Jun 28 '21 at 17:44

Drew Dormann · Answer 2 · 2021-06-28T17:53:19.157

2

How do I make the program print "hello world this is my string" without touching and changing the main ?

You are asking for sneaky tricks, I presume? Macros are infamous for being sneaky.

int main(){
  std::cout << "hello world this is my string\n";
  return 0;
}
#define main not_main

// "the main", below, has not been touched.

int main(){
  std::cout << "hello world\n";
  return 0;
}

edited Jun 28 '21 at 17:53

answered Jun 28 '21 at 16:23

Drew Dormann

59,987
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technically the second is UB (https://stackoverflow.com/a/2726221/4117728), the first is fine I think – 463035818_is_not_an_ai Jun 28 '21 at 16:42
Favorite answer! – kebs Jun 28 '21 at 16:44
1

Love the first one. The behaviour of the second one is undefined. (I'll upvote if you remove it.) – Bathsheba Jun 28 '21 at 17:04
@Bathsheba I've avoided macros so well, I don't know if I could `#define 0` to have the same horrific effect. – Drew Dormann Jun 28 '21 at 17:55

Bathsheba · Answer 3 · 2021-06-28T17:08:11.097

It is possible to do this in a defined manner. You've made it difficult due to the newline character which we somehow need to workaround. (We can't rely on a destructor for example). What we need to do is to make our own output stream class, and set it to be the one that std::cout uses:

Derive a class from std::ostream.
Implement operator<< for const char*. That needs to replace \n with the extra text you want, or something along those lines.
Imbue that stream into std::cout using rdbuf: https://en.cppreference.com/w/cpp/io/basic_ios/rdbuf

Clearly all this fun can take place outside main. (3) needs some thought: you can use a constructor and instantiate a static instance of that class above main. That's an idiomatic way of running code before main.

About operator << overloading

3 Answers3