Can't std::ostream output a const char array?

Question

For the fun and experience of it, I'm modifying and exploring the source code for Blobby Volley 2 1.0 (Linux).

Well... I would be modifying the source code, but I can't even get the program to compile. (Sad, isn't it?)

Here's the code that causes the error:

std::ostream& operator<<(std::ostream& stream, const ServerInfo& val) {
    return stream << val.name << " (" << val.hostname << ":" << val.port << ")";
    }

Trying to compile this with g++ 5.4.0 gives the following (simplified output--the original output is ~443 lines) error message:

error: no match for ‘operator<<’ (operand types are ‘std::ostream {aka std::basic_ostream}’ and ‘const char [32]’)

return stream << val.name << " (" << val.hostname << ":" << val.port << ")";

I simplified the code to this:

std::ostream& operator<<(std::ostream& stream, const ServerInfo& val) {
    stream << "hello"; //can't get simpler than this, right?
    return stream;
    }

and got

error: no match for ‘operator<<’ (operand types are ‘std::ostream {aka std::basic_ostream}’ and ‘const char [6]’)

stream << "hello";

The code that calls it looks like this:

std::cout << "duplicate server entry\n";
std::cout << info << "\n"; //it's called here

---

The thing I find most surprising is that we all know that std::cout and its ilk can handle char arrays.

For instance,

#include <iostream>
#include <string>

int main () {
    const char a[6] = "hello";
    std::cout << a << std::endl; //No problem here!
    return 0;
    }

works without a hitch.

---

Oh, one more thing.

If I include <string>, this works:

std::ostream& operator<<(std::ostream& stream, const ServerInfo& val) {
    stream << std::string("hello");
    return stream;
    }

---

Does anyone know what I'm missing?

---

PS: Here's a pastebin of the errors.

PPS: Here's the headers that were requested:

/* header include */
#include "NetworkMessage.h"

/* includes */
#include <cstring>

#include "UserConfig.h"
#include "SpeedController.h"

PPS: If you are wondering why I didn't get an error about std::ostream not being defined, check the 3rd paragraph of Sam's answer.

Freehanded

Can you post, in addition to what you have here, the full error log? There might be something else in there that's important. – Wallpaper

@templatetypedef: Are you positive you want it all? – Freehanded

You're reporting an error that shouldn't normally be happening, so I suspect something else might be up as well. So... yes! :-) – Wallpaper

The most likely answer is that the translation unit in question is missing an include of <iostream>. – Layamon

@templatetypedef: I'm getting you your error log. I have to format it first so that you can actually read it though... :D – Freehanded

@JefréN.: Post the headers as well which you have included in the file which contains this code. – Indult

@templatetypedef: Posted pastebin. – Freehanded

Sure looks like the errors you get if you didn't #include <iostream> - the only overload it knows about is the custom one you wrote. – Wallpaper

@templatetypedef: I'll try it, although I don't think it is theoretically necessary. – Freehanded

@templatetypedef: That was the problem ... Stunned. Post it as an answer and I'll accept it, if you want. – Freehanded

Give credit to @SamVarshavchik - he was the first to figure that out. :-) – Wallpaper

@SamVarshavchik: Congratulations! You're hypothesis proved correct. Do you care to give it as an answer? (Since iostream was included in 'main.cpp', I'm still not sure why it was necessary, though.) – Freehanded

@Wallpaper and everyone else: Thanks for the help and brainstorming. I guess I just learned another lesson in never assuming. :DD – Freehanded

Every C++ file is compiled independently of all others, so just because you included a header in one file doesn't mean you've included it everywhere. – Wallpaper

Anyone think I should delete this question? – Freehanded

@templatetypedef: In truth, I include every header I need in each file. However, I didn't think it was strictly necessary. Guess it is. Thanks again for the help and explanation. :D – Freehanded

@JefréN. Deleting the question now would probably save reviewers' work in the long run, since I suspect that's the ultimate fate of this question anyways. But it's up to you. Glad you figured it out though. – Empyrean

@JasonC: If not for the fact that Sam Varshavchik, the guy who initially guessed what the problem was, gave an answer like I asked him to, I would delete the question. But as it is, I think I'm gonna let stackoverflow take its course ... :D – Freehanded

Answer 1

The fact that #include <iostream> was likely missing was deduced using the Sherlock Holmes approach to debugging: "when you have eliminated the impossible, whatever remains, however improbable, must be the truth".

Clearly. std::ostream should've have had no problems accepting a const char * overload.

Therefore, an overload resolution complaint must mean that <iostream> wasn't included. Most C++ library classes are forward-declared all over the place. Including some random header file is likely to get you a forward declaration of std::ostream, as a free bonus. So the compiler will not complain about this class not being defined.

But unless <iostream> is included, the compiler will not know about all the overloads that are defined there. That's it.

Layamon

#include <ostream> (which declares the const char* overload - 27.7.3.1) would suffice instead of iostream (which includes ostream - 27.4.1). – Marion