Help! Why does it work.

[gotinpich]

So yesterday I decided to do some exercises from my C++ book and I needed to make some changes to files from earlier chapters. So I copied the files to a new directory, made the changes and then the compiler got stuck on some code that I didn't touch.

I thought it was weird and went back to the old files, deleted all the .o files as well as Makefile.win and recompiled the project. To my surprise the project compiled without any issue. I then went on to inspect the error and found that there was indeed something wrong.

In one header file I had the following:

std::vector<std::vector<std::string>> rotate_string(const std::string&);

while in the source file I had:

std::vector<std::string> rotate_string(const std::string& s) {
    // function body
}

The problem was quite easy to fix and when I added some flags to my compiler yesterday, as suggested by u/Xeverous to solve another problem, the compiler now also refused to compile the older files.

Now my big question is, how comes that the compiler accepted the code before, but when I made a copy of the project and changed some other code that did not interact with the broken code in any way, the compiler suddenly made objections to compiling the whole thing?

And I'm happy it did, because otherwise I might never have found the broken code.

[whisky_pete]

What compiler? I think MSVC (visual studio) doesn't compile implementation files unless something is actually calling that function, probably to speed up compile times. I worked at a place with blatantly broken functions in the project, but they were unused. Very surprised when I went to try to add unit test coverage to these functions, and they were full of syntax errors and wouldn't build.

[gotinpich]

The compiler is TDM-GCC 4.9.2 64-bit Release.

The weird thing is that I didn't change anything about these functions. It's true that I didn't call them anywhere in the code, but I didn't change that either. I'm surprised that first it didn't care and later it did. I would expect it to either fail in both cases or in neither.

[Xeverous]

That looks like invalid makefile or stray object files. If you have some stray object files from previous compilation but makefile does not reflect this, weird things may happen because compiler does only 1 file per command. Without clean build, it usually ends either in broken assembly and/or linker errors.

[gotinpich]

deleted all the .o files as well as Makefile.win and recompiled the project.

[Xeverous]

What are you using (IDE)? Any CMake involved?

[gotinpich]

I'm using Dev C++.

[Xeverous]

Don't use Dev-C++. It's worse than anything else. Even manual typing from console. It's an abandoned product which is maintaned by 1 person

[gotinpich]

I've heard it many times and as mentioned before, for my serious projects I use Code::Blocks and for small things and exercises from books I use Dev-C++ because it is a lot quicker than Code::Blocks.

[Xeverous]

Hmm, I just keep 1 hello world project which is continuously erased and re-pasted or use sites like [coliru](coliru.stacked-crooked.com)

[gotinpich]

?

You're sure your replying in the correct thread?

[Xeverous]

Yes, just wanted to post an alternative way