Show compilation errors in "#+RESULTS" code blocks

Question

The problem

I'm currently writing notes on C++ classes. Sometimes I annotate what I shouldn't do (because they cause compilation errors) so it would be really helpful if I could automatically insert compilation errors in the #+RESULTS section of source code blocks.

Consider the following example: I would like the following code block to produce

#+begin_src cpp
class A {
  static int n = 0;
};

int main() { return 0; }
#+end_src

this #+RESULTS code block

#+RESULTS:
#+begin_example
/tmp/babel-NYqDkg/C-src-HslBxN.cpp:11:14: error: ISO C++ forbids in-class initialization of non-const static member ‘A::n’
   11 |   static int n = 0;
      |              ^
#+end_example

Additional context

In sh source code blocks, I can accomplish a similar behavior by using the following header arguments

#+begin_src sh :prologue "exec 2>&1" :epilogue ":"
ls "this path doesn't exist"
#+end_src

#+RESULTS:
#+begin_example
ls: cannot access "this path doesn't exist": No such file or directory
#+end_example

Answer 1

Yes, you can do so by using the :post() header argument.

I will present two solutions. The first one is simpler but has two disadvantages. I would consider the second solution the way to go.

First solution

Consider the following Org Mode file. We can call the dash code block after the c++ code block is executed by executing org-babel-execute-src-block, which is executed by pressing C-c C-c when the element at point is a source code block.

* Compilation errors in C++

#+NAME: compile-source-cpp
#+begin_src dash :epilogue ":" :prologue "exec 2>&1"
g++ main.cpp && echo "Success"
#+end_src

#+begin_src cpp yes :main yes :tangle main.cpp :post compile-source-cpp()
int 12 = "a";
#+end_src

#+RESULTS:
#+begin_example
main.cpp: In function ‘int main()’:
main.cpp:2:5: error: expected unqualified-id before numeric constant
    2 | int 12 = "a";
      |     ^~
#+end_example

There are two disadvantages of this solution:

1. The C++ source file would be executed twice: The first one because of C-c C-c is pressed and the second one because g++ is executed in the dash code block but at least the #+RESULTS of the corresponding block would show the compilation errors.

2. Before pressing C-c C-c, you will need to ensure that org-babel-tangle is executed beforehand on the C++ code block so that the source file is created and g++ can then compile it.

Second solution

This solution gets rid of the two disadvantages of the first solution.

#+PROPERTY: header-args:cpp :tangle main.cpp :eval no

* Utilities

#+begin_src elisp
(defun org-babel-tangle-previous-src ()
  "Tangles the backward nearest source code block."
  (interactive)
  (save-excursion
    (goto-char
     (search-backward-regexp "^[[:space:]]*#\\+begin_src\\>"))
    (let ((current-prefix-arg '(4)))
      (call-interactively 'org-babel-tangle))))
#+end_src

#+NAME: org-babel-tangle-previous-src
#+begin_src elisp
(org-babel-tangle-previous-src)
#+end_src

#+NAME: compile-source-cpp
#+begin_src dash :prologue "exec 2>&1" :epilogue ":"
g++ main.cpp && echo "Success"
#+end_src

* Compilation errors in C++

#+begin_src cpp
int main
#+end_src

#+CALL: org-babel-tangle-previous-src() :post compile-source-cpp()

#+RESULTS:
#+begin_example
main.cpp: In function ‘int main()’:
main.cpp:3:1: error: expected initializer before ‘return’
    3 | return 0;
      | ^~~~~~
#+end_example

#+begin_src cpp
int number = "12 + 13";
#+end_src

#+CALL: org-babel-tangle-previous-src() :post compile-source-cpp()

#+RESULTS:
#+begin_example
main.cpp: In function ‘int main()’:
main.cpp:2:14: error: invalid conversion from ‘const char*’ to ‘int’ [-fpermissive]
    2 | int number = "12 + 13";
      |              ^~~~~~~~~
      |              |
      |              const char*
#+end_example

Pressing C-c C-c in the #+CALL statements will tangle the nearest source code block and then will execute g++ on the tangled file.

Answer 2

If you look at ob-C.el function org-babel-C-execute, you'll see that it compiles and links a program to tmp-bin-file, then runs it and captures its output to results.

So you need a different source-file type eg cpp-wrong that you would execute using shell that invokes g++ and captures its stderr as you hinted above.

Maybe copy ob-C.el and try to modify it according ot the above blueprint?