C++ is incredibly powerful but notoriously difficult to debug. While we have valgrind for memory leaks and gdb for complex logic, a simple syntax error can produce a wall of text that obscures the actual issue. An experienced developer learns to filter this “noise” through years of exposure, but for a junior developer, a single missing semicolon or type mismatch can result in a “wall of text” that feels more like a curse than a diagnostic.

After years of watching junior developers (and honestly, myself) drown in compiler output, I built stlfilt-go — a real-time filter that transforms C++ error logs into something human-readable. stlfilt-go is a command-line filter (written in Go) that processes stderr from GCC or Clang in real-time. It’s a modern take on Leor Zolman’s original STL Error Decryptor, updated for C++11 through C++20.

1. The “Aka” Heuristic (The Benjen Stark Rule)

In Game of Thrones, Benjen Stark says: “Nothing someone says before the word ‘but’ really counts.” In C++ logs, that word is “aka”. Clang loves to give you a complex internal type and then whisper the real name at the end. We should flip this logic and jump straight to the point.

  • Raw: note: candidate expects 'value_type' (aka 'std::basic_string<char32_t>')
  • Simplified: note: expected 'u32string'

2. Collapsing I/O Stream Bloat

I/O streams are some of the wordiest templates in the library. Because std::basic_ifstream is actually a complex hierarchy of traits and character types, a simple mismatch can produce lines of text that obscure the actual filename and line number.

  • Raw: std::basic_ifstream<char, std::char_traits<char>>
  • Simplified: ifstream

This applies to istringstream, ofstream, and streambuf as well. Stripping the basic_ prefix and the character traits allows you to see the actual stream type you defined in your code.

3. Killing “Allocator Bloat”

Almost every STL container has a hidden std::allocator parameter. While technically accurate, it’s 99% irrelevant during a syntax error. By treating these as invisible defaults, a 100-character line shrinks significantly.

  • Raw: std::map<int, MyClass, std::less<int>, std::allocator<std::pair<const int, MyClass>>>
  • Simplified: map<int, MyClass>

4. Deciphering Internal Iterators and Traits

Compilers often leak the “plumbing” of the STL. For example, a vector iterator is often a __normal_iterator wrapping a raw pointer, and modern C++ introduces traits like __unwrap_ref_decay_t.

  • Iterators: __gnu_cxx::__normal_iterator<int*, vector<int>> to vector<int>::iterator
  • Traits: __unwrap_ref_decay_t<T> to T
  • Smart Pointers: unique_ptr<int, default_delete<int>> to unique_ptr<int>

5. Path Shortening (The “Signals over Noise” Rule)

When you are deep in a project, you don’t need to see the full absolute path of every system header. Seeing /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/vector tells you nothing new—you just need to know the error happened in vector. stlfilt-go strips these absolute paths down to the base filename.

  • Raw: /usr/include/c++/11/bits/stl_vector.h:1234:20:
  • Simplified: vector:1234:20:

6. Namespace and Attribute Stripping

Compilers decorate function signatures with internal macros and namespaces that aren’t part of the code you actually wrote. Attributes like _LIBCPP_HIDE_FROM_ABI or internal namespaces like __gnu_cxx add horizontal bulk without adding diagnostic value.

  • Raw: _LIBCPP_HIDE_FROM_ABI void push_back(const value_type& __x)
  • Simplified: void push_back(const value_type&)

Repo: https://github.com/ozacod/stlfilt-go