How to catch C++ exceptions properly

[AllMeatball]

Hi there! writing a C++ codebase and would like to know how to catch exceptions safely. it should catch the exception correctly without crashing the program.

Following example(s) below shows what is expected:

Example (basic pseudocode):

std::vector<GFX::Image> images;

for (const std::string image_path : image_paths) {
    try {
        GFX::Image image(image_path); // potential point where the program could throw an exception
        images.push_back(image);
    } catch (std::exception &e) {
        std::cerr << "Failed to load image: " << e.what() << std::endl;
    }
}

Expected output (e.g. desktop with GCC or clang):

Failed to load image: GFX::Image: 'player.png' not found

Actual output (e.g. Emscripten):

Uncaught (in promise) std::runtime_error: GFX::Image: 'player.png' not found

Note the difference that it shows Uncaught (in promise) std::runtime_error: on the actual output in emscripten, on desktop C++ this wouldn't happen.

[sbc100]

Did you build with -sDISABLE_EXCEPTION_CATCHING=0?

[sbc100]

Can you share your full link command?

[AllMeatball]

Here's the snippet from my CMakeLists.txt file:

# NOTE: `-O3` doesn't change anything related to exceptions to my knowledge. this is only really enabled for speed and probably could be omitted
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -O3")

set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -s USE_GLFW=3 -s ASSERTIONS=1 -s WASM=1 -s ASYNCIFY -s GL_ENABLE_GET_PROC_ADDRESS=1 -v --shell-file \"${CMAKE_SOURCE_DIR}/shell.html\" -s EXCEPTION_CATCHING_ALLOWED=[..] -sINITIAL_MEMORY=268435456")

I did try with other options too -fwasm-exceptions, -fexceptions with whatever else is in C++ Exceptions Support page. I could try to figure out how to implement the Handling C++ Exceptions from JavaScript section.

[CedricConday]

Exception catching is off by default, and the flag has to be present at compile and link time.
The second half is what usually bites: the build compiles fine, then links without exception
support.

Two options.

-fexceptions — Emscripten's JavaScript-based implementation. Works on every engine with Wasm
support, noticeable size and speed cost. -sDISABLE_EXCEPTION_CATCHING=0 is the older spelling of
the same thing.

-fwasm-exceptions — native WebAssembly exception handling. Smaller and faster, not implemented by
every engine yet; current major browsers have it.

Pass it to both phases:

emcc -fwasm-exceptions -c image.cpp -o image.o
emcc -fwasm-exceptions image.o -o app.js

In CMake that means target_compile_options and target_link_options, or CMAKE_CXX_FLAGS plus
CMAKE_EXE_LINKER_FLAGS. Every translation unit and prebuilt library that throws or catches needs
the same flag — mixing a -fexceptions archive into a -fwasm-exceptions link is its own failure
mode.

If it still doesn't catch: build with -sASSERTIONS --profiling-funcs for a stack trace with real
function names, and use getExceptionMessage(e) in the JS catch block if the throw crosses into
JavaScript. Also check GFX::Image isn't aborting rather than throwing — an abort is not an
exception and no catch will see it.

[AllMeatball]

Adding either -fwasm-exceptions or -fexceptions for the compiler and linker works. Used the same flag for each (-fwasm-exceptions), -fexceptions also works. may consider using -fexceptions instead if there's problems for the end user.