# Known limitations Over time, some limitations of Catch2 emerged. Some of these are due to implementation details that cannot be easily changed, some of these are due to lack of development resources on our part, and some of these are due to plain old 3rd party bugs. ## Implementation limits ### Sections nested in loops If you are using `SECTION`s inside loops, you have to create them with different name per loop's iteration. The recommended way to do so is to incorporate the loop's counter into section's name, like so: ```cpp TEST_CASE( "Looped section" ) { for (char i = '0'; i < '5'; ++i) { SECTION(std::string("Looped section ") + i) { SUCCEED( "Everything is OK" ); } } } ``` or with a `DYNAMIC_SECTION` macro (that was made for exactly this purpose): ```cpp TEST_CASE( "Looped section" ) { for (char i = '0'; i < '5'; ++i) { DYNAMIC_SECTION( "Looped section " << i) { SUCCEED( "Everything is OK" ); } } } ``` ### Tests might be run again if last section fails If the last section in a test fails, it might be run again. This is because Catch2 discovers `SECTION`s dynamically, as they are about to run, and if the last section in test case is aborted during execution (e.g. via the `REQUIRE` family of macros), Catch2 does not know that there are no more sections in that test case and must run the test case again. ### MinGW/CygWin compilation (linking) is extremely slow Compiling Catch2 with MinGW can be exceedingly slow, especially during the linking step. As far as we can tell, this is caused by deficiencies in its default linker. If you can tell MinGW to instead use lld, via `-fuse-ld=lld`, the link time should drop down to reasonable length again. ## Features This section outlines some missing features, what is their status and their possible workarounds. ### Thread safe assertions Catch2's assertion macros are not thread safe. This does not mean that you cannot use threads inside Catch's test, but that only single thread can interact with Catch's assertions and other macros. This means that this is ok ```cpp std::vector threads; std::atomic cnt{ 0 }; for (int i = 0; i < 4; ++i) { threads.emplace_back([&]() { ++cnt; ++cnt; ++cnt; ++cnt; }); } for (auto& t : threads) { t.join(); } REQUIRE(cnt == 16); ``` because only one thread passes the `REQUIRE` macro and this is not ```cpp std::vector threads; std::atomic cnt{ 0 }; for (int i = 0; i < 4; ++i) { threads.emplace_back([&]() { ++cnt; ++cnt; ++cnt; ++cnt; CHECK(cnt == 16); }); } for (auto& t : threads) { t.join(); } REQUIRE(cnt == 16); ``` We currently do not plan to support thread-safe assertions. ### Process isolation in a test Catch does not support running tests in isolated (forked) processes. While this might in the future, the fact that Windows does not support forking and only allows full-on process creation and the desire to keep code as similar as possible across platforms, mean that this is likely to take significant development time, that is not currently available. ### Running multiple tests in parallel Catch2 keeps test execution in one process strictly serial, and there are no plans to change this. If you find yourself with a test suite that takes too long to run and you want to make it parallel, you have to run multiple processes side by side. There are 2 basic ways to do that, * you can split your tests into multiple binaries, and run those binaries in parallel * you can run the same test binary multiple times, but run a different subset of the tests in each process There are multiple ways to achieve the latter, the easiest way is to use [test sharding](command-line.md#test-sharding). ## 3rd party bugs This section outlines known bugs in 3rd party components (this means compilers, standard libraries, standard runtimes). ### Visual Studio 2017 -- raw string literal in assert fails to compile There is a known bug in Visual Studio 2017 (VC 15), that causes compilation error when preprocessor attempts to stringize a raw string literal (`#` preprocessor directive is applied to it). This snippet is sufficient to trigger the compilation error: ```cpp #include TEST_CASE("test") { CHECK(std::string(R"("\)") == "\"\\"); } ``` Catch2 provides a workaround, by letting the user disable stringification of the original expression by defining `CATCH_CONFIG_DISABLE_STRINGIFICATION`, like so: ```cpp #define CATCH_CONFIG_DISABLE_STRINGIFICATION #include TEST_CASE("test") { CHECK(std::string(R"("\)") == "\"\\"); } ``` _Do note that this changes the output:_ ``` catchwork\test1.cpp(6): PASSED: CHECK( Disabled by CATCH_CONFIG_DISABLE_STRINGIFICATION ) with expansion: ""\" == ""\" ``` ### Clang/G++ -- skipping leaf sections after an exception Some versions of `libc++` and `libstdc++` (or their runtimes) have a bug with `std::uncaught_exception()` getting stuck returning `true` after rethrow, even if there are no active exceptions. One such case is this snippet, which skipped the sections "a" and "b", when compiled against `libcxxrt` from the master branch ```cpp #include TEST_CASE("a") { CHECK_THROWS(throw 3); } TEST_CASE("b") { int i = 0; SECTION("a") { i = 1; } SECTION("b") { i = 2; } CHECK(i > 0); } ``` If you are seeing a problem like this, i.e. weird test paths that trigger only under Clang with `libc++`, or only under very specific version of `libstdc++`, it is very likely you are seeing this. The only known workaround is to use a fixed version of your standard library.