# 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); ``` Because C++11 provides the necessary tools to do this, we are planning to remove this limitation in the future. ### 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 Catch's test execution is strictly serial. If you find yourself with a test suite that takes too long to run and you want to make it parallel, there are 2 feasible solutions * You can split your tests into multiple binaries and then run these binaries in parallel. * You can have Catch list contained test cases and then run the same test binary multiple times in parallel, passing each instance list of test cases it should run. Both of these solutions have their problems, but should let you wring parallelism out of your test suite. ## 3rd party bugs This section outlines known bugs in 3rd party components (this means compilers, standard libraries, standard runtimes). ### Visual Studio 2015 -- `GENERATE` does not compile if it would deduce char array VS 2015 refuses to compile `GENERATE` statements that would deduce to a char array with known size, e.g. this: ```cpp TEST_CASE("Deducing string lit") { auto param = GENERATE("start", "stop"); } ``` A workaround for this is to use the `as` helper and force deduction of either a `char const*` or a `std::string`. ### 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 is applied to it). This snippet is sufficient to trigger the compilation error: ```cpp #define CATCH_CONFIG_MAIN #include "catch.hpp" TEST_CASE("test") { CHECK(std::string(R"("\)") == "\"\\"); } ``` Catch provides a workaround, it is possible to disable stringification of original expressions by defining `CATCH_CONFIG_DISABLE_STRINGIFICATION`: ```cpp #define CATCH_CONFIG_FAST_COMPILE #define CATCH_CONFIG_DISABLE_STRINGIFICATION #include "catch.hpp" TEST_CASE("test") { CHECK(std::string(R"("\)") == "\"\\"); } ``` _Do note that this changes the output somewhat_ ``` catchwork\test1.cpp(6): PASSED: CHECK( Disabled by CATCH_CONFIG_DISABLE_STRINGIFICATION ) with expansion: ""\" == ""\" ``` ### Visual Studio 2015 -- Alignment compilation error (C2718) VS 2015 has a known bug, where `declval` can cause compilation error if `T` has alignment requirements that it cannot meet. A workaround is to explicitly specialize `Catch::is_range` for given type (this avoids code path that uses `declval` in a SFINAE context). ### Visual Studio 2015 -- Wrong line number reported in debug mode VS 2015 has a known bug where `__LINE__` macro can be improperly expanded under certain circumstances, while compiling multi-file project in Debug mode. A workaround is to compile the binary in Release mode. ### 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 master ```cpp #define CATCH_CONFIG_MAIN #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. a 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. ### Clang/G++ -- `Matches` string matcher always returns false This is a bug in `libstdc++-4.8`, where all matching methods from `` return false. Since `Matches` uses `` internally, if the underlying implementation does not work, it doesn't work either. Workaround: Use newer version of `libstdc++`. ### libstdc++, `_GLIBCXX_DEBUG` macro and random ordering of tests Running a Catch2 binary compiled against libstdc++ with `_GLIBCXX_DEBUG` macro defined with `--order rand` will cause a debug check to trigger and abort the run due to self-assignment. [This is a known bug inside libstdc++](https://stackoverflow.com/questions/22915325/avoiding-self-assignment-in-stdshuffle/23691322) Workaround: Don't use `--order rand` when compiling against debug-enabled libstdc++.