FAQ

How is doctest different from Catch?

Pros of doctest:

  • including the doctest header is over 50 times lighter on compile times than that of Catch
  • the asserts in doctest can be many times lighter on compile times than those of Catch
  • everything testing-related can be removed from the binary by defining the DOCTEST_CONFIG_DISABLE identifier
  • doesn't drag any headers when included (except for in the translation unit where the library gets implemented)
  • 0 warnings even on the most aggressive warning levels for MSVC/GCC/Clang
  • per commit tested with 300+ builds on much more compilers - and through valgrind/sanitizers
  • test cases can be written in headers - the framework will still register the tests only once - no duplicates

Aside from everything mentioned so far doctest has some features (like templated test cases) which Catch doesn't.

Missing stuff:

  • a reporter/listener system - to a file, to xml, ability for the user to write their own reporter, etc.
  • matchers and generators
  • other small stuff

But these things (and more!) are planned in the roadmap!

doctest can be thought of as a very polished, light, stable and clean subset (or reimplementation) of Catch but this might change in the future as more features are added.

A quick and easy way to migrate most of your Catch tests to doctest is to change the TEST_CASE (if using tags) and SECTION macros as follows:

#include "path/to/doctest.h"

#undef TEST_CASE
#define TEST_CASE(name, tags) DOCTEST_TEST_CASE(tags " " name) // will concatenate the tags and test name string literals to one
#define SECTION(name) DOCTEST_SUBCASE(name)
using doctest::Approx; // catch exposes this by default outside of its namespace

How to get the best compile-time performance with the framework?

Using the fast asserts in combination with DOCTEST_CONFIG_SUPER_FAST_ASSERTS yelds the fastest compile times.

There are only 2 drawbacks of this approach:

  • using fast asserts (40-80% faster than CHECK(a==b)) means that there is no try/catch block in each assert so if an expression throws the whole test case ends.
  • defining the DOCTEST_CONFIG_SUPER_FAST_ASSERTS config identifier will result in even faster fast asserts (30-80%) at the cost of only one thing: when an assert fails and a debugger is present - the framework will break inside a doctest function so the user will have to go 1 level up in the callstack to see where the actual assert is in the source code.

These 2 things can be considered negligible if you are dealing mainly with arithmetic (expressions are unlikely to throw exceptions) and all the tests usually pass (you don't need to often navigate to a failing assert with a debugger attached)

If you want better aliases for the asserts instead of the long ones you could use DOCTEST_CONFIG_NO_SHORT_MACRO_NAMES and then define your aliases like this: #define CHECK_EQ DOCTEST_FAST_CHECK_EQ (example in here).

Is doctest thread-aware?

Currently no. Asserts cannot be used in multiple threads and test cases cannot be ran in parallel. These are long-term features that are planned on the roadmap.

For now tests are ran serially and doing asserts in multiple user threads will lead to crashes.

There is an option to run a range of tests from an executable - so tests can be ran in parallel with multiple process invocations - see the example python script.

Is mocking supported?

doctest doesn't support mocking but should be easy to integrate with third-party libraries such as:

by using the logging macros such as ADD_FAIL_AT(file, line, message)

Why are my tests in a static library not getting registered?

This is a common problem among libraries with self-registering code and it affects all modern compilers on all platforms.

The problem is that when a static library is being linked to a binary (executable or dll) - only object files from the static library that define a symbol being required from the binary will get pulled in (this is a linker/dependency optimization).

A way to solve this in CMake is to use object libraries instead of static libraries - like this:

add_library(with_tests OBJECT src_1.cpp src_2.cpp src_3.cpp ...)

add_library(dll SHARED $<TARGET_OBJECTS:with_tests> dll_src_1.cpp ...)
add_executable(exe $<TARGET_OBJECTS:with_tests> exe_src_1.cpp ...)

Thanks to pthom for suggesting this.

As an alternative I have created a CMake function that forces every object file from a static library to be linked into a binary target - it is called doctest_force_link_static_lib_in_target(). It is unintrusive - no source file gets changed - everything is done with compiler flags per source files. An example project using it can be found here - the commented part of the CMakeLists.txt file.

It doesn't work in 2 scenarios:

  • either the target or the library uses a precompiled header - see this issue for details
  • either the target or the library is an imported target (pre-built) and not built within the current cmake tree

You can also checkout this repository for a different solution: pthom/doctest_registerlibrary.

Why is comparing C strings (char*) actually comparing pointers?

doctest by default treats char* as normal pointers. Using the DOCTEST_CONFIG_TREAT_CHAR_STAR_AS_STRING changes that.

How to write tests in header-only libraries?

There are 2 options:

  • just include the doctest header in your headers and write the tests - the doctest header should be shipped with your headers and the user will have to implement the doctest runner in one of his source files.
  • don't include the doctest header and guard your test cases with #ifdef DOCTEST_LIBRARY_INCLUDED and #endif - that way your tests will be compiled and registered if the user includes the doctest header before your headers (and he will also have to implement the test runner somewhere).

Also note that it would be a good idea to add a tag in your test case names (like this: TEST_CASE("[the_lib] testing foo")) so the user can easily filter them out with --test-case-exclude=*the_lib* if he wishes to.

Does the framework use exceptions?

Yes - but they can be disabled - see the DOCTEST_CONFIG_NO_EXCEPTIONS config identifier.

Why do I get compiler errors in STL headers when including the doctest header?

Try using the DOCTEST_CONFIG_USE_IOSFWD configuration identifier.

Can different versions of the framework be used within the same binary (executable/dll)?

Currently no. Single header libraries like stb have this as an option (everything gets declared static - making it with internal linkage) but it isn't very logical for doctest - the main point is to write tests in any source file of the project and have the test runner implemented in only one source file.

Why is doctest using macros?

Aren't they evil and not modern? - Check out the answer Phil Nash gives to this question here (the creator of Catch).

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