Test Fixtures With CMake/CTest

NOTE: This article refers to CMake 3.7.0 which is currently in release candidate stage.

New test properties were added in CMake 3.7.0 to support test fixtures. Put simply, a fixture allows setup and cleanup tasks to be associated with a group of tests. On their own, fixtures are quite useful, but they also integrate well with the existing RESOURCE_LOCK and DEPENDS test properties to support some interesting use cases. This article explains the new fixture features and provides examples showing how they complement the existing resource and dependency functionality.
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Avoiding Copies And Moves With auto

When C++11 introduced auto, it opened up a whole range of useful techniques and improved the life of C++ developers in a variety of ways. There’s no shortage of simple examples and tutorials teaching how to start using auto, but as is often the case, many of these well intentioned examples have left readers with an incomplete picture of how to use auto effectively and in some cases have resulted in poor habits becoming common practice. Since auto essentially hides away some of the details of how objects are created, there is a risk that developers don’t think as carefully about what is actually taking place. This article aims to arm the reader with information often omitted so that common pitfalls can be avoided.
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Using ccache with CMake

Updated 31st May 2016

Working with very large C/C++ code bases will sometimes test your patience. Build times in particular can be a sore point, especially if the development team do not have a great understanding of how to minimise dependencies between source files, headers, etc. The problem gets worse if the developer frequently switches between branches of their source control system, resulting in source files often changing their contents and/or timestamps. The ccache tool aims to minimise that pain by caching compilation outputs (i.e. object files) and reusing them instead of compiling the source again if it gets a compilation request it has seen before and cached. It supports GCC or any compiler that looks like GCC (eg clang). When rebuilding a large project that ccache has mostly compiled before already, the time saving can be significant, even sometimes reducing many minutes down to seconds.

Getting ccache to work with CMake is not overly complicated, but the way to do it is not always obvious. This is especially true with Xcode builds. This article demonstrates how to set up a CMake project to use ccache with Unix Makefiles, Ninja or Xcode generators, with Xcode receiving special attention. The techniques presented do not require any changes to the host system. Specifically, no symlinks need to be set up for ccache, making it suitable for use in CI systems, etc. where the developer may not be in control of how/where ccache is installed.
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Scripting CMake builds

Sometimes you just want a single way to build a software project, regardless of what platform or build tool you are using. The promise of CMake is that this should be possible, but in practice, it sometimes doesn’t always seem that way. One particular area where this becomes apparent is scripted builds, especially for things like continuous integration systems, automated testing processes, etc. Since each platform typically has its own commonly used build tool and developers tend to be more familiar with that tool than with CMake, the tendency is to invoke that tool directly in scripts. Unfortunately, this means such scripts end up handling each platform’s build tool separately. But it doesn’t have to be that way. This article will address this and a few other smaller details associated with setting up a platform independent scripted CMake build.
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Enhanced source file handling with target_sources()

In all but trivial CMake projects, it is common to find targets built from a large number of source files. These files may be distributed across various subdirectories, which may themselves be nested multiple levels deep. In such projects, traditional approaches usually either list all source files at the top-most level or build up the list of source files in a variable and pass that to add_library(), add_executable(), etc. With CMake 3.1, a new command target_sources() was introduced which provides the missing piece among the various target_... commands. While the CMake documentation succintly describes what target_sources() does, it fails to highlight just how useful the new command is and why it promotes better CMake projects:

  • It can lead to cleaner and more concise CMakeLists.txt project files.
  • Dependency information can be specified closer to where the actual dependencies exist in the directory heirarchy.
  • Source files gain the ability to become part of a target’s interface.
  • Source files can be added to third party project targets without having to modify the third party project files.

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Member Function Overloading: Choices You Didn’t Know You Had

Let’s explore your understanding of member function overloading. For a given class, how many different non-template overloads can you define for a given function where the function takes no arguments? Let’s make this a multiple choice, pick your answer from: 1, 2, 4 or 8. I’ll even provide a clue that the return types of the functions don’t matter. For extra points, how many of the function overloads are likely to be useful?
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Move constructors can copy and std::move doesn’t move anything

I recently came across an interesting use of std::move which looked something like the following:

void MyObject::processItems()
  std::vector<int>  items(std::move(m_items));
  for (auto item : items)
    // Do things which may add new items to m_items

The intent of the code was that every time the member function processItems() was called, it would perform some operation on each item held in the member variable m_items. Each processed item should be removed from m_items. The operation to be performed might generate new items which would be added to m_items, so care had to be taken about how to iterate over the set of items.

To ensure robust iteration over the items to be processed, the code transfers the contents of m_items to a local object and then iterates over that local object. Thus, if new items are created during processing, they would be added to m_items and the container being iterated over (items) would not be affected. All good, right? Well, probably yes, but by no means is it guaranteed.
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