C++ [[maybe_unused]]


The [[maybe_unused]] attribute is created for indicating in code that certain logic might not be used. This if often linked to preprocessor conditions where this might be used or might not be used. As compilers can give warnings on unused variables, this is a way of suppressing them by indicating intent.

A typical example of variables which are needed in debug builds while unneeded in production are return values indicating success. In the debug builds, the condition should be asserted, though in production these asserts have been removed.

[[maybe_unused]] auto mapInsertResult = configuration.emplace("LicenseInfo", stringifiedLicenseInfo);
assert(mapInsertResult.second); // We only get called during startup, so we can't be in the map

A more complex example are different kind of helper functions which are in an unnamed namespace. If these functions aren't used during compilation, a compiler might give a warning on them. Ideally you would like to guard them with the same preprocessor tags as the caller, though as this might become complex the [[maybe_unused]] attribute is a more maintainable alternative.

namespace {
    [[maybe_unused]] std::string createWindowsConfigFilePath(const std::string &relativePath);
    // TODO: Reuse this on BSD, MAC ...
    [[maybe_unused]] std::string createLinuxConfigFilePath(const std::string &relativePath);

std::string createConfigFilePath(const std::string &relativePath) {
#if OS == "WINDOWS"
      return createWindowsConfigFilePath(relativePath);
#elif OS == "LINUX"
      return createLinuxConfigFilePath(relativePath);
#error "OS is not yet supported"

See the proposal for more detailed examples on how [[maybe_unused]] can be used.