gistfile1.cpp

#define _X_WITH_ID_(x) _x_with_once_id_ ## x ## _
#define _X_WITH_ID(x) _X_WITH_ID_(x)
#define with(decl) \
    if (bool _X_WITH_ID(__LINE__) = true) \
        for (decl; _X_WITH_ID(__LINE__); _X_WITH_ID(__LINE__) = false)


with (std::lock_guard<std::mutex> lock(m_mutex)) {
    // Do stuff
    if (failed)
        break;

    // Do other stuff
}