C++Guns – RoboBlog blogging the bot

Goal

Overload operator +=, << etc. as free function for a templated class with private member variables without boilerplate code.
Choose free function to achieve less coupling than with member functions.
In the example you can imagine the Type as Array2D for a use case.

Solution

template<typename T>
class Type {
public:
    // free function form of the operator
    // no repeat of the template parameters for class Type necessary
    // with the friend keyword this is still a free function but can 
    // access private variables like a member function
    friend Type& operator+=(Type& lhs, double value) {
        lhs.data += value; // can access private member variable
        return x;
    }

private:
    double data = 0;
};


void f(Type<double>& x) {
    x += 2.0;
}

Not a Solution

// forward declaration, not usual
template<typename T>
class Type;

// Definition of the function is placed bevor the definition of the type. 
// This is also not usual but needed for the friend declaration later in the code.
// Repeat of the template parameter.
template<typename T>
inline Type<T>& operator+=(Type<T>& lhs, double value) {
    lhs.data += value;    
    return lhs;
}

template<typename T>
class Type {

private:
    double data = 0;

    // templated friend declaration. watch the necessary but very unusual <> here.    
    friend Type& operator+= <>(Type& lhs, double value);
};

void f(Type<double>& x) {
    x += 2.0;
}

C.4: Make a function a member only if it needs direct access to the representation of a class https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rc-member
Sources: https://stackoverflow.com/questions/4660123/overloading-friend-operator-for-class-template

Mal ausprobieren ob damit Exception einfacher in der Handhabung werden...
Aus Exceptionally Bad: The Misuse of Exceptions in C++ & How to Do Better - Peter Muldoon - CppCon 2023

Conclusion of the talk:

What should they used for ?
• Error tracing (logging)
• Stack unwinding (reset / termination)
• Data passing / Control flow (when necessary)

When should they used ?
• As “Rarely” as possible
• Serious/infrequent/unexpected errors
• With as few exception types as possible (as determined by catch functionality)

template<typename DATA_T>
class OmegaException {
public:
  OmegaException(std::string str, DATA_T data, const std::source_location& loc = std::source_location::current(), std::stacktrace trace = std::stacktrace::current())
:err_str_{std::move(str)}, user_data_{std::move(data)}, location_{loc}, backtrace_{trace} { }

  std::string& what() { return err_str_; }
  const std::string& what() const noexcept { return err_str_; }

  const std::source_location& where() const noexcept { return location_; }
  const std::stacktrace& stack() const noexcept { return backtrace_; }

  DATA_T& data(){ return user_data_;}
  const DATA_T& data() const noexcept { return user_data_; }

private:
  std::string err_str_;
  DATA_T user_data_;
  const std::source_location location_;
  const std::stacktrace backtrace_;
}

std::ostream& operator << (std::ostream& os, const std::source_location& location) {
  os << location.file_name() << "("
  << location.line() << ":"
  << location.column() << "), function `"
  << location.function_name() << "`";
  return os;
}

std::ostream& operator << (std::ostream& os, const std::stacktrace& backtrace) {
  for(auto iter = backtrace.begin(); iter != (backtrace.end()-3); ++iter) {
    os << iter->source_file() << "(" << iter->source_line()
   << ") : " << iter->description() << "\n";
  }
  return os;
}

///////////////

enum Errs1 { bad = 1, real_bad };
enum class Errs2 { not_bad = 10, not_good };

using MyExceptionErrs1 = OmegaException<Errs1>;
using MyExceptionErrs2 = OmegaException<Errs2>;

throw MyExceptionErrs1(“Bad Order id", real_bad);

catch(const MyExceptionErrs1& e) {
  std::cout << "Failed to process with code (" << e.data() << ") : "
  << e.what() << “\n" << e.where() << std::endl;
}

/*
Failed to process with code (2) : Bad Order id
/app/example.cpp(76:69), function `Order findOrder(unsigned int
*/


//////////////

struct bucket {
  int id_;
  Msg msg_;
};

using MyExceptionBucket = OmegaException< bucket >;

throw MyExceptionBucket ("bad error", bucket{cliendId, amsg});

catch(MyExceptionBucket& eb) {
  std::cout << "Failed to process id (" << e.data().id_ << ") : "
  << e.what() << "\n" << e.stack() << std::endl;
  send_error(eb.data().msg_);
}

/*
Failed to process id (222) : Bad Order id
example.cpp(76) : findOrder(unsigned int)
example.cpp(82) : processOrder(unsigned int)
example.cpp(97) : main
*/

Mal ausprobieren ob damit Exception einfacher in der Handhabung werden...
Aus Exceptionally Bad: The Misuse of Exceptions in C++ & How to Do Better - Peter Muldoon - CppCon 2023

Conclusion of the talk:

What should they used for ?
• Error tracing (logging)
• Stack unwinding (reset / termination)
• Data passing / Control flow (when necessary)

When should they used ?
• As “Rarely” as possible
• Serious/infrequent/unexpected errors
• With as few exception types as possible (as determined by catch functionality)

template<typename DATA_T>
class OmegaException {
public:
  OmegaException(std::string str, DATA_T data, const std::source_location& loc = std::source_location::current(), std::stacktrace trace = std::stacktrace::current())
:err_str_{std::move(str)}, user_data_{std::move(data)}, location_{loc}, backtrace_{trace} { }

  std::string& what() { return err_str_; }
  const std::string& what() const noexcept { return err_str_; }

  const std::source_location& where() const noexcept { return location_; }
  const std::stacktrace& stack() const noexcept { return backtrace_; }

  DATA_T& data(){ return user_data_;}
  const DATA_T& data() const noexcept { return user_data_; }

private:
  std::string err_str_;
  DATA_T user_data_;
  const std::source_location location_;
  const std::stacktrace backtrace_;
}

std::ostream& operator << (std::ostream& os, const std::source_location& location) {
  os << location.file_name() << "("
  << location.line() << ":"
  << location.column() << "), function `"
  << location.function_name() << "`";
  return os;
}

std::ostream& operator << (std::ostream& os, const std::stacktrace& backtrace) {
  for(auto iter = backtrace.begin(); iter != (backtrace.end()-3); ++iter) {
    os << iter->source_file() << "(" << iter->source_line()
   << ") : " << iter->description() << "\n";
  }
  return os;
}

///////////////

enum Errs1 { bad = 1, real_bad };
enum class Errs2 { not_bad = 10, not_good };

using MyExceptionErrs1 = OmegaException<Errs1>;
using MyExceptionErrs2 = OmegaException<Errs2>;

throw MyExceptionErrs1(“Bad Order id", real_bad);

catch(const MyExceptionErrs1& e) {
  std::cout << "Failed to process with code (" << e.data() << ") : "
  << e.what() << “\n" << e.where() << std::endl;
}

/*
Failed to process with code (2) : Bad Order id
/app/example.cpp(76:69), function `Order findOrder(unsigned int
*/


//////////////

struct bucket {
  int id_;
  Msg msg_;
};

using MyExceptionBucket = OmegaException< bucket >;

throw MyExceptionBucket ("bad error", bucket{cliendId, amsg});

catch(MyExceptionBucket& eb) {
  std::cout << "Failed to process id (" << e.data().id_ << ") : "
  << e.what() << "\n" << e.stack() << std::endl;
  send_error(eb.data().msg_);
}

/*
Failed to process id (222) : Bad Order id
example.cpp(76) : findOrder(unsigned int)
example.cpp(82) : processOrder(unsigned int)
example.cpp(97) : main
*/

Convert from a broken down date time structure from UTC to localtime in C++.

See Stackoverflow: How to convert from UTC to local time in C?
I converted the code from C to C++ and make it shorter.

// replace non std function strptime with std::get_time

#include <iostream>
#include <sstream>
#include <iomanip>
#include <ctime>

/*
  Convert from a broken down date time structure from UTC to localtime.
  See https://stackoverflow.com/questions/9076494/how-to-convert-from-utc-to-local-time-in-c
*/
std::tm UTC2localtime(std::tm tp) {
  // make sure Daylight saving time flag is not accidentally switched on in UTC time
  tp.tm_isdst = 0;

  // get seconds since EPOCH for this time
  const time_t utc = std::mktime(&tp);
  std::cout << "UTC date and time in seconds since EPOCH: " << utc << "\n";

  // convert UTC date and time (Jan. 1, 1970) to local date and time
  std::tm e0{};
  e0.tm_mday = 1;
  e0.tm_year = 70;

  // get time_t EPOCH value for e0. This handles daylight saving stuff.
  // The value is e.g. -3600 for 1h difference between the timezones
  const time_t diff = std::mktime(&e0);

  // calculate local time in seconds since EPOCH
  const time_t local = utc - diff;
  std::cout << "local date and time in seconds since EPOCH: " << local << "\n";

  // convert seconds since EPOCH for local time into local_tm time structure
  std::tm local_tm;
  if(localtime_r(&local, &local_tm) == nullptr) {
    throw std::system_error(errno, std::generic_category(), "UTC2localtime(): in conversion vom UTC to localtime");
  }
  return local_tm;
}

int main() {
  // hard coded date and time in UTC
  std::string datetime = "2013 11 30 23 30 26";
  std::cout << "UTC date and time to be converted in local time: " << datetime << "\n";

  // put values of datetime into time structure
  std::tm UTC_tm{};
  std::istringstream ss(datetime);
  ss >> std::get_time(&UTC_tm, "%Y %m %d %H %M %S");

  if(ss.fail()) {
    throw std::runtime_error("Can not parse datetime from datetime '" + datetime + "' to format %Y %m %d %H %M %S");
  }

  const std::tm local_tm = UTC2localtime(UTC_tm);
  std::cout << "local date and time: " << std::put_time(&local_tm, "%Y-%m-%d %H:%M:%S %Z") << "\n";
}

This example throw and catch all standard exceptions just for fun
exception list from https://en.cppreference.com/w/cpp/error/exception
sorted after C++ Standard

searching a stack trace? look at https://en.cppreference.com/w/cpp/utility/basic_stacktrace

/* This example throw and catch all standard exceptions just for fun
 * exception list from https://en.cppreference.com/w/cpp/error/exception
 * sorted after C++ Standard
 *
 * searching a stack trace? look at https://en.cppreference.com/w/cpp/utility/basic_stacktrace
 */

#include <iostream>
#include <exception>
#include <future>
#include <regex>
#include <filesystem>
#include <chrono>
#include <any>
#include <optional>
#include <variant>
//#include <format> // C++20. Not implemented in GCC 12
// #include <expected> // C++23


int nCatched = 0;

template<typename T>
void check(const T& ex, std::string_view exceptionType) {
  nCatched++;
  std::string_view what(ex.what());
  // only compare the first chars from what() message, we are not intereseting in stuff after the exception type
  if(what.substr(0,exceptionType.size()) != exceptionType) {
      std::cerr << exceptionType << " what(): " << ex.what() << "\n";
      std::cerr << "\nERROR: Not all exception derived from " << exceptionType << " catched...\n";
      ::exit(EXIT_FAILURE);
    }
}

int main() {

  bool finish = false;
  int nThrow = 0;
  while(not finish){
    try {
      switch(nThrow) {
        case  1: ++nThrow; throw std::exception();
        case  2: ++nThrow; throw std::logic_error("std::logic_error");
        case  3: ++nThrow; throw std::invalid_argument("std::invalid_argument");
        case  4: ++nThrow; throw std::domain_error("std::domain_error");
        case  5: ++nThrow; throw std::length_error("std::length_error");
        case  6: ++nThrow; throw std::out_of_range("std::out_of_range");
        case  7: ++nThrow; throw std::runtime_error("std::runtime_error");
        case  8: ++nThrow; throw std::range_error("std::range_error");
        case  9: ++nThrow; throw std::overflow_error("std::overflow_error");
        case 10: ++nThrow; throw std::underflow_error("std::underflow_error");
        case 11: ++nThrow; throw std::bad_typeid();
        case 12: ++nThrow; throw std::bad_alloc();
        case 13: ++nThrow; throw std::bad_exception();
        case 14: ++nThrow; throw std::regex_error(std::regex_constants::error_collate); // C++11
        case 15: ++nThrow; throw std::system_error(ENOENT, std::system_category(), "std::system_error"); // C++11
        case 16: ++nThrow; throw std::ios_base::failure("std::ios_base::failure"); // C++11
        case 17: ++nThrow; throw std::future_error(std::future_errc::broken_promise); // C++11
        case 18: ++nThrow; throw std::bad_weak_ptr(); // C++11
        case 19: ++nThrow; throw std::bad_function_call(); // C++11
        case 20: ++nThrow; throw std::bad_array_new_length(); // C++11
        case 21: ++nThrow; throw std::filesystem::filesystem_error("std::filesystem::filesystem_error", std::error_code(ENOENT, std::system_category())); // C++17
        case 22: ++nThrow; throw std::bad_any_cast(); // C++17
        case 23: ++nThrow; throw std::bad_optional_access(); // C++17
        case 24: ++nThrow; throw std::bad_variant_access(); // C++17
        // case 25: throw std::chrono::nonexistent_local_time(); // C++ 20. Not implemented in GCC 12
        // case 26: throw std::chrono::ambiguous_local_time(); // C++20. Not implemented in GCC 12
        // case 27: throw std::format_error(); // C++20. Not implemented in GCC 12
        // case 28: throw std::bad_expected_access(); // C++23
        // case 29: throw std::tx_exception(); TODO
        default: {
          finish = true;
        }
      }
    }
    catch(std::bad_variant_access& ex) {
      check(ex, "bad variant access");
    }
    catch(std::bad_exception& ex) {
      check(ex, "std::bad_exception");
    }
    catch(std::bad_array_new_length& ex) {
      check(ex, "std::bad_array_new_length");
    }
    catch(std::bad_alloc& ex) {
      check(ex, "std::bad_alloc");
    }
    catch(std::bad_function_call& ex) {
      check(ex, "bad_function_call");
    }
    catch(std::bad_weak_ptr& ex) {
      check(ex, "bad_weak_ptr");
    }
    catch(std::bad_optional_access& ex) {
      check(ex, "bad optional access");
    }
    catch(std::bad_any_cast& ex) {
      check(ex, "bad any_cast");
    }
    catch(std::bad_typeid& ex) {
      check(ex, "std::bad_typeid");
    }
    catch(std::filesystem::filesystem_error& ex) {
      check(ex, "filesystem error");
    }
    catch(std::ios_base::failure& ex) {
      check(ex, "std::ios_base::failure");
    }
    catch(std::system_error& ex) {
      check(ex, "std::system_error");
    }
    catch(std::regex_error& ex) {
      check(ex, "Invalid collating element in regular expression"); // regex_error.what does not print the exception type first...
    }
    catch(std::underflow_error& ex) {
      check(ex, "std::underflow_error");
    }
    catch(std::overflow_error& ex) {
      check(ex, "std::overflow_error");
    }
    catch(std::range_error& ex) {
      check(ex, "std::range_error");
    }
    catch(std::runtime_error& ex) {
      check(ex, "std::runtime_error");
    }
    catch(std::future_error& ex) {
      check(ex, "std::future_error");
    }
    catch(std::out_of_range& ex) {
      check(ex, "std::out_of_range");
    }
    catch(std::length_error& ex) {
      check(ex, "std::length_error");
    }
    catch(std::domain_error& ex) {
      check(ex, "std::domain_error");
    }
    catch(std::invalid_argument& ex) {
      check(ex, "std::invalid_argument");
    }
    catch(std::logic_error& ex) {
      check(ex, "std::logic_error");
    }
    catch(std::exception& ex) {
      check(ex, "std::exception");
    }
  } // while


  if(nThrow != nCatched) {
    std::cerr << nThrow << " exception thrown but " << nCatched << " catched\n";
  } else {
    std::cout << "All exceptions which was thrown was catched\n";
  }

  return EXIT_SUCCESS;
}

$ ./a.out
All exceptions which was thrown was catched

Display which format flags are currently set e.g. fixed, scientific, dec, hex

std::ostream& operator<<(std::ostream& s, const std::ios::fmtflags f) {
   if(f & std::ios::boolalpha) s << "boolalpha ";
   if(f & std::ios::dec) s << "dec ";
   if(f & std::ios::hex) s << "hex ";
   if(f & std::ios::oct) s << "oct ";
   if(f & std::ios::fixed) s << "fixed ";
   if(f & std::ios::scientific) s << "scientific ";
   if(f & std::ios::right) s << "right ";
   if(f & std::ios::left) s << "left ";
   if(f & std::ios::internal) s << "internal ";
   if(f & std::ios::showbase) s << "showbase ";
   if(f & std::ios::showpoint) s << "showpoint ";
   if(f & std::ios::showpos) s << "showpos ";
   if(f & std::ios::uppercase) s << "uppercase ";
   if(f & std::ios::unitbuf) s << "unitbuf ";
   if(f & std::ios::skipws) s << "skipws ";
   return s;
}

std::cout << std::cout.flags() << "\n";

Example output

dec fixed skipws 

https://en.cppreference.com/w/cpp/io/ios_base/flags

Das Beispiel habe ich von https://www.mpi-forum.org/docs/mpi-3.1/mpi31-report/node425.htm
Jeder Thread erstellt einen MPI Datentyp welcher die Offsett Addressen der struct Member Variablen hat.
Thread 1 sendet Daten zu Thread 0
Thread 0 empfaenge Daten von Thread1 und seine eigenen Daten, so dass alle in einem Array dann liegen.

// This example is based on https://www.mpi-forum.org/docs/mpi-3.1/mpi31-report/node425.htm
// Jeder Thread erstellt einen MPI Datentyp welcher die Offsett Addressen der struct Member Variablen hat.
// Thread 1 sendet Daten zu Thread 0
// Thread 0 empfaenge Daten von Thread1 und seine eigenen Daten, so dass alle in einem Array dann liegen.

#include <iostream>
#include <array>

#include <mpi.h>

int my_rank;
int nThreads;

struct basetype_t {
  double volstep = 0;
  double volTot = 0;
};

// Vererbung ist nicht erlaubt
struct type_t  {
  char ID[50];
  int i = 0;
  float x = 0;
  // Ein Container wie std::vector zwischen den Datentypen scheint wohl zu funktionieren, ist in Fortran aber explizit nicht erlaubt.
//   std::vector<int> unused;
  bool l = false;
  double d = 0;
  basetype_t base;
};

// Check MPI Error code
void check(int ierr) {
  if (ierr != MPI_SUCCESS) {
    char err_recvbuffer[MPI_MAX_ERROR_STRING];
    int resultlen;
    MPI_Error_string(ierr, err_recvbuffer, &resultlen);
    std::cerr << err_recvbuffer << "\n";
    MPI_Finalize();
  }
}

// create new MPI datatype based on the addresses of the member variables of the type we want to send
MPI_Datatype createMPItyp() {
  type_t foo;
  MPI_Aint base;
  check(MPI_Get_address(&foo, &base));

  // Fuer jede member Variable die gesendet werden soll, Typ und Addresse bestimmen
  const int nMembervarsToSend = 7;
  std::array<MPI_Datatype, nMembervarsToSend> types;
  std::array<int,nMembervarsToSend> blocklen;
  std::array<MPI_Aint, nMembervarsToSend> disp;

  types[0] = MPI_INT;
  blocklen[0] = 1;
  check(MPI_Get_address(&foo.i, &disp[0]));

  types[1] = MPI_FLOAT;
  blocklen[1] = 1;
  check(MPI_Get_address(&foo.x, &disp[1]));

  types[2] = MPI_LOGICAL;
  blocklen[2] = 1;
  check(MPI_Get_address(&foo.l, &disp[2]));

  types[3] = MPI_DOUBLE;
  blocklen[3] = 1;
  check(MPI_Get_address(&foo.d, &disp[3]));

  types[4] = MPI_CHAR;
  blocklen[4] = sizeof(foo.ID);
  check(MPI_Get_address(&foo.ID, &disp[4]));

  types[5] = MPI_DOUBLE;
  blocklen[5] = 1;
  check(MPI_Get_address(&foo.base.volstep, &disp[5]));

  types[6] = MPI_DOUBLE;
  blocklen[6] = 1;
  check(MPI_Get_address(&foo.base.volTot, &disp[6]));

  if(my_rank == 0) {
      std::cout << "Base Address " << std::hex << base << "\n";
      std::cout << "Addresses   ";
      for(auto& x : disp) {
        std::cout << " " << std::hex << x;
      }
      std::cout << std::dec << "\n";
  }

  // Addresse zu Offset umrechnen
  for(auto& x : disp) {
    x -= base;
  }

  if(my_rank == 0) {
    std::cout << "Displacement";
    for(auto& x : disp) {
      std::cout << " " << x;
    }
    std::cout << "\n";
  }

  MPI_Datatype newMPItype;
  check(MPI_Type_create_struct(nMembervarsToSend, blocklen.data(), disp.data(), types.data(), &newMPItype));
  check(MPI_Type_commit(&newMPItype));

  return newMPItype;
}

void doRank0(MPI_Datatype newMPItype) {
    type_t sendbuffer;
    strcpy(sendbuffer.ID, "Kreis100");
    sendbuffer.i = 10;
    sendbuffer.x = 1.2f;
    sendbuffer.d = 1.23;
    sendbuffer.l = true;
    sendbuffer.base.volstep = 1.34;
    sendbuffer.base.volTot = 1.56;

    int  displacements[nThreads], counts[nThreads];

    std::vector<type_t> recvbuffer(2);
    std::cout << my_rank << " Receiving...\n";

    int root_rank = 0;
    displacements[0] = 0;
    displacements[1] = 1;
    counts[0] = 1;
    counts[1] = 1;
    // MPI_Gatherv(recvbuffer_send,count_send, datatype_send, recvbuffer_recv, counts_recv, displacements, datatype_recv, root,comm)
    check(MPI_Gatherv(&sendbuffer, 1, newMPItype, recvbuffer.data(), counts, displacements, newMPItype, root_rank, MPI_COMM_WORLD));
    std::cout << my_rank << " Done receiving\n";

    std::cout << my_rank << " content of struct:\n";
    for(const type_t& buf : recvbuffer) {
      std::cout << "ID "  << buf.ID << "\n";
      std::cout << "i "  << buf.i << "\n";
      std::cout << "x "  << buf.x << "\n";
      std::cout << "d "  << buf.d << "\n";
      std::cout << "l "  << buf.l << "\n";
      std::cout << "volstep "  << buf.base.volstep << "\n";
      std::cout << "volTot "  << buf.base.volTot << "\n\n";
    }
}

void doRank1(MPI_Datatype newMPItype) {
    type_t sendbuffer;
    int  displacements[nThreads], counts[nThreads];

    strcpy(sendbuffer.ID, "Kreis200");
    sendbuffer.i = 20;
    sendbuffer.x = 2.2;
    sendbuffer.d = 2.23;
    sendbuffer.l = true;
    sendbuffer.base.volstep = 2.34;
    sendbuffer.base.volTot = 2.56;

    std::cout << my_rank << " Sending...\n";
    // MPI_Gatherv(recvbuffer_send,count_send, datatype_send, recvbuffer_recv, counts_recv, displacements, datatype_recv, root,comm)
    int root_rank = 0;
    check(MPI_Gatherv(&sendbuffer, 1, newMPItype, NULL, counts, displacements, newMPItype, root_rank, MPI_COMM_WORLD));
    std::cout << my_rank << " Done sending\n";
}

int main(int argc, char* argv[]) {
    MPI_Init(&argc, &argv);

    // Get number of processes and check only 2 processes are used
    MPI_Comm_size(MPI_COMM_WORLD, &nThreads);
    if(nThreads != 2) {
        std::cout << "Start with 2 threads.\n";
        MPI_Abort(MPI_COMM_WORLD, EXIT_FAILURE);
    }

    MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);
    MPI_Datatype newMPItype = createMPItyp();

    switch(my_rank) {
      case 0: doRank0(newMPItype); break;
      case 1: doRank1(newMPItype); break;
    };

    MPI_Finalize();

    return EXIT_SUCCESS;
}

$ mpic++ -g -ggdb -Wall test_MPI_struct.cpp

$ mpiexec -n 2 ./a.out
1 Sending...
1 Done sending
Base Address 7ffd83f7e180
Addresses 7ffd83f7e1b4 7ffd83f7e1b8 7ffd83f7e1bc 7ffd83f7e1c0 7ffd83f7e180 7ffd83f7e1c8 7ffd83f7e1d0
Displacement 52 56 60 64 0 72 80
0 Receiving...
0 Done receiving
0 content of struct:
ID Kreis100
i 10
x 1.2
d 1.23
l 1
volstep 1.34
volTot 1.56

ID Kreis200
i 20
x 2.2
d 2.23
l 1
volstep 2.34
volTot 2.56