I have tried to create a program in C++ that can catch and handle bad_alloc exception.
While writing the code, I have noticed no errors but when I have tried to compile this code I got C2276 error code and C3876 in Microsoft Visual Studio Community 2017.
The error itself seems to appear in the catch{...} code block.
#include "pch.h"
#include <iostream>
#include <string>
using namespace std;
class CanGoWrong {
public:
CanGoWrong() {
char *pMemory = new char[9999999999];
delete[] pMemory;
}
};
int main()
{
try {
CanGoWrong wrong;
}
catch(std::bad_alloc &e){
cout << "Caught exception: "<< e.what << e << endl;
}
cout << "Still running" << endl;
return 0;
}
The error with code C3867 ("non standart syntax, use &") I fixed this way :
from
cout << "Caught exception: "<< e.what << e << endl;
to:
cout << "Caught exception: "<< &e.what << e << endl;
Still, the error C2276 won't disappear. I guess there is something to do using basic std::exception class.
I want to understand how to do it right, this code is an course sample that I must follow and understand.
Related
I am writing a small application that modifies a text file. It first creates a copy of the file in case something goes wrong.
The following function creates this copy in the same directory. It takes the file's name as an argument and returns true if the copy is successfully created, and false if it fails.
#include <iostream>
#include <filesystem>
#include <fstream>
#include <string>
using std::ifstream;
using std::ofstream;
using std::string;
using std::cerr;
using std::cin;
using std::cout;
using std::endl;
bool backupFile(string FileName) {
cout << "Creating backup for " << FileName << "..." << endl;
try { // for debugging purposes
string NewName = "bkp_" + FileName;
string CurLine;
ifstream FileCopy(FileName);
ofstream FileBackup(NewName);
if (FileCopy.fail()) { // Could specify how file copy failed?
cerr << "Error opening file " << FileName << ".";
return false;
}
while (getline(FileCopy, CurLine)) { // Copy lines to new file
//cout << "Copying " << CurLine << "\" to " << NewName << "." << endl;
FileBackup << CurLine << "\n";
}
cout << "File successfully backed up to " << NewName << endl;
return true;
}
catch (const ifstream::failure& iE) {
cerr << "Exception thrown opening original file: " << iE.what() << endl;
return false;
}
catch (const ofstream::failure& oE) {
cerr << "Exception thrown outputting copy: " << oE.what() << endl;
}
catch (...) {
cerr << "Unknown exception thrown copying file." << endl;
return false;
}
}
I've used a few catch statements to indicate if there is an issue with the input (ifstream::failure), the output (ofstream::failure), or neither.
During compilation, however, the following error appears:
error C2312: 'const std::ios_base::failure &': is caught by 'const std::ios_base::failure &' on line 42
To me, the error implies that both ifstream::failure and ofstream::failure are caught on ifstream::failure, which seems strange. When I remove the catch for ofstream::failure, it runs fine.
Why is this the case?
ifstream::failure and ofstream::failure are both the same type defined in the std::ios_base base class std::ios_base::failure, you can't catch the same type in two separate catch clauses.
Note that neither of your streams will actually throw any exceptions, by default std::fstream doesn't throw any exceptions. You have to turn exceptions on by calling exceptions:
FileCopy.exceptions(f.failbit);
FileBackup.exceptions(f.failbit);
The above will cause an std::ios_base::failure to be thrown when the stream enters the failed state. As you are already checking for FileCopy.fail() you could just expand that checking to cover other failure cases (e.g. check that FileCopy doesn't fail during getline and that FileBackup also doesn't fail) rather than enabling exceptions.
I'm testing protobuf with zlib compression.
I wrote some c++ sample code using protobuf 3.8.0, but the following error occurred at calling ParseFromZeroCopyStream() at Ubuntu.
terminate called after throwing an instance of 'std::system_error'
what(): Unknown error -1
(core dumped)
what can I do?
I tried to replace ParseFromZeroCopyStream() with ParseFromBoundedZeroCopyStream().
That results in no core dump, but ParseFromBoundedZeroCopyStream() returned false.
test.proto
syntax = "proto2";
package test;
message Msg
{
required uint32 data = 1;
}
test.cc
#include <iostream>
#include <google/protobuf/io/zero_copy_stream_impl.h>
#include <google/protobuf/io/zero_copy_stream_impl_lite.h>
#include <google/protobuf/io/gzip_stream.h>
#include "test.pb.h"
using namespace std;
using namespace google::protobuf;
using namespace test;
int main(void)
{
Msg srcMsg;
srcMsg.set_data(1);
long sSize = srcMsg.ByteSizeLong();
cout << "SerializedSize = " << sSize << endl;
char * compressedMsg = new char[sSize];
io::ArrayOutputStream aos(compressedMsg, sSize);
io::GzipOutputStream gos(&aos);
long cSize;
if (srcMsg.SerializeToZeroCopyStream(&gos) == true)
{
gos.Close();
cSize = aos.ByteCount();
cout << "compression success : " << cSize << " bytes" << endl;
}
else
{
cout << "compression error" << endl;
return 1;
}
Msg targetMsg;
io::ArrayInputStream ais(compressedMsg, cSize);
io::GzipInputStream gis(&ais);
if (targetMsg.ParseFromZeroCopyStream(&gis) == false)
{
cout << "decompression error" << endl;
}
else
{
cout << "decompression success : " << targetMsg.ByteSizeLong() << " bytes" << endl;
cout << "data = " << targetMsg.data() << endl;
}
delete[] compressedMsg;
return 0;
}
I expect that decompression succeeds.
You will need to learn to use a debugger to investigate further why exactly this "unknown error: -1" is thrown - if possible.
That being said, unknown library errors is sometimes caused by a failed memory allocation or in rarer cases some other ressource constraint like failing to start a thread/process, etc.
Visual studio is hiding my exception messages. Take the following code example:
#include <stdio.h>
#include <iostream>
#include <exception>
void exceptional_method(){
throw std::runtime_error("Hello from exceptional_method!");
}
int main(){
std::cout << "Hello world!" << std::endl;
exceptional_method();
std::cin.get();
}
Visual studio gives me some vague addresses:
Unhandled exception at 0x76A9DDC2 in ExceptionTest.exe: Microsoft C++ exception: std::runtime_error at memory location 0x006FFD34.
Whereas linux mint gives me the following output on the terminal:
Hello world!
terminate called after throwing an instance of 'std::runtime_error'
what(): Hello from exceptional_method!
Aborted (core dumped)
I've googled a bunch, messed around with the settings in Visual studio, but cannot figure this out. My current workaround is writing the exception message to console before throwing so that I can at least catch the message so I know which exception was thrown.
inline void throw_exception(string& message)
{
cout << message << endl;
throw runtime_error(message);
}
This is not ideal. Any help would be greatly appreciated.
edit:
Getting the debugger to break on the actual exception instead of a few lines ahead was the problem, causing me to investigate the wrong code.
The following solution is what I was looking for.
#ifndef DEBUG_ASSERT_H
#define DEBUG_ASSERT_H
#include <iostream>
#include <string>
using std::cout;
using std::endl;
using std::string;
inline void log_failed_assert(const string message, const string expression, const string file, const long line) {
cout << "Assert failed." << endl;
cout << "Expression: " << expression << endl;
cout << "Message : " << message << endl;
cout << "File : " << file << endl;
cout << "Line : " << line << endl;
}
inline void windows_break()
{
#ifdef _WIN32
__debugbreak();
#endif
}
//do {
//} while (0)
#ifdef _DEBUG
#ifdef _WIN32
#define DEBUG_ASSERT(expr, s) do{\
if(!(expr)){\
log_failed_assert(s, #expr, __FILE__, __LINE__);\
__debugbreak();\
}\
} while(0)
#else
#define DEBUG_ASSERT(expr, s) do{\
if(!(expr)){\
log_failed_assert(s, #expr, __FILE__, __LINE__);\
}\
} while(0)
#endif
#else
#define DEBUG_ASSERT(expr, s)
#endif
#endif
Exceptions are there to be catched. If you dont catch it your program will terminate. If this is what you want, there are easier ways to terminate. If you catch the exception in main you can use the message to eg print it:
#include <exception>
#include <iostream>
void exceptional_method(){
throw std::runtime_error("Hello from exceptional_method!");
}
int main(){
std::cout << "Hello world!" << std::endl;
try {
exceptional_method();
} catch (std::exception& e) {
std::cout << e.what();
}
std::cin.get();
}
As RichardCritten pointed out, it is mint being nice to you rather than visual studio "hiding" the message, as there is no requirement to print the message when your program terminates.
This simple example works as expected in g++, but in MS VS 2010 or 2013 shows a runtime library debug error (Invalid allocation size) before the error is caught (clicking Ignore does then flow through the error handler showing it correctly to be a bad_alloc).
Any ideas about why VS behaves this way?
#include <iostream>
#include <exception>
using namespace std;
int main() {
int x;
cout << "Enter -1 for bad_alloc: ";
cin >> x;
try
{
int* myarray = new int[x];
}
catch (exception& e)
{
cout << "Standard exception: " << e.what() << endl;
}
return 0;
}
That's so that you can analyse the exception at the point it is thrown, before the stack is unwound.
It's a feature that the debugger does this for you.
Nothing to worry about here.
I am currently catching errors from boost::filesystem::is_directory and showing the error to the user by calling "what()" on the exception. This gives the reason for failure but the error is strange to the user. For example:
boost::filesystem::is_directory: Access is denied
How can I catch the boost error and figure out what the actual cause is, so I can show a nicer error message?
By "nicer error message" would you mean something like
#include <iostream>
#include <boost/filesystem.hpp>
int main()
{
boost::filesystem::path p("/proc/1/fd/1");
try {
boost::filesystem::is_directory(p);
} catch(const boost::filesystem::filesystem_error& e)
{
if(e.code() == boost::system::errc::permission_denied)
std::cout << "Search permission is denied for one of the directories "
<< "in the path prefix of " << p << "\n";
else
std::cout << "is_directory(" << p << ") failed with "
<< e.code().message() << '\n';
}
}