I currently read the book Effective C++ from Scott Meyers. It says I should prefer inline
functions over #define
for function-like macros.
Now I try to code an inline function to replace my exception macro. My old macro looks like this:
#define __EXCEPTION(aMessage) \
{ \
std::ostringstream stream; \
stream << "EXCEPTION: " << aMessage << ", file " <<__FILE__ << " line " << __LINE__; \
throw ExceptionImpl(stream.str()); \
}
My new inline function is this:
inline void __EXCEPTION(const std::string aMessage)
{
std::ostringstream stream;
stream << "EXCEPTION: " << aMessage << ", file " <<__FILE__ << " line " << __LINE__;
throw ExceptionImpl(stream.str());
}
As probably some people already expect, now the __FILE__
and __LINE__
macros are useless, because they refer always to the C++-file with the definition of the inline function.
Is there any way to circumvent this behaviour or should I stick with my old macro? I read this threads here, and I already suspect that there is probably no way of my second example to work fine:
Don't use __
(double underscore) as it's reserved. Having an inline
function is better.
However, here you need a mix of macro and the function, hence you can do following:
#define MY_EXCEPTION(aMessage) MyException(aMessage, __FILE__, __LINE__)
inline void MyException(const std::string aMessage,
const char* fileName,
const std::size_t lineNumber)
{
std::ostringstream stream;
stream << "EXCEPTION: " << aMessage << ", file " << fileName << " line " << lineNumber;
throw ExceptionImpl(stream.str());
}
Please consider that there is another difference between using the #define
function-like macro in your case in comparison to inline
functions. You could have used streaming operators and parameters in your macro's invocation to be composed as your message's text:
__EXCEPTION( "My message with a value " << val )
But most times I've needed something like this, it was to check on a certain condition (like an assertion). So you could extend @iammilind's example with something like:
#define MY_EXCEPTION_COND( cond ) \
if (bool(cond) == false) \
{ \
std::string _s( #cond " == false" ); \
MyException(_s, __FILE__, __LINE__); \
}
Or something a little more specialized where the values are also printed:
template <typename T>
inline void MyExceptionValueCompare(const T& a,
const T& b,
const char* fileName,
const std::size_t lineNumber)
{
if (a != b)
{
std::ostringstream stream;
stream << "EXCEPTION: " << a << " != " << b << ", file " << fileName << " line " << lineNumber;
throw ExceptionImpl(stream.str());
}
}
#define MY_EXCEPTION_COMP( a, b ) MyExceptionValueCompare(a, b, __FILE__, __LINE__)
Another approach you may want to take a look at is Microsoft's usage of their __LineInfo
class in the Microsoft::VisualStudio::CppUnitTestFramework
namespace (VC\\UnitTest\\Include\\CppUnitTestAssert.h). See https://msdn.microsoft.com/en-us/library/hh694604.aspx
I see this is an old question but I think that the approach of printing the line in the exception macro is fundamentally flawed and I think I have a better alternative. I assume that the macro is used similar to the following code:
try {
/// code
throw;
}
catch (...) { __EXCEPTION(aMessage); }
With this approach the macro prints the location where the exception was catch'ed
. But for troubleshooting and debugging the location where it was throw'n
is usually more useful.
To get that information, we can attach the __FILE__
and __LINE__
macros to the exception. However, we still can't get completely rid of macros, but we get at least the exact throw location:
#include <iostream>
#include <exception>
#include <string>
#define MY_THROW(msg) throw my_error(__FILE__, __LINE__, msg)
struct my_error : std::exception
{
my_error(const std::string & f, int l, const std::string & m)
: file(f)
, line(l)
, message(m)
{}
std::string file;
int line;
std::string message;
char const * what() const throw() { return message.c_str(); }
};
void my_exceptionhandler()
{
try {
throw; // re-throw the exception and capture the correct type
}
catch (my_error & e)
{
std::cout << "Exception: " << e.what() << " in line: " << e.line << std::endl;
}
}
int main()
{
try {
MY_THROW("error1");
} catch(...) { my_exceptionhandler(); }
}
There is one additional improvement possible if we are willing to use boost::exception
: We can get rid of macro definitons at least in our own code. The whole program gets shorter and the locations of code execution and error handling can be nicely separated:
#include <iostream>
#include <boost/exception/all.hpp>
typedef boost::error_info<struct tag_error_msg, std::string> error_message;
struct error : virtual std::exception, virtual boost::exception { };
struct my_error: virtual error { };
void my_exceptionhandler()
{
using boost::get_error_info;
try {
throw;
}
catch(boost::exception & e)
{
char const * const * file = get_error_info<boost::throw_file>(e);
int const * line = get_error_info<boost::throw_line>(e);
char const * const * throw_func = get_error_info<boost::throw_function>(e);
std::cout << diagnostic_information(e, false)
<< " in File: " << *file << "(" << *line << ")"
" in Function: " << *throw_func;
}
}
int main()
{
try {
BOOST_THROW_EXCEPTION(my_error() << error_message("Test error"));
} catch(...) { my_exceptionhandler(); }
}
With std::experimental::source_location , you might do:
#include <experimental/source_location>
void THROW_EX(const std::string_view& message,
const std::experimental::source_location& location
= std::experimental::source_location::current())
{
std::ostringstream stream;
stream << "EXCEPTION: " << message
<< ", file " << location.file_name()
<< " line " << location.line();
throw ExceptionImpl(stream.str());
}
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