How to throw std::exceptions with variable messages?

This is an example of what I often do when I want to add some information to an exception:

std::stringstream errMsg;
errMsg << "Could not load config file '" << configfile << "'";
throw std::exception(errMsg.str().c_str());

Is there a nicer way to do it?

The standard exceptions can be constructed from a std::string:

#include <stdexcept>

char const * configfile = "hardcode.cfg";
std::string const anotherfile = get_file();

throw std::runtime_error(std::string("Failed: ") + configfile);
throw std::runtime_error("Error: " + anotherfile);

Note that the base class std::exception can not be constructed thus; you have to use one of the concrete, derived classes.

Here is my solution:

#include <stdexcept>
#include <sstream>

class Formatter
{
public:
    Formatter() {}
    ~Formatter() {}

    template <typename Type>
    Formatter & operator << (const Type & value)
    {
        stream_ << value;
        return *this;
    }

    std::string str() const         { return stream_.str(); }
    operator std::string () const   { return stream_.str(); }

    enum ConvertToString 
    {
        to_str
    };
    std::string operator >> (ConvertToString) { return stream_.str(); }
    
private:
    std::stringstream stream_;

    Formatter(const Formatter &);
    Formatter & operator = (Formatter &);
};

Example:

throw std::runtime_error(Formatter() << foo << 13 << ", bar" << myData);   // implicitly cast to std::string
throw std::runtime_error(Formatter() << foo << 13 << ", bar" << myData >> Formatter::to_str);    // explicitly cast to std::string

There are different exceptions such as runtime_error, range_error, overflow_error, logic_error, etc.. You need to pass the string into its constructor, and you can concatenate whatever you want to your message. That's just a string operation.

std::string errorMessage = std::string("Error: on file ")+fileName;
throw std::runtime_error(errorMessage);

You can also use boost::format like this:

throw std::runtime_error(boost::format("Error processing file %1") % fileName);

The following class might come quite handy:

struct Error : std::exception
{
    char text[1000];

    Error(char const* fmt, ...) __attribute__((format(printf,2,3))) {
        va_list ap;
        va_start(ap, fmt);
        vsnprintf(text, sizeof text, fmt, ap);
        va_end(ap);
    }

    char const* what() const throw() { return text; }
};

Usage example:

throw Error("Could not load config file '%s'", configfile.c_str());

Use string literal operator if C++14 (operator ""s)

using namespace std::string_literals;

throw std::exception("Could not load config file '"s + configfile + "'"s);

or define your own if in C++11. For instance

std::string operator ""_s(const char * str, std::size_t len) {
    return std::string(str, str + len);
}

Your throw statement will then look like this

throw std::exception("Could not load config file '"_s + configfile + "'"_s);

which looks nice and clean.

Maybe this?

throw std::runtime_error(
    (std::ostringstream()
        << "Could not load config file '"
        << configfile
        << "'"
    ).str()
);

It creates a temporary ostringstream, calls the << operators as necessary and then you wrap that in round brackets and call the .str() function on the evaluated result (which is an ostringstream) to pass a temporary std::string to the constructor of runtime_error.

Note: the ostringstream and the string are r-value temporaries and so go out of scope after this line ends. Your exception object's constructor MUST take the input string using either copy or (better) move semantics.

Additional: I don't necessarily consider this approach "best practice", but it does work and can be used at a pinch. One of the biggest issues is that this method requires heap allocations and so the operator << can throw. You probably don't want that happening; however, if your get into that state your probably have way more issues to worry about!

There are two points to answer in regards to what you want:

The first point is that the nicer way is creating special types (classes) for custom exceptions and passing parameters as fields of the classes.

Something like the following:

class BaseFor_Exceptions : public std::exception {
protected:
    BaseFor_Exceptions();
};

class Exception1 : public BaseFor_Exceptions {
public:
    Exception1(uint32_t value1);
private:
	uint32_t value1;
};

throw Exception1(0);

The second point is that you are performing memory allocations when preparing the exception object because of trying to pass a value of variable size (filename).

There is a possibility (when changing objects of the both std::string and std::stringstream ) of std::bad_alloc exception to be thrown in the process of it, so that you fail to prepare or throw (*) your exception – you will lose the info and the state.

In a well-designed program it is easy to avoid memory allocation when preparing or handling an exception. All that you need is just:

• either guarantee the value is still life when handling the exception and pass some kind of link to the value as a part of the exception – either reference or some kind (most likely smart) of pointer,
• or get the value when handling the exception using the exception type info or/and fixed-size values; for example,

} catch (const ConfigurationLoadError & ex) {

	std::cerr
        << “Some message 1 ”
        << serviceLocator1.SomeGetMethod1().Get_ConfigurationFileName();

} catch (const SomeException & ex) {

    std::cerr
        << “Some message 2 ”
        << serviceLocator1.SomeGetMethod2().GetEventDetailsString(ex.Get_Value1());

}

Of course, you always have an option to accept buffer size limitations and use a pre-allocated buffer.

Also, please note that the type (classes) used for exceptions are not permitted to throw exceptions out of their copy constructors since, if the initial exception is attempted to be caught by value, a call of copy constructor is possible (in case is not elided by the compiler) and this additional exception will interrupt the initial exception handling before the initial exception is caught, which causes calling std::terminate. Since C++11 compilers are permitted to eliminate the copying in some cases when catching, but both the elision is not always sensible and, if sensible, it is only permission but not obligation (see https://en.cppreference.com/w/cpp/language/copy_elision for details; before C++11 the standards of the language didn’t regulate the matter).

'*' Also, you should avoid exceptions (will call them the additional) to be thrown out of constructors and move constructors of your types (classes) used for exceptions (will call them initial) since the constructors and move constructors could be called when throwing objects of the types as initial exceptions, then throwing out an additional exception would prevent creation of an initial exception object, and the initial would just be lost. As well as an additional exception from a copy constructor, when throwing an initial one, would cause the same.

Whenever I need a custom message to be thrown in an exception, I construct a C-style string with snprintf() and pass it to the exception constructor.

if (problem_occurred) {
    char buffer[200];
    snprintf(buffer, 200, "Could not load config file %s", configfile);
    string error_mesg(buffer);
    throw std::runtime_error(error_mesg);
}

I'm not sure if the extra string string error_mesg(buffer) is necessary. I reason that the buffer is on stack memory, and if the exception catcher keeps running, then allowing the catcher to keep a reference to a stack-allocated C string is problematic. Instead, passing a string to the exception will invoke copy-by-value, and the buffer array will be deep-copied.

Ran into a similar issue, in that creating custom error messages for my custom exceptions make ugly code. This was my solution:

class MyRunTimeException: public std::runtime_error
{
public:
      MyRunTimeException(const std::string &filename):std::runtime_error(GetMessage(filename)) {}
private:
      static std::string GetMessage(const std::string &filename)
     {
           // Do your message formatting here. 
           // The benefit of returning std::string, is that the compiler will make sure the buffer is good for the length of the constructor call
           // You can use a local std::ostringstream here, and return os.str()
           // Without worrying that the memory is out of scope. It'll get copied
           // You also can create multiple GetMessage functions that take all sorts of objects and add multiple constructors for your exception
     }
}

This separates the logic for creating the messages. I had originally thought about overriding what(), but then you have to capture your message somewhere. std::runtime_error already has an internal buffer.