How to throw std::exceptions with variable messages?

Question

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          


        

Answer 1

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.

Answer 2

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!

Answer 3

A really nicer way would be creating a class (or classes) for the exceptions.

Something like:

class ConfigurationError : public std::exception {
public:
    ConfigurationError();
};

class ConfigurationLoadError : public ConfigurationError {
public:
    ConfigurationLoadError(std::string & filename);
};

The reason is that exceptions are much more preferable than just transferring a string. Providing different classes for the errors, you give developers a chance to handle a particular error in a corresponded way (not just display an error message). People catching your exception can be as specific as they need if you use a hierarchy.

a) One may need to know the specific reason

} catch (const ConfigurationLoadError & ex) {
// ...
} catch (const ConfigurationError & ex) {

a) another does not want to know details

} catch (const std::exception & ex) {

You can find some inspiration on this topic in https://books.google.ru/books?id=6tjfmnKhT24C Chapter 9

Also, you can provide a custom message too, but be careful - it is not safe to compose a message with either std::string or std::stringstream or any other way which can cause an exception.

Generally, there is no difference whether you allocate memory (work with strings in C++ manner) in the constructor of the exception or just before throwing - std::bad_alloc exception can be thrown before the one which you really want.

So, a buffer allocated on the stack (like in Maxim's answer) is a safer way.

It is explained very well at http://www.boost.org/community/error_handling.html

So, the nicer way would be a specific type of the exception and be avoiding composing the formatted string (at least when throwing).

Answer 4

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.

Answer 5

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

Answer 6

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);