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  lt  lt   Could not load config file     lt  lt  configfile  lt  lt       throw std  exception errMsg str   c str       Is there a nicer way to do it

User · Answer

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());

User · Answer

The standard exceptions can be constructed from a std  string    include  lt stdexcept gt   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

User · Answer

Maybe this  throw std  runtime error       std  ostringstream            lt  lt   quot Could not load config file   quot           lt  lt  configfile          lt  lt   quot   quot        str       It creates a temporary ostringstream  calls the  lt  lt  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  quot best practice quot   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  lt  lt  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

User · Answer

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  amp  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  amp  ex             catch  const ConfigurationError  amp  ex     a  another does not want to know details   catch  const std  exception  amp  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

User · Answer

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  amp filename  std  runtime error GetMessage filename      private        static std  string GetMessage const std  string  amp 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

User · Answer

Here is my solution    include  lt stdexcept gt   include  lt sstream gt   class Formatter   public      Formatter           Formatter           template  lt typename Type gt      Formatter  amp  operator  lt  lt   const Type  amp  value                stream   lt  lt  value          return  this             std  string str   const           return stream  str          operator std  string    const     return stream  str           enum ConvertToString                to str            std  string operator  gt  gt   ConvertToString    return stream  str       private      std  stringstream stream        Formatter const Formatter  amp        Formatter  amp  operator    Formatter  amp         Example   throw std  runtime error Formatter    lt  lt  foo  lt  lt  13  lt  lt     bar   lt  lt  myData        implicitly cast to std  string throw std  runtime error Formatter    lt  lt  foo  lt  lt  13  lt  lt     bar   lt  lt  myData  gt  gt  Formatter  to str         explicitly cast to std  string

User · Answer

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

User · Answer

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

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