Question

There’s probably a Google search that’ll answer this question but for the life of me, I can’t think of one that doesn’t get millions of unrelated answers.

So,

In MSVC text inside quotes “like this” is taken, I think, as a std::string or, maybe, std::string&. In g++/gcc it always seems to be taken as const char*. True?

I found a code snippet I wanted to play with and it contains

if(NULL == key)
   throw exception("Empty key");

compiles just fine in MSVC/VC++(2008( but when I try it on g++ (4.4.3) I get

no matching functions for calls to std::exception::exception(const char&) 

I got this to work:

if (NULL == key)
{
   std::string   estr   ("Empty key");
   throw exception("Empty key");
}

But that’s just plain ugly.

This got me different errors:

std::string   estr   ("");
if (NULL == key)
{
   estr = "Empty key";
   throw exception("Empty key");
}

I have no clue what exception() expects as its input. I did find something that suggested std::string or maybe std::string& but I lost that page and the millions of unhelpful pages I’ve found since are, well, useless. Have all kinds of info on exception class, exception use,….

Short of my ugly fix, is there a simple way to tell g++ that “this is a std::string” not a const char& and still keep VC++ happy? (obviously I’m trying to do cross compilable code from single source.)

And for that matter, how id

   throw exception("Empty key");

different from

   throw "Empty key";

Thanks,

Wes

Answer 1

A string literal always has the type of an array of char, with a size just large enough to contain the characters in the literal with a null terminator. So "Empty key" has the type char[10]. This can be implicitly converted to either char const * or std::string if required.

The error is because exception is intended as a base class for exception types, not something you instantiate directly. You should throw one of the types defined in <stdexcept> such as std::runtime_error (which can be constructed using a string), or define your own type that inherits std::exception and overrides what().

I’m guessing that Microsoft has added a non-standard constructor to std::exception. They like to extend the language in strange ways.