What type should I catch if I throw a string literal?

Question

I am writing a pretty simple application in C++ using g++ under Linux and I am trying to throw some raw strings as exceptions (yes, I know, its not a good practise).

I have the following code (simplified):

int main()
{
  try
  {
    throw "not implemented";

  }
  catch(std::string &error)
  {
    cerr<<"Error: "<<error<<endl;
  }
  catch(char* error)
  {
    cerr<<"Error: "<<error<<endl;
  }
  catch(...)
  {
    cerr<<"Unknown error"<<endl;
  }
}

And I get Unknow error on the console. But if I static cast the literal string to either std::string or char * it prints Error: not implemented as expected. My question is: so what is the type I should catch if I don't want to use static casts?

Stupid question: does anyone throw things that do not inherit from `std::exception` in real code ? (Apart from the `SuicideException`, and no, it's not derision :p) — Matthieu M., Jan 28 '11 at 19:02

Johannes Schaub - litb · Accepted Answer · 2011-01-28T18:17:38.053

You need to catch it with char const* instead of char*. Neither anything like std::string nor char* will catch it.

Catching has restricted rules with regard to what types it match. The spec says (where "cv" means "const/volatile combination" or neither of them).

A handler is a match for an exception object of type E if

The handler is of type cv T or cv T& and E and T are the same type (ignoring the top-level cv-qualifiers), or

the handler is of type cv T or cv T& and T is an unambiguous public base class of E, or

the handler is of type cv1 T* cv2 and E is a pointer type that can be converted to the type of the handler by either or both of

a standard pointer conversion (4.10) not involving conversions to pointers to private or protected or ambiguous classes

a qualification conversion

A string literal has type char const[N], but throwing an array will decay the array and actually throws a pointer to its first element. So you cannot catch a thrown string literal by a char*, because at the time it matches, it needs to match the char* to a char const*, which would throw away a const (a qualification conversion is only allowed to add const). The special conversion of a string literal to char* is only considered when you need to convert a string literal specifically.

luke · Answer 2 · 2011-01-28T18:14:06.787

10

Try adding const to the types you're catching, const char* (possibly const char* const).

edited Jan 28 '11 at 18:14

answered Jan 28 '11 at 18:06

luke

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Or my personal fav: `char const*` – Martin York Jan 28 '11 at 18:25
@Martin: English speakers seem to have some difficulty with the `const` in suffix position for some reason :) – Matthieu M. Jan 28 '11 at 19:00

score 4 · Answer 3 · edited Sep 20 '18 at 13:16

4

The exact type of a string literal is an array of const characters (const char [15] for your example, since the NUL terminator is included). The array decays to const char* when thrown, which is independent of the length.

edited Sep 20 '18 at 13:16

JFMR

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answered Jan 28 '11 at 18:07

Ben Voigt

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score 2 · Answer 4 · answered Jan 28 '11 at 18:11

The type should be const char[15] or const char*.

However, while the language does not forbids you throwing any type value, you should not be raising native data types as exception. Instead, you want to raise an std::exception() instance, or creating your own exception class.

score 2 · Answer 5 · answered Jan 28 '11 at 18:14

The type of a string literal is char const *. There's a (deprecated) conversion to char * provided for backward compatibility with existing code (but you still have to treat it as const -- any attempt at modification gives UB).

As such, code like this should work:

#include <iostream>
using namespace std;

int main()
{
  try
  {
    throw "not implemented";

  }
  catch(char const *error)
  {
    cerr<<"Error: "<<error<<endl;
  }
  return 0;
}

score 1 · Answer 6 · answered Jan 28 '11 at 18:12

1

The problem is that you're trying to catch something that is a const. The following will work:

catch(const char* error) { cerr

answered Jan 28 '11 at 18:12

Wes Hardaker

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score 1 · Answer 7 · answered Jan 28 '11 at 18:19

Check out the section 2.14.5 of the standard specification, it treats types and kinds of string literals on 3 pages. Don't do what you started to do, just say:

throw std::exception("not implemented");

along with proper

catch (std::exception& pEx)

Is there something wrong with this "normal" approach...?

What type should I catch if I throw a string literal?

7 Answers7

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