(Very) Basic question re: exception

[Michael Satterwhite]

I haven't programmed C++ in years, so please forgive this very simple
question.

In function A i have:

void A(int l) {
if(l == 0)
throw "This is a simple exception";
...

In function B I have

try {
A(0);
}
catch (char * s) {
<do something>
}
catch (...) {
<do something else>
}

The general exception handler receives control every time (FWIW, I'm using
gcc in Linux). If I remove the general exception handler, the program
aborts.

Question: Why isn't the (char *s) handler being activated. I know I'm
missing the obvious.

Thanks in advance

 

[Claudio Puviani]

I haven't programmed C++ in years, so please forgive this very simple
question.

In function A i have:

void A(int l) {
if(l == 0)
throw "This is a simple exception";
...

In function B I have

try {
A(0);
}
catch (char * s) {
<do something>
}
catch (...) {
<do something else>
}

The general exception handler receives control every time (FWIW, I'm using
gcc in Linux). If I remove the general exception handler, the program
aborts.

Question: Why isn't the (char *s) handler being activated. I know I'm
missing the obvious.

Thanks in advance

Because 'char *' handlers can't catch 'const char *' or 'const char [N]' types.

Claudio Puviani

 

[Mike Wahler]

I haven't programmed C++ in years, so please forgive this very simple
question.

In function A i have:

void A(int l) {
if(l == 0)
throw "This is a simple exception";
...

In function B I have

try {
A(0);
}
catch (char * s) {

catch (const char *s) {

<do something>
}
catch (...) {
<do something else>
}

The general exception handler receives control every time (FWIW, I'm using
gcc in Linux). If I remove the general exception handler, the program
aborts.

Question: Why isn't the (char *s) handler being activated. I know I'm
missing the obvious.

-Mike

 

[Michael Satterwhite]

catch (const char *s) {

AAAAAAAAAAARRRRRRRRRGGGGGGGGHHHHHHHHH

Bad case of tunnel vision. The subtle I usually see, the obvious, never.

Thanks much!

 

[John Carson]

AAAAAAAAAAARRRRRRRRRGGGGGGGGHHHHHHHHH

Bad case of tunnel vision. The subtle I usually see, the obvious,
never.

Thanks much!

I don't think it is obvious at all. Given a function taking a pointer to
const char, you are allowed to pass it a pointer to non-const char. It is
not "obvious" why it should be different with a catch.

 

[Michiel Salters]

[SNIP]

Given a function taking a pointer to
const char, you are allowed to pass it a pointer to non-const char. It is
not "obvious" why it should be different with a catch.

You're right, and it isn't different.
Nor is there a difference for the reverse conversion: You can't pass
a char const* to a function taking char*, nor can you pass a char const*
exception to a handler catching char*.

E.g.

try { foo() }
catch( char* e ) { if(e) e[0]=\0; }

should be legal, as e is writeable. If foo throws "X", the handler
should be skipped as it would modify the "X" literal.

Regards,
Michiel Salters

 

[John Carson]

[SNIP]

Given a function taking a pointer to
const char, you are allowed to pass it a pointer to non-const char.
It is not "obvious" why it should be different with a catch.

You're right, and it isn't different.
Nor is there a difference for the reverse conversion: You can't pass
a char const* to a function taking char*, nor can you pass a char
const*
exception to a handler catching char*.

E.g.

try { foo() }
catch( char* e ) { if(e) e[0]=\0; }

should be legal, as e is writeable. If foo throws "X", the handler
should be skipped as it would modify the "X" literal.

Regards,
Michiel Salters

Thanks. I clearly got confused about which direction of conversion we were
discussing.

Starting the discussion again from scratch, it does appear that the catch
and function argument treatments of string literals are not the same. The
following compiles on Comeau online:

#include <iostream>

void print(char *ptr)
{
std::cout << ptr << '\n';
}

int main ()
{
print("String Literal");
return 0;
}

Thus a string literal is accepted as a char *. (It isn't if it is assigned
to a pointer to const char and you try to use that pointer as the function
argument.)

Accordingly, it seems that the rule that string literals are pointers to
const char is strictly enforced for catches but not for function arguments
(presumably because of a C-compatibility issue with functions that is not
relevant for catches).