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Subject: [std-proposals] Re-purposing the function exception specification
From: andrei_at_[hidden]
Date: 2020-06-20 09:10:53


In some situations I want checked exceptions (ALMOST Java style) to make
sure thay are handled.
I also want all my existing C++ code to keep working without any changes.
I also want the use of "checked"  exceptions to be a choice, not an
enforced practice.

What if we re-purpose the "<function header> throw(list-of-types)"
syntax to mean "these exceptions
are checked FOR THAT FUNCTION", i.e. make the list-of-checked-exceptions
a property of a function
(method, function type) - an exception class by itself is not "checked"
or "unchecked":

============================================================
class base_ex : public std::exception { ... }
class derived_ex : public base_ex { ... }

void foo() throws() { ... }             // no exceptions thrown at all
(== noexcept(true))
void zee() { ... }                      // may throw anything, no
exceptions are "checked"
void bla() throws (base_ex) { ... }     // may throw anything, also
"base_ex" is "checked"
void umf() throws (derived_ex) { ... }  // may throw anything, also
"derived_ex" is "checked"

void (*pfoo1)() throws() = foo;   // OK
void (*pfoo2)() throws() = zee;   // Not OK, "zee" may throw but
"*pfoo2" must not
void (*pfoo3)() throws() = bla;   // Not OK, "bla" may throw but
"*pfoo3" must not
void (*pfoo4)() throws() = umf;   // Not OK, "umf" may throw but
"*pfoo4" must not

void (*pzee1)() = foo;   // OK, "*pzee1" may throw, but "foo" will never
do so
void (*pzee2)() = zee;   // OK
void (*pzee3)() = bla;   // Not OK, "*pzee3" has no "checked"
exceptions, but "bla" has a "checked base_ex"
void (*pzee4)() = umf;   // Not OK, "*pzee4" has no "checked"
exceptions, but "umf" has a "checked derived_ex"

void (*pbla1)() throws(base_ex) = foo;   // OK, "*pbla1" has a "checked
base_ex", but "foo" will never throw it
void (*pbla2)() throws(base_ex) = zee;   // OK, "*pbla2" has a "checked
base_ex", and "zee" may or may not throw it
void (*pbla3)() throws(base_ex) = bla;   // OK
void (*pbla4)() throws(base_ex) = umf;   // OK, "umf" has a "checked
derived_ex" but "*pbla4" has checked "base_ex"

void (*pumf1)() throws(derived_ex) = foo;   // OK, "*pumf1" has a
"checked derived_ex", but "foo" will never throw it
void (*pumf2)() throws(derived_ex) = zee;   // OK, "*pumf2" has a
"checked derivede_ex", and "zee" may or may not throw it
void (*pumf3)() throws(derived_ex) = bla;   // Not OK, "*pumf" has a
"checked derived_ex" but "bla" has checked "base_ex"
void (*pumf4)() throws(derived_ex) = umf;   // OK

void huh1() { bla(); }                    // Not OK, the "checked
base_ex" of "bla" is not handled or propagated
void huh2() throw(base_ex) { umf(); }     // OK, the "checked
derived_ex" of "umf" is propagated out of "huh2"
void huh3() { try { umf() } catch (const base_ex &) {}; } // OK, the
"checked derived_ex" is handled by "catch"
============================================================

The old "pound the function senseless if it throws something non on its
throw(...) list" has been
removed as of C++17 (I think), so there's no harm to any code written in
C++17-or-later.

Any pre-C++11 code that uses the old-style "throw(...)" semantics will
still keep working, because
when e.g. a "g++" is called the C++ version is specified explicitly with
e.g. "-std=c++17".

What do you think?

Thanks in advance,
 Â Â Â  Andrey Kapustin
 Â Â Â  andrei_at_[hidden]


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