Re: Trying to call a function pointer causes a compile error with GNU G++

From:

Alex Buell <alex.buell@munted.org.uk>

Newsgroups:

comp.lang.c++

Date:

Fri, 26 Dec 2008 18:06:43 +0000

Message-ID:

<20081226180643.34cf6e9b@lithium.local.net>

On Fri, 26 Dec 2008 16:44:26 +0100, I waved a wand and this message
magically appears in front of Rolf Magnus:

Since the class is only going to be used once, and it's not going
to be copied or assigned, maybe I can explicitly define these with
= 0 to forbid these operations.

If you don't want to support copying the object, simply declare a copy
constructor and assignment operator as private, and don't implement
them. That'll do the trick. If an accidental attempt to copy the
object slips in, you'll get an error from the compiler or from the
linker.

Right, I see.

std::ostream& operator<<(std::ostream& os, const
Command::ELEMENT& rhs) {
os << rhs.command;
return os;
}

Command::Command(std::vector<std::string>& arguments)
{
typedef std::pair<ELEMENT, int> command;
commands = new std::map<ELEMENT, int>;
cmd = commands->end(); // make sure it's set to the end

for (int i = 0; i < COMMANDS; i++)
commands->insert(command(command_table[i], i));

You can use std::make_pair here.

What's the difference between that and what I've used?

You don't need the typedef. You can just write:

commands->insert(std::make_pair(command_table[i], i));

I think that does read a lot better than the other approach.

int Command::execute()
{
if (cmd->first.function != NULL)
return (cmd->first.function)();

cmd is set to end() in the constructor. You must not dereference
this iterator, if it still is end(). Checking the function pointer
for NULL does not help.

Is there an alternative? I use NULL in the struct ELEMENT's command
member to indicate that the command doesn't have a callable
function.

The point is that commands->end() is an iterator to one past the end,
i.e. to a non-existing element. Therefore, cmd->first is invalid in
that case. You also have to check that cmd != commands->end().

OK, all that's needed is the check != commands->end(). Thanks.
--
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Fearsome grindings.