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

From:

Rolf Magnus <ramagnus@t-online.de>

Newsgroups:

comp.lang.c++

Date:

Fri, 26 Dec 2008 16:44:26 +0100

Message-ID:

<gj2u4h$uoa$02$1@news.t-online.com>

Alex Buell wrote:

#include <iostream>
#include <map>
#include <vector>
#include <string>
#include <cassert>

class Command
{
public:
Command(std::vector<std::string>& arguments);
~Command();

You have a destructor, but no copy constructor and no copy assignment
operator. If you have one, you usually need all three.

Google for "c++ rule of three".

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.

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));

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().