Use typedef within struct for naming and indexing text commands

Question

I am working with a simple command line application that takes in ASCI text and interprets it as a command.

I have attempted to minimize the redundancy in this appli

Answer 1

Question:

Is there any method using the C preprocessor that would allow me to create a macro creates the "command" struct (with string and int members), and auto-numbers the int value (command_idx) as I add more commands via the COMMAND() macro?

Yes, and since you have tagged the question also as C++:

#include <iostream>
#include <map>
#include <string>
using namespace std;

map< string, int >     commands;

bool register_cmd( int id, string const& name )
{
    commands[name] = id;
    return true;
}

#define COMMAND( name ) \
    int const name ## _cmd = __LINE__; \
    bool const name ## _reg = register_cmd( name ## _cmd, #name )

COMMAND( exit );
COMMAND( help );
COMMAND( do_stuff );

int cmd_id( string const& name )
{
    auto const it = commands.find( name );
    return (it == commands.end()? -1 : it->second );
}

int main()
{
    for( auto it = commands.begin();  it != commands.end();  ++it )
    {
        cout << it->first << " => " << it->second << endl;
    }

    cout << "Gimme a command, please: ";
    string cmd;  getline( cin, cmd );
    switch( cmd_id( cmd ) )
    {
    case exit_cmd:
        cout << "You typed an EXIT command, which has id " << exit_cmd << endl;
        break;
    default:
        cout << "Hey, why not try an 'exit' command?" << endl;
    }
}

I just used map instead of fancy new C++11 hash table because map works with older compilers and no real need for shaving nano-seconds here.

Cheers & hth.,

Answer 2

You can use macro redefinition to achieve this. First, you create a file that simply lists your commands called commands.inc:

COMMAND(quit)
COMMAND(help)
...

Then, in your C source you can #include "commands.inc" multiple times, with different definitions of COMMAND() in effect to control how it works. For example:

struct command
{
   char *name;
   int command_idx;
};

#define COMMAND(NAME) CMD_ ## NAME,

enum command_enum {
#include "commands.inc"
};

#undef COMMAND

#define COMMAND(NAME) { #NAME, CMD_ ## NAME },

struct command commands[] =
{
#include "commands.inc"
};

#undef COMMAND

(Note that this particular example relies on a C99 improvement that allows a trailing , at the end of the lists in the enum declaration and compound initialiser - you can easily work around that in C89 by adding a dummy entry at the end).