C++, efficient way to call many possible functions from user - c++

Question

I'm relatively new to c++, mostly worked with python.
I have a scenario where a user(me) uses a GUI to send commands to a microcontroller via serial, and then the microcontroller processes them.
Right now i have 10 commands, but as the project develops (some form of modular robot) I can envision having 50-100 possible commands.
Is there a better way for my c++ handleCommands function to select which one of the possible 100 functions to run without doing massive case switches or if else statements?
Extract of the code:
char cmd = 1; // example place holder
int value = 10; //example place holder
switch (cmd){
case '1':
toggleBlink(value);
break;
case '2':
getID(value); // in this case value gets ignored by the function as its not required
break;
This works fine for 3-4 functions but doesn't seem to me like the best way to do it for more functions.
I've heard of lookup tables but as each function is different and may require arguments or not I'm consumed on how to implement them.
Some background on the set-up:
The commands are mainly diagnostic ,< ID > ect and a couple of functional ones that require parameters like, <blink,10> <runto,90> <set-mode,locked>
The validation is done in python against a csv file and the actual serial message sent to the microcontroller is sent as <(index of comand in csvfile),parameter> with < > and , being delimiters.
So the user would type blink,10 and the python app will send <1,10> over serial as blink is found at index 1 of the csv file.
The microcontroller reads these in and i am left over with 2 char arrays, the command array containing a number, and the value array containing the value sent.(also a number)
As I'm running this on a microcontroller i don't really want to have to store a long file of possible commands in flash, hence the validation done on the python gui side.
Note that in the case of a possible multi argument function, say <move,90,30> i.e move 90 degrees in 30 seconds eat, the actual function would only receive one argument "30,90" and then split that up as needed.

Answer 1

If you have the commands comming over the serial line in the format
<command-mapped-to-a-number,...comma-separated-parameters...>
we can simulate that like so:
#include <iostream>
#include <sstream> // needed for simple parsing
#include <string>
#include <unordered_map> // needed for mapping of commands to functors
int main() {
std::cout << std::boolalpha;
// example commands lines read from serial:
for (auto& cmdline : {"<1,10>", "<2,10,90>", "<3,locked>", "<4>"}) {
std::cout << exec(cmdline) << '\n';
}
}
exec above is the interpreter that will return true if the command line was parsed and executed ok. In the examples above, command 1 takes one parameter, 2 takes two, 3 takes one (string) and 4 doesn't have a parameter.
The mapping from command-mapped-to-a-number could be an enum:
// uint8_t has room for 256 commands, make it uint16_t to get room for 65536 commands
enum class command_t : uint8_t {
blink = 1,
take_two = 2,
set_mode = 3,
no_param = 4,
};
and exec would make the most basic validation of the command line (checking < and >) and put it in a std::istringstream for easy extraction of the information on this command line:
bool exec(const std::string& cmdline) {
if(cmdline.size() < 2 || cmdline.front() != '<' || cmdline.back() != '>' )
return false;
// put all but `<` and `>` in an istringstream:
std::istringstream is(cmdline.substr(1,cmdline.size()-2));
// extract the command number
if (int cmd; is >> cmd) {
// look-up the command number in an `unordered_map` that is mapped to a functor
// that takes a reference to an `istringstream` as an argument:
if (auto cit = commands.find(command_t(cmd)); cit != commands.end()) {
// call the correct functor with the rest of the command line
// so that it can extract, validate and use the arguments:
return cit->second(is);
}
return false; // command look-up failed
}
return false; // command number extraction failed
}
The only tricky part left is the unordered_map of commands and functors.
Here's a start:
// a helper to eat commas from the command line
struct comma_eater {} comma;
std::istream& operator>>(std::istream& is, const comma_eater&) {
// next character must be a comma or else the istream's failbit is set
if(is.peek() == ',') is.ignore();
else is.setstate(std::ios::failbit);
return is;
}
std::unordered_map<command_t, bool (*)(std::istringstream&)> commands{
{command_t::blink,
[](std::istringstream& is) {
if (int i; is >> comma >> i && is.eof()) {
std::cout << "<blink," << i << "> ";
return true;
}
return false;
}},
{command_t::take_two,
[](std::istringstream& is) {
if (int a, b; is >> comma >> a >> comma >> b && is.eof()) {
std::cout << "<take-two," << a << ',' << b << "> ";
return true;
}
return false;
}},
{command_t::set_mode,
[](std::istringstream& is) {
if (std::string mode; is >> comma && std::getline(is, mode,',') && is.eof()) {
std::cout << "<set-mode," << mode << "> ";
return true;
}
return false;
}},
{command_t::no_param,
[](std::istringstream& is) {
if (is.eof()) {
std::cout << "<no-param> ";
return true;
}
return false;
}},
};
If you put that together you'll get the below output from the successful parsing (and execution) of all command lines received:
<blink,10> true
<take-two,10,90> true
<set-mode,locked> true
<no-param> true
Here's a live demo.

Answer 2

Given an integer index for each "command" a simple function pointer look-up table can be used. For example:
#include <cstdio>
namespace
{
// Command functions (dummy examples)
int examleCmdFunctionNoArgs() ;
int examleCmdFunction1Arg( int arg1 ) ;
int examleCmdFunction2Args( int arg1, int arg2 ) ;
int examleCmdFunction3Args( int arg1, int arg2, arg3 ) ;
int examleCmdFunction4Args( int arg1, int arg2, int arg3, int arg4 ) ;
const int MAX_ARGS = 4 ;
const int MAX_CMD_LEN = 32 ;
typedef int (*tCmdFn)( int, int, int, int ) ;
// Symbol table
#define CMD( f ) reinterpret_cast<tCmdFn>(f)
static const tCmdFn cmd_lookup[] =
{
0, // Invalid command
CMD( examleCmdFunctionNoArgs ),
CMD( examleCmdFunction1Arg ),
CMD( examleCmdFunction2Args ),
CMD( examleCmdFunction3Args ),
CMD( examleCmdFunction4Args )
} ;
}
namespace cmd
{
// For commands of the form: "<cmd_index[,arg1[,arg2[,arg3[,arg4]]]]>"
// i.e an angle bracketed comma-delimited sequence commprising a command
// index followed by zero or morearguments.
// e.g.: "<1,123,456,0>"
int execute( const char* command )
{
int ret = 0 ;
int argv[MAX_ARGS] = {0} ;
int cmd_index = 0 ;
int tokens = std::sscanf( "<%d,%d,%d,%d,%d>", command, &cmd_index, &argv[0], &argv[1], &argv[2], &argv[3] ) ;
if( tokens > 0 && cmd_index < sizeof(cmd_lookup) / sizeof(*cmd_lookup) )
{
if( cmd_index > 0 )
{
ret = cmd_lookup[cmd_index]( argv[0], argv[1], argv[2], argv[3] ) ;
}
}
return ret ;
}
}
The command execution passes four arguments (you can expand that as necessary) but for command functions taking fewer arguments they will simply be "dummy" arguments that will be ignored.
Your proposed translation to an index is somewhat error prone and maintenance heavy since it requires you to maintain both the PC application symbol table and the embedded look up table in sync. It may not be prohibitive to have the symbol table on the embedded target; for example:
#include <cstdio>
#include <cstring>
namespace
{
// Command functions (dummy examples)
int examleCmdFunctionNoArgs() ;
int examleCmdFunction1Arg( int arg1 ) ;
int examleCmdFunction2Args( int arg1, int arg2 ) ;
int examleCmdFunction3Args( int arg1, int arg2, arg3 ) ;
int examleCmdFunction4Args( int arg1, int arg2, int arg3, int arg4 ) ;
const int MAX_ARGS = 4 ;
const int MAX_CMD_LEN = 32 ;
typedef int (*tCmdFn)( int, int, int, int ) ;
// Symbol table
#define SYM( c, f ) {#c, reinterpret_cast<tCmdFn>(f)}
static const struct
{
const char* symbol ;
const tCmdFn command ;
} symbol_table[] =
{
SYM( cmd0, examleCmdFunctionNoArgs ),
SYM( cmd1, examleCmdFunction1Arg ),
SYM( cmd2, examleCmdFunction2Args ),
SYM( cmd3, examleCmdFunction3Args ),
SYM( cmd4, examleCmdFunction4Args )
} ;
}
namespace cmd
{
// For commands of the form: "cmd[ arg1[, arg2[, arg3[, arg4]]]]"
// i.e a command string followed by zero or more comma-delimited arguments
// e.g.: "cmd3 123, 456, 0"
int execute( const char* command_line )
{
int ret = 0 ;
int argv[MAX_ARGS] = {0} ;
char cmd[MAX_CMD_LEN + 1] ;
int tokens = std::sscanf( "%s %d,%d,%d,%d", command_line, cmd, &argv[0], &argv[1], &argv[2], &argv[3] ) ;
if( tokens > 0 )
{
bool cmd_found = false ;
for( int i = 0;
!cmd_found && i < sizeof(symbol_table) / sizeof(*symbol_table);
i++ )
{
cmd_found = std::strcmp( cmd, symbol_table[i].symbol ) == 0 ;
if( cmd_found )
{
ret = symbol_table[i].command( argv[0], argv[1], argv[2], argv[3] ) ;
}
}
}
return ret ;
}
}
For very large symbol tables you might want a more sophisticated look-up, but depending on the required performance and determinism, the simple exhaustive search will be sufficient - far faster than the time taken to send the serial data.
Whilst the resource requirement for the symbol table is somewhat higher that the indexed look-up, it is nonetheless ROM-able and will can be be located in Flash memory which on most MCUs is a less scarce resource than SRAM. Being static const the linker/compiler will most likely place the tables in ROM without any specific directive - though you should check the link map or the toolchain documentation n this.
In both cases I have defined the command functions and executer as returning an int. That is optional of course, but you might use that for returning responses to the PC issuing the serial command.

Answer 3

What you are talking about are remote procedure calls. So you need to have some mechanism to serialize and un-serialize the calls.
As mentioned in the comments you can make a map from cmd to the function implementing the command. Or simply an array. But the problem remains that different functions will want different arguments.
So my suggestion would be to add a wrapper function using vardiac templates.
Prefix every command with the length of data for the command so the receiver can read a block of data for the command and knows when to dispatch it to a function. The wrapper then takes the block of data, splits it into the right size for each argument and converts it and then calls the read function.
Now you can make a map or array of those wrapper function, each one bound to one command and the compiler will generate the un-serialize code for you from the types. (You still have to do it once for each type, the compiler only combines those for the full function call).