Call function with parameters extracted from string - c++

Question

I'm looking at the following problem:
I get strings that are formatted like this:
functionname_parameter1_parameter2_parameter3
otherfunctionname_parameter1_parameter2
.
.
.
and i would like to call the function with the given parameters.
So let's say i have a function test:
void test(int x, float y, std::string z) {}
and i get a message:
test_5_2.0_abc
then i would like the function test to be automatically invoked like this:
test(5, 2.0, "abc");
Do you have any hints on how to accomplish this in C++?

Answer 1

Update: Updated stream_function to fix the argument-evaluation-order problem #Nawaz mentioned in the comments, and also removed the std::function for improved efficiency. Note that the evaluation-order fix only works for Clang, as GCC doesn't follow the standard here. An example for GCC, with manual order-enforcement, can be found here.
This is generally not that easy to accomplish. I wrote a little wrapper class around std::function once that extracts the arguments from a std::istream. Here's an example using C++11:
#include <map>
#include <string>
#include <iostream>
#include <sstream>
#include <functional>
#include <stdexcept>
#include <type_traits>
// for proper evaluation of the stream extraction to the arguments
template<class R>
struct invoker{
R result;
template<class F, class... Args>
invoker(F&& f, Args&&... args)
: result(f(std::forward<Args>(args)...)) {}
};
template<>
struct invoker<void>{
template<class F, class... Args>
invoker(F&& f, Args&&... args)
{ f(std::forward<Args>(args)...); }
};
template<class F, class Sig>
struct stream_function_;
template<class F, class R, class... Args>
struct stream_function_<F, R(Args...)>{
stream_function_(F f)
: _f(f) {}
void operator()(std::istream& args, std::string* out_opt) const{
call(args, out_opt, std::is_void<R>());
}
private:
template<class T>
static T get(std::istream& args){
T t; // must be default constructible
if(!(args >> t)){
args.clear();
throw std::invalid_argument("invalid argument to stream_function");
}
return t;
}
// void return
void call(std::istream& args, std::string*, std::true_type) const{
invoker<void>{_f, get<Args>(args)...};
}
// non-void return
void call(std::istream& args, std::string* out_opt, std::false_type) const{
if(!out_opt) // no return wanted, redirect
return call(args, nullptr, std::true_type());
std::stringstream conv;
if(!(conv << invoker<R>{_f, get<Args>(args)...}.result))
throw std::runtime_error("bad return in stream_function");
*out_opt = conv.str();
}
F _f;
};
template<class Sig, class F>
stream_function_<F, Sig> stream_function(F f){ return {f}; }
typedef std::function<void(std::istream&, std::string*)> func_type;
typedef std::map<std::string, func_type> dict_type;
void print(){
std::cout << "print()\n";
}
int add(int a, int b){
return a + b;
}
int sub(int a, int b){
return a - b;
}
int main(){
dict_type func_dict;
func_dict["print"] = stream_function<void()>(print);
func_dict["add"] = stream_function<int(int,int)>(add);
func_dict["sub"] = stream_function<int(int,int)>(sub);
for(;;){
std::cout << "Which function should be called?\n";
std::string tmp;
std::cin >> tmp;
auto it = func_dict.find(tmp);
if(it == func_dict.end()){
std::cout << "Invalid function '" << tmp << "'\n";
continue;
}
tmp.clear();
try{
it->second(std::cin, &tmp);
}catch(std::exception const& e){
std::cout << "Error: '" << e.what() << "'\n";
std::cin.ignore();
continue;
}
std::cout << "Result: " << (tmp.empty()? "none" : tmp) << '\n';
}
}
Compiles under Clang 3.3 and works as expected (small live example).
Which function should be called?
a
Invalid function 'a'
Which function should be called?
add
2
d
Error: 'invalid argument to stream_function'
Which function should be called?
add
2
3
Result: 5
Which function should be called?
add 2 6
Result: 8
Which function should be called?
add 2
6
Result: 8
Which function should be called?
sub 8 2
Result: 6
It was fun to hack that class together again, hope you enjoy. Note that you need to modify the code a little to work for your example, since C++ IOstreams have whitespace as delimiter, so you'd need to replace all underscores in your message with spaces. Should be easy to do though, after that just construct a std::istringstream from your message:
std::istringstream input(message_without_underscores);
// call and pass 'input'

Answer 2

You pretty much can't, C++ doesn't have any kind of reflection on functions.
The question then is how close you can get. An interface like this is pretty plausible, if it would suit:
string message = "test_5_2.0_abc";
string function_name = up_to_first_underscore(message);
registered_functions[function_name](message);
Where registered_functions is a map<string,std::function<void,string>>, and you have to explicitly do something like:
registered_functions["test"] = make_registration(test);
for each function that can be called in this way.
make_registration would then be a fairly hairy template function that takes a function pointer as a parameter and returns a std::function object that when called splits the string into chunks, checks that there are the right number there, converts each one to the correct parameter type with a boost::lexical_cast, and finally calls the specified function. It would know the "correct type" from the template argument to make_registration -- to accept arbitrarily many parameters this would have to be a C++11 variadic template, but you can fake it with:
std::function<void,string> make_registration(void(*fn)(void));
template <typename T>
std::function<void,string> make_registration(void(*fn)(T));
template <typename T, U>
std::function<void,string> make_registration(void(*fn)(T, U));
// etc...
Dealing with overloads and optional parameters would add further complication.
Although I don't know anything about them, I expect that there are C++ support frameworks out there for SOAP or other RPC protocols, that might contain some relevant code.

Answer 3

What you are looking for is reflection. And it is not possible in C++. C++ is designed with speed in mind. If you require inspection of a library or code and then identify the types in it and invoke methods associated with those types (usually classes) then I am afraid it is not possible in C++.
For further reference you can refer to this thread.
How can I add reflection to a C++ application?
http://en.wikibooks.org/wiki/C%2B%2B_Programming/RTTI
Why does C++ not have reflection?

Answer 4

You could parse the string, separate the arguments and send them to the function with no problem, but what you cannot do is reference the function with its name on a string, because the function doesn't have a name anymore at runtime.
You could have a if-else if chain that checks for the function name, and then parse the arguments and call the specific function.

Answer 5

I modified #Xeo's code to work with gcc properly, so it ensures the parameters are pulled in the right order. I'm only posting this since it took me a while to understand the original code and splice in the order-enforcement. Full credit should still go to #Xeo. If I find anything wrong with my implementation I'll come back and edit, but thus far in my testing I haven't seen any problems.
#include <map>
#include <string>
#include <iostream>
#include <sstream>
#include <functional>
#include <stdexcept>
#include <type_traits>
#include <tuple>
template<class...> struct types{};
// for proper evaluation of the stream extraction to the arguments
template<class ReturnType>
struct invoker {
ReturnType result;
template<class Function, class... Args>
invoker(Function&& f, Args&&... args) {
result = f(std::forward<Args>(args)...);
}
};
template<>
struct invoker<void> {
template<class Function, class... Args>
invoker(Function&& f, Args&&... args) {
f(std::forward<Args>(args)...);
}
};
template<class Function, class Sig>
struct StreamFunction;
template<class Function, class ReturnType, class... Args>
struct StreamFunction<Function, ReturnType(Args...)>
{
StreamFunction(Function f)
: _f(f) {}
void operator()(std::istream& args, std::string* out_opt) const
{
call(args, out_opt, std::is_void<ReturnType>());
}
private:
template<class T>
static T get(std::istream& args)
{
T t; // must be default constructible
if(!(args >> t))
{
args.clear();
throw std::invalid_argument("invalid argument to stream_function");
}
return t;
}
//must be mutable due to const of the class
mutable std::istream* _args;
// void return
void call(std::istream& args, std::string*, std::true_type) const
{
_args = &args;
_voidcall(types<Args...>{});
}
template<class Head, class... Tail, class... Collected>
void _voidcall(types<Head, Tail...>, Collected... c) const
{
_voidcall<Tail...>(types<Tail...>{}, c..., get<Head>(*_args));
}
template<class... Collected>
void _voidcall(types<>, Collected... c) const
{
invoker<void> {_f, c...};
}
// non-void return
void call(std::istream& args, std::string* out_opt, std::false_type) const {
if(!out_opt) // no return wanted, redirect
return call(args, nullptr, std::true_type());
_args = &args;
std::stringstream conv;
if(!(conv << _call(types<Args...>{})))
throw std::runtime_error("bad return in stream_function");
*out_opt = conv.str();
}
template<class Head, class... Tail, class... Collected>
ReturnType _call(types<Head, Tail...>, Collected... c) const
{
return _call<Tail...>(types<Tail...>{}, c..., get<Head>(*_args));
}
template<class... Collected>
ReturnType _call(types<>, Collected... c) const
{
return invoker<ReturnType> {_f, c...} .result;
}
Function _f;
};
template<class Sig, class Function>
StreamFunction<Function, Sig> CreateStreamFunction(Function f)
{
return {f};
}
typedef std::function<void(std::istream&, std::string*)> StreamFunctionCallType;
typedef std::map<std::string, StreamFunctionCallType> StreamFunctionDictionary;
This also works with Visual Studio 2013, have not tried earlier versions.