Automatic collection of std :: map with function pointers

I am developing a script language in C ++ that uses functions that are "built" into the interpreter. I use the following construct to map function names to their corresponding pointers:

typedef void(*BuiltInFunction)(Context*);
typedef std::unordered_map<std::string, BuiltInFunction> BuiltinFunctionsMap;

      

where Context

is the custom class.

Then I have function declarations:

namespace BuiltIns {
    void _f_print(Context* context);
    void _f_error(Context* context);
    void _f_readline(Context* context);
    void _f_read(Context* context);
    void _f_readchar(Context* context);
    void _f_eof(Context* context);
    ...
}

      

And finally, a procedure that populates the map with actual pointers:

BuiltinFunctionsMap BuiltIns::populateFunctions() {
    BuiltinFunctionsMap funcMap;
    // Standard I/0
    funcMap["print"] = &BuiltIns::_f_print;
    funcMap["error"] = &BuiltIns::_f_error;
    funcMap["readline"] = &BuiltIns::_f_readline;
    funcMap["read"] = &BuiltIns::_f_read;
    funcMap["readchar"] = &BuiltIns::_f_readchar;
    funcMap["eof"] = &BuiltIns::_f_eof;
    ...
    return funcMap;
}

      

I am asking if there is a way to automatically generate a population function from a function declaration using templates or something similar. I currently use regex, which is simple enough, but I have to do it every time I add new features, and it's cumbersome.

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2 answers


I don't know if this is really a useful answer, but you can use a preprocessor to do some pretty protein stuff :

#include <iostream>
#include <map>
#include <string>

class Context {}; 
typedef void (*BuiltInFunction)(Context*);

// a list of your function names
#define FN_NAMES \
X(foo)  \
X(bar)  \
X(baz)

// this declares your functions (you also have to define them 
// somewhere else, e.g. below or in another file)
#define X(a) void _f_ ## a ## _print(Context *context);
namespace BuiltIns {
  FN_NAMES
}
#undef X

// a global map initialized (using C++11 initializer syntax)
// to strings mapped to functions
#define X(a) {#a, &BuiltIns::_f_ ## a ## _print},
std::map<std::string, BuiltInFunction> g_fns = { 
  FN_NAMES
};
#undef X

int main() {
  g_fns["foo"](NULL);  // calls BuiltIns::_f_foo_print(NULL) 
}

// (these have to be defined somewhere)
namespace BuiltIns {
  void _f_foo_print(Context *context) {
    std::cout << "foo\n";
  }
  void _f_bar_print(Context *context) {
    std::cout << "bar\n";
  }
  void _f_baz_print(Context *context) {
    std::cout << "baz\n";
  }
}

      



This approach has the advantage of automatically generating, for example, a string "foo"

and binding it to a function _f_foo_print

. The downsides are horrible preprocessing trickery and the fact that you still have to deal with foo

in two places.

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You can automatically detect some kinds of functions through template metaprogramming, for example given the type of T, you can programmatically answer the question "Does T have operator+

?" But in general, you cannot do what you are doing above automatically in the language.



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