external/bsd/lutok/dist/examples/bindings.cpp

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  28
  29 /// \file examples/bindings.cpp
  30 /// Showcases how to define Lua functions from C++ code.
  31 ///
  32 /// A major selling point of Lua is that it is very easy too hook native C and
  33 /// C++ functions into the runtime environment so that Lua can call them.  The
  34 /// purpose of this example program is to show how this is done by using Lutok.
  35
  36 #include <cassert>
  37 #include <cstdlib>
  38 #include <iostream>
  39 #include <map>
  40 #include <sstream>
  41 #include <stdexcept>
  42 #include <string>
  43
  44 #include <lutok/exceptions.hpp>
  45 #include <lutok/operations.hpp>
  46 #include <lutok/state.ipp>
  47
  48
  49 /// Calculates the factorial of a given number.
  50 ///
  51 /// \param i The postivie number to calculate the factorial of.
  52 ///
  53 /// \return The factorial of i.
  54 static int
  55 factorial(const int i)
  56 {
  57     assert(i >= 0);
  58
  59     if (i == 0)
  60         return 1;
  61     else
  62         return i * factorial(i - 1);
  63 }
  64
  65
  66 /// A custom factorial function for Lua.
  67 ///
  68 /// \pre stack(-1) contains the number to calculate the factorial of.
  69 /// \post stack(-1) contains the result of the operation.
  70 ///
  71 /// \param state The Lua state from which to get the function arguments and into
  72 ///     which to push the results.
  73 ///
  74 /// \return The number of results pushed onto the stack, i.e. 1.
  75 ///
  76 /// \throw std::runtime_error If the input parameters are invalid.  Note that
  77 ///     Lutok will convert this exception to lutok::error.
  78 static int
  79 lua_factorial(lutok::state& state)
  80 {
  81     if (!state.is_number())
  82         throw std::runtime_error("Argument to factorial must be an integer");
  83     const int i = state.to_integer();
  84     if (i < 0)
  85         throw std::runtime_error("Argument to factorial must be positive");
  86     state.push_integer(factorial(i));
  87     return 1;
  88 }
  89
  90
  91 /// Program's entry point.
  92 ///
  93 /// \param argc Length of argv.  Must be 2.
  94 /// \param argv Command-line arguments to the program.  The first argument to
  95 ///     the tool has to be a number.
  96 ///
  97 /// \return A system exit code.
  98 int
  99 main(int argc, char** argv)
 100 {
 101     if (argc != 2) {
 102         std::cerr << "Usage: bindings <number>\n";
 103         return EXIT_FAILURE;
 104     }
 105
 106     // Create a new Lua session and load the print() function.
 107     lutok::state state;
 108     state.open_base();
 109
 110     // Construct a 'module' that contains an entry point to our native factorial
 111     // function.  A module is just a Lua table that contains a mapping of names
 112     // to functions.  Instead of creating a module by using our create_module()
 113     // helper function, we could have used push_cxx_function on the state to
 114     // define the function ourselves.
 115     std::map< std::string, lutok::cxx_function > module;
 116     module["factorial"] = lua_factorial;
 117     lutok::create_module(state, "native", module);
 118
 119     // Use a little Lua script to call our native factorial function providing
 120     // it the first argument passed to the program.  Note that this will error
 121     // out in a controlled manner if the passed argument is not an integer.  The
 122     // important thing to notice is that the exception comes from our own C++
 123     // binding and that it has been converted to a lutok::error.
 124     std::ostringstream script;
 125     script << "print(native.factorial(" << argv[1] << "))";
 126     try {
 127         lutok::do_string(state, script.str());
 128         return EXIT_SUCCESS;
 129     } catch (const lutok::error& e) {
 130         std::cerr << "ERROR: " << e.what() << '\n';
 131         return EXIT_FAILURE;
 132     }
 133 }