Re: [Boost-users] boost::proto user guide - compile error creating a lazy function

Hi Tom, comments inline ... Tom Babbin wrote:

Hi,

I am going through proto user guide. I am getting compiling error for the following piece code (it's pretty much copy and pasted from the guide). I used 1.40 and gcc.

#include <iostream> #include #include #include #include #include <cmath> #include <vector>

namespace proto = boost::proto;

template<int I> struct placeholder {};

// Define some placeholders proto::terminal< placeholder< 1 > >::type const _1 = {{}}; proto::terminal< placeholder< 2 > >::type const _2 = {{}};

struct calculator_context : proto::callable_context< calculator_context const > { // Values to replace the placeholders std::vector<double> args;

// Define the result type of the calculator. // (This makes the calculator_context "callable".) typedef double result_type;

// Handle the placeholders: template<int I> double operator()(proto::tag::terminal, placeholder) const

Should be placeholder<I> here ----------------^^^^^^^^^^^

{ return this->args[I]; } };

// Define a pow_fun function object template<int Exp> struct pow_fun { typedef double result_type; double operator()(double d) const { return std::pow(d, Exp); } };

// Define a lazy pow() function for the calculator DSEL. // Can be used as: pow< 2 >(_1) template typename proto::function< typename proto::terminal::type , Arg const &

...

::type

Return type needs a const.

pow1(Arg const &arg) { typedef typename proto::function< typename proto::terminal::type , Arg const & >::type result_type;

result_type result = {{{}}, arg}; return result; }

int main() { calculator_context ctx; ctx.args.push_back(3);

std::cout << boost::proto::eval( pow1<2>( _1 ), ctx );

The problem is that proto::eval() takes its first argument by non-const reference. The temporary expression returned by pow1 doesn't bind to it as it's defined in the users' guide. Adding a const-qualification to the return type solves the problem. I'll update the docs for 1.41. Thanks for the report. -- Eric Niebler BoostPro Computing http://www.boostpro.com