将C ++ 11“自动”转换为非C ++ 11编译器

Question

I have download the source from

http://matt.eifelle.com/2012/07/17/just-a-small-example-of-numerical-optimization-in-c/

and try to compile in g++ 4.4.6 , which don't know about auto of the follwoing source :

  auto optimizer = Optimization::Local::build_simplex( 
  fun,
  Optimization::Local::make_and_criteria(Optimization::Local::IterationCriterion(max_iterations),
      Optimization::Local::RelativeValueCriterion<float>(ftol))); 

Reading the webpage , I know it is because auto is supported in C++11 , so I try to search the source what I downloaded , there is a simplex.h which has the following source code :

template<class Function, class Criterion>
static Simplex<typename Function::DataType, typename Function::ParameterType, Function, Criterion> build_simplex(const Function& fun, const Criterion& criterion)
{
  return Simplex<typename Function::DataType, typename Function::ParameterType, Function, Criterion>(criterion);
}

then I change the

auto optimizer  

to

Simplex<typename Function::DataType, typename Function::ParameterType, Function, Criterion> optimizer

or to

Optimization::Simplex<typename Function::DataType, typename Function::ParameterType, Function, Criterion> optimizer

none would pass the compiler !!

I don't know much about template usage of c++ , except for get a c++11 compile , What returned datatype modification should I do for this auto ?

Update :

Thanks for kindly help, I modify to int optimizer and compile , I got : test_rosenbrock_simplex.cpp:44: error: cannot convert Optimization::Local::Simplex<float, Eigen::Matrix<float, 2, 1, 0, 2, 1>, Rosenbrock, Optimization::Local::AndCriteria<Optimization::Local::IterationCriterion, Optimization::Local::RelativeValueCriterion<float> > > to int in initialization

then I change the code to :

Optimization::Local::Simplex<float, Eigen::Matrix<float, 2, 1, 0, 2, 1>, Rosenbrock, Optimization::Local::AndCriteria<Optimization::Local::IterationCriterion, Optimization::Local::RelativeValueCriterion<float> > > optimizer

it works !!!!!

g++ -O2 -march=native -msse2 -m64 -DEIGEN_NO_DEBUG test_rosenbrock_simplex.cpp -I/home/usrA/tools/eigen-eigen-5097c01bcdc4 -o test_rosenbrock_simplex.exe

./test_rosenbrock_simplex.exe

-2.75  -0.5     0
2.1875 1.125     0
Starting point: 10
10
Starting value: 8181
Best point: 1
1
Best value: 5.68434e-14

Answer 1

You have to substitute in the actual template arguments, merely the declared return type doesn't cut it.

As Drew mentioned in a comment, you can convince the compiler to tell you this.

Simply write

int optimizer = .....;

and inspect the resulting error.

Answer 2

Boost has a macro that implements a c++11 auto lookalike. It's called BOOST_AUTO .

It would look like:

BOOST_AUTO(optimizer, Optimization::Local::build_simplex(
    fun,
    Optimization::Local::make_and_criteria(Optimization::Local::IterationCriterion(max_iterations),
    Optimization::Local::RelativeValueCriterion<float>(ftol))); 

Answer 3

Indeed, either you use the Boost macro, or you write the full type yourself. I'm sorry I didn't put everything, but I would code this only with a C++11 compiler. Otherwise, stick with non-template code. (I'm the blog author).