const-correctness improvement suggestion

[MaciejPatro]

Hello,

  /* Getter enabled for all template types other than std::vector and std::list   
   * @throws std::logic_error in case of an invalid argument name                 
   * @throws std::logic_error in case of incompatible types                       
   */                                                                             
  template <typename T = std::string> T get(std::string_view aArgumentName) {     
    return (*this)[aArgumentName].get<T>();                                       
  }                                                                               

Function is providing a copy of value - but is not const - which leaks this to other classes that have ArgumentParser as a member.

Example of problem:

struct program_arguments
{
  int age()  /* const is not possible here */ { return parser.get<int>("age"); }
  argparse::ArgumentParser parser;
};

There seems to be no reason besides usage of [] operator for the function not to be const. It could be made const for example by using .at on std::map

[skrobinson]

Was this addressed in #74 previously? Or have you found a new issue?

[MaciejPatro]

Yes, you are fully right - sorry for the confusion!