changing a variable value according to it's name

[ilanhak]

hello,

I was wondering if there is a way to change a certain variable's value if I know it's name?

For example,say you have a struct called 'profile' who has sveral variable in it: id, address, height, weight, and so on. Assume all variable have default values and the value of 'height' is 60. Now let say someone gave you a string as an input:"height=80". What I want to do is to be able to read to input, parse it and change the value to 'height' to 80. The first two parts I can do (read and parse), what I don't know is how the change the value.

Any advice? I would appreciate it if you could add a code sample.


P.S. working with cpp

[Peter87]

No. The variable names are just there to help you as a programmer make sense of the program. After the code has been compiled the variable names are of no importance.

You probably could use a map (std::map or std::unordered_map) instead.
http://en.cppreference.com/w/cpp/container/map
http://en.cppreference.com/w/cpp/container/unordered_map

[keskiverto]

In other words, the 'profile' should have a method, which takes input, and acts as necessary.

input := "height=80 foo=bar"
tokenize
=>
token0 := "height"
value0 := "80"
token1 := "foo"
value1 := "bar"

for each token:
IF "height" == token
THEN
  IF value is an integer
  THEN this->height := toInt(value)
  ENDIF
ELSE IF ...

And yes, a map could be used to implement the IF .. ELSE IF ...


Boost has code to parse command line parameters into option variables.
Qt has "meta object" properties.

[ilanhak]

I tought about using a map but I don't know how to do it in case of a struct.

Would be glad to see some code sample in c++

[JLBorges]

> Would be glad to see some code sample in c++

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

struct profile
{
    void set( std::string name, std::string value )
    {
        auto iter = setters.find(name) ;
        if( iter != setters.end() ) iter->second( this, value ) ;
    }

    std::string id = "none" ;
    int height = 0 ;
    // etc.

    using setter_type = std::function< void( profile*,  std::string ) > ;
    static const std::map< std::string, setter_type > setters ;
};

const std::map< std::string, profile::setter_type > profile::setters
{
    { "id", []( profile* This, std::string value ) { This->id = value ; } },
    { "height", []( profile* This, std::string value ) { This->height = std::stoi(value) ; } }
    // etc.
};

int main()
{
    profile p ;
    std::cout << "id: " << p.id << "   height: " << p.height << '\n' ;
    
    p.set( "id", "this is a local profile" ) ;
    std::cout << "id: " << p.id << "   height: " << p.height << '\n' ;

    p.set( "height", "3456789" ) ;
    std::cout << "id: " << p.id << "   height: " << p.height << '\n' ;
}

http://coliru.stacked-crooked.com/a/4ac217c3ab2aa1b6

[ilanhak]

Hi,

First, thanks for the help and for the code.
Second, from some reason, when I try to run your code I'm getting errors:
'personal:: id' : only static const integral data members can be initialized within a class
'Setter_type' : symbol cannot be used in a using declartion
//Etc

I managed to get rid of the first one by not initialzing it.

I couldn't get rid from the second. I tried to change it to:
Typedef std::function<void(persona*, std::string)> setter_type;
But now I get: error c2039: 'function' : is not a member of 'std'

Any advice?

[keskiverto]

C++11. Your compiler did not have the support for C++11 enabled.

[JLBorges]

> Any advice?

Well, switch to a more modern compiler; Visual C++ 12.0 which ships with Visual Studio 2013 is a good option.

Otherwise, we would have to restrict our code so that it conforms to C++98.

Try this on your current compiler; unless it is ancient, this would compile cleanly:

#include <iostream>
#include <map>
#include <string>
#include <cstdlib> // std::strtol

// we wouldn't have std::stoi, so we will write a function
int string_to_int( std::string str ) // error checks elided
{ return std::strtol( str.c_str(), 0, 10 ) ; }

struct profile
{
    // the setter_type is now a pointer to member function
    typedef void (profile::*setter_type)( std::string ) ;

    void set( std::string name, std::string value )
    {
        std::map< std::string, setter_type >::const_iterator iter = setters.find(name) ;
        if( iter != setters.end() )
        {
            setter_type pfn = iter->second ; // get the pointer to member function
            ( this->*pfn )(value) ; // and call it with value as the argument
        }
    }

    // a constructor to initialize members
    profile() : id("none"), height(0), width(0) {}

    std::string id ;
    int height ;
    int width ;
    // etc.

    // member functions to set the values
    void set_id( std::string str_id ) { id = str_id ; }
    void set_height( std::string str_height ) { height = string_to_int(str_height) ; }
    void set_width( std::string str_width ) { width = string_to_int(str_width) ; }
    // etc.

    static std::map< std::string, setter_type > setters ;
    static bool init_helper() ;
    static const bool init ;
};

std::map< std::string, profile::setter_type > profile::setters ;

// helper function to populate the map
bool profile::init_helper()
{
    setters["id"] = &profile::set_id ;
    setters["height"] = &profile::set_height ;
    setters["width"] = &profile::set_width ;
    // etc.
    return true ;
}

const bool profile::init = init_helper() ; // populates map by calling init_helper()

std::ostream& operator<< ( std::ostream& stm, const profile& p )
{
    return stm << "profile { id:" << p.id << ", height:" << p.height
                << ", width:" << p.width << " }" ;
}

int main()
{
    profile p ;
    std::cout << p << '\n' ;

    p.set( "id", "local" ) ;
    std::cout << p << '\n' ;

    p.set( "height", "3456" ) ;
    std::cout << p << '\n' ;

    p.set( "width", "999" ) ;
    std::cout << p << '\n' ;
}

http://coliru.stacked-crooked.com/a/e225a9b2c7e0ef84