Re: How to make code accepting differet types work?

"Earl Purple" <>
12 Jul 2006 07:02:12 -0700
Victor Bazarov wrote:

Jim Langston wrote:

"Ian Collins" <> wrote in message

Jim Langston wrote:

This is something I've been thinking about creating, and am trying
to get the pieces together.

I want to be able to assign values in a method accepting different
types. I.E.

MyInstance.MyMethod("IntField") = 1;
MyInstance.MyMethod("FloatField") = 2.34f;
MyInstance.MyMethod("StringField") = std::string("Hello");

Is this possible?

Assign to what?

Could you use a map and have something like MyInstance["IntField"] =

MyInstance will, in fact, have a map, but the values will be
std::string. If I was using method overloading it would be something
like (untested code):

void MyInstance::MyMethod( std::string key, int value )
  std::map<std::string, std::string>::iterator it = MyMap.find(key);
  if ( it != MyMap.end() )
     it.second = jml::StrmConvert( value );

StrmConvert is a template that uses stringstream to convert between
types, in this case to a std::string.

Yes, I know I can actually use this, but I would prefer to use
operator= as it just seems more natural to me.

I didn't see the beginning of the conversation, but here is my take on
what you're asking about (as I understand it):
#include <string>
#include <map>

class MyClass {
    std::map<std::string,int> i;
    std::map<std::string,float> f;
    std::map<std::string,std::string> str;
    class MyProxy {
        MyClass& mc;
        std::string key;
        MyProxy(MyClass& m, std::string const& k) : mc(m), key(k) {}
        void operator =(int i) { mc.i[key] = i; }
        void operator =(float f) { mc.f[key] = f; }
        void operator =(std::string const &s) { mc.str[key] = s; }

    MyProxy MyMethod(std::string const& k) {
        return MyProxy(*this,k);

int main() {
    MyClass MyInstance;
    MyInstance.MyMethod("OneInt") = 0;
    MyInstance.MyMethod("TwoInt") = 42;
    MyInstance.MyMethod("OneFloat") = 1.0f;
    MyInstance.MyMethod("TwoFloat") = 3.14159f;
    MyInstance.MyMethod("OneString") = "blah";
    MyInstance.MyMethod("TwoString") = "blahblah";



Such a class would not be extensible though. An extensible version
would use templates and an "any" type class. Any type you wanted to
support would have to be registered but you'd be able to register more
types without modifying existing code.

A possible solution is a getMap<T>() function that is overloaded for
every map you support. Or you can have a single map to an "any" type
(which avoids the problem that having multiple maps would allow you to
reuse names as long as they were different types).

Once again you use overloaded functions to determine how the data is
actually stored.

Generally I solve this problem by not using an "any" class but instead
have an "opaque" class which is simply binary data, and then overloaded
converter functions You can also make each type registered with some
enumeration to check compatibility. It does rely on anything you want
to store having conversions to and from an opaque.

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