Object prints out its variable name

[garf13ld]

Hi!

I wanted to do something like

MyClass var1, var2;

cout << var1.name() << var2.name() << endl;

This should printout

var1 var2

Note: I am not explicitly passing the object what variable name its assigned. Is there any way the object can figure out this information on its own..?

Thanks!

[brewbuck]

Note: I am not explicitly passing the object what variable name its assigned. Is there any way the object can figure out this information on its own..?

It's impossible.

[garf13ld]

I mean, I know as far as the language is concerned its impossible.. But could there be any ugly compiler hack that may allow this..?

[matsp]

You could, however, write a macro that generates a string from it's input value:

#define mkstring(x) #x

Using this, we can do something like this:

class MyObject
{
private:
   char *iName
 ....
public:
   MyObject(char *name) : iname(name) { ... };
   ...
}

#define INSTANCIATE(type, name) type name(#name)

int main()
{
    INSTANCIATE(MyObject, var1);
    INSTANCIATE(MyObject, var2);

    cout << var1.name << var2.name << endl;
}

--
Mats

[DougDbug]

Normally, variable names don't even exist in the compiled EXE file. The machine code just keeps track of the variable's address in memory, it doesn't need a name. (If you use a de-compiler, it will make-up it's own variable names.)

I've never tried using <typeinfo>, but it might work for you. I think it can give you an object name, but I have no idea if it also keeps track of variable names.

[cpjust]

Why would you need to print out the variable name?

You could add a string parameter in the constructor that takes the name... but again, what's the point?

[grumpy]

I've never tried using <typeinfo>, but it might work for you. I think it can give you an object name, but I have no idea if it also keeps track of variable names.

typeinfo only gives information about types, not about particular variables.

[Blackroot]

You could add a string or char* which holds the name in your class, but there is no default way to get a variable name.

[CodeMonkey]

matsp's solution is the only feasible way to do it with the language, but as he would agree it is an ugly, ugly, unnecessary hack.

[garf13ld]

Well the reason I wanted to do something like this is because I am designing a matrix class and when I do something like

Matrix jacobian;
...
cout << jacobian;

I want it to print out

jacobian = [.....]

This is similar to what Matlab does. Obviously I could include a string within the class and make the developer explain what the matrix represents, but I figured if something like this was indeed possible it would encourage the developer to use descriptive variable names.

[Prelude]

>This is similar to what Matlab does.
Matlab is much higher level than C++. If you want a scripting language, use one or write an interpreter for some pseudo-C++. If you want C++, you have to realize that compile-time symbols are generally going to be lost during compilation. The only way to retain them is to store them separately at runtime with your own machinery.

[matsp]

Well, explaining your purpose and how you want to use it will also help us give you the best help.

In this particular case, you can solve the problem in a manner like what I explained above (or, of corurse just adding it's name to the creation of it).

However, there are multiple problems with "naming" a variable. Consider:

int func(type &var1)
{
 ... 
}

int func2(type &var2)
{
   ... 
    func1(var2);
    ... 
}

int main()
{
   type someVariable;
   type **ptr;
   int x;
   
   x = func1(someVariable);
   ptr = new type[10];
   ... 
   func2(*ptr[5]); 
}

Now, what is the "name" of var1 inside func1:
- when it's called with someVariable?
- when it's been called from func2 with a pointer indexed?


Having "meaningfull" variable names is about style, and a goor programmer/sw engineer will use good style - what the definition of good style is of course also slightly depending on several factors, and what one person thinks is a good name isn't necessarily what everyone else thinks is a good name.

--
Mats

[cpjust]

#define PRINT_NAME_AND_VAR( var )   cout << #var << " = " << var;
...
Matrix jacobian;
...
PRINT_NAME_AND_VAR( jacobian );