Why does toString() exist? It seems useless.

[Max Samukha]

On Mon, 18 Aug 2008 15:15:51 +0200, Don <nospam at nospam.com.au> wrote:

>bearophile wrote:
>> Don Wrote:
>>> I cannot understand the rationale for a toString() member function which 
>>> doesn't support formatting.
>> 
>> I think the rationale is that it's a simple function that performs a simple thing with an easy to use and easy to learn syntax, I have found it useful often.
>
>I think it encourages a very bad programming style. It makes ONE case 
>very easy, but does nothing for the other cases. It doesn't scale at all.
>
>> For more complex purposes you can write your own printing methods, 
>
>> or you can write a method that changes the printing state of the class/struct, 
> > that is later read and used when you call toString() and used to 
>change its behavior.
>
>But the way it is formatted should be a property of the output stream, I 
>think, not a property of the object. Certainly this is the way C and C++ 
>  I/O works.
>
>> 
>> So I have created a graph class that by default when you call toString() gives a compact and simple textual representation of the graph. 
>Then there are other methods, one that returns a string that shows the 
>matrix of links, another that shows the weights of the arcs too, etc.
>
>  There is also the possibility to modify the printing defaults with 
>another method, to then later toString() acts differently.
>
>Then I got suspicious that this might have come from Java or .NET.

FWIW, .NET provides IFormattable interface that allows objects to
imlement custom formatting:

interface IFormattable
{ 
	string toString(string  fmt, IFormatProvider fProvider);
}

class C : IFormattable
{
	string toString(string fmt, IFormatProvider fProvider)
	{
		if (fProvider !is null)
		{
			// try to get a formater for this type
			// from fProvider
		
		}
		else
		{		
			switch (fmt)
			{
				// do formatting based on the format
string  
			}
		}		
	}
}


>I googled for toString in Java and found:
>---
>http://www.javapractices.com/topic/TopicAction.do?Id=55
>---
>"The toString method is widely implemented. It provides a simple, 
>convenient mechanism for debugging classes during development. It is 
>also widely used for logging, and for passing informative error messages 
>to Exception constructors and assertions. When used in these informal 
>ways, the exact format of toString is not part of the contract of the 
>method, and callers should not rely on the exact format of the returned 
>String."
>---
>Now this makes sense. It's a useful hack. If it's just for debugging and 
>error messages, that's fine. But if that is what it's for, the normal 
>runtime I/O should discourage its use elsewhere (eg, by appending the 
>class name in front).
>
>OTOH if it is intended for general I/O, I think it's a horribly broken 
>design.