Re: [boost] Re: lexical_cast with default value

From: Dave Dribin (dave-ml_at_[hidden])
Date: 2005-05-09 16:14:48

On May 5, 2005, at 2:55 PM, Andrey Melnikov wrote:
> Semantically convert_to is a different operation. Safe C++ way to do 
> itoa has nothing to do with serialization.

Then why does the lexical cast documentation specifically compare 
itself to atoi and strtol:

   http://www.boost.org/libs/conversion/lexical_cast.htm

As stated in the first paragraph on the motivation:

"Sometimes a value must be converted to a literal text form, such as an 
int represented as a string, or vice-versa, when a string is 
interpreted as an int. Such examples are common when converting between 
data types internal to a program and representation external to a 
program, such as windows and configuration files."

Working with windows and configuration files, in my experience, at 
least, requires handling of invalid values.  They are the normal case, 
not an exceptional case.  And from reading the lexical_cast 
documentation, you'd think this is the library to use.  Apart from the 
name containing "cast", lexical_cast does what I need: converting 
numbers to and from strings.

> I think we can leave lexical_cast alone as it is, but add separate 
> functions for safe conversions between ascii and binary representation 
> of numbers. These functions should be fast and properly named 
> according to their semantic. Performance requirement can also force us 
> to implement additional interface with error flag instead of throwing 
> exceptions. convert_to<int>("abc") can be so fast that in some 
> applications overhead of throwing an exception will be very 
> noticeable.

That would work, as well.  But I'm thinking that convert_to would need 
to fall back to lexical_cast for conversion that do not contain a 
string.  If we want to concentrate on the conversion to and from 
strings, maybe names like string_to and to_string would be more 
appropriate:

   int n = string_to<int>("50");
   int httpPort = string_to<int>("invalid conversion", 80);

And the converse:

   string message = "Connecting to server: " + host + ":" + 
to_string(80);

Actually, to_string cannot have an invalid conversion case, can it?  It 
seems only going from a string can cause that.

And as for performance being an issue, it probably is in some cases.  
In the cases where I've used string to int conversions and vice vera, 
performance has not been critical and stringstreams have been just 
fine, though.  I started using lexical_cast because it made my 
stringstream code easier to read.

-Dave