[SOLVED] Why boost::format cannot be converted directly to std::string?

Why boost::format cannot be converted directly to std::string?

Following is not possible:

std::string s = boost::format("%d") % 1; // error

You have to explicitely call the method str():

std::string s = (boost::format("%d") % 1).str(); // OK

It would only be syntactic sugar, but why not just add the conversion?

Solution

It is not a very good thing if an implicit conversion can throw exceptions. Conversion to string will by default throw an exception if less arguments are fed to format than needed. E.g.

std::string f()
{
   boost::format fmt("%d");
   // forgot to feed an argument
   std::string s = fmt;  // throws boost::io::too_few_args
   widget.set_title( fmt );  // throws boost::io::too_few_args
   return fmt;  // throws boost::io::too_few_args
}

Such implicit conversions make it hard to spot and analyze the parts of code that can throw exceptions. But explicit .str() calls provide a hint of such possible exceptions, which makes life easier when ensuring the surrounding code's exception safety, as well as (in this particular case) hinting to double-check preceding code to prevent said exception from happening in the first place.