Wire-Cell News

Throwing exceptions is now a SOP for indicating some error.

As you write configurable components, be particularly liberal with throwing exceptions inside of the configure() method. The wire-cell CLI will catch these and exit. Other applications of the WCT that do not, will at least fail early if any exception is thrown and thus the user can fix the problem without delay.

WCT uses Boost exception support hidden with a thin layer to keep things looking simple. When you program WCT code with exceptions do like:

#include "WireCellUtil/Exceptions.h"  

void MyComponent::configure(Configuration cfg)
{
    std::string mytool_tn = get<std::string>(cfg, "mytool");
    if (mytool_tn.empty()) {
	THROW(ValueError() << errmsg{"You must set \"mytool\" to something"});
    }
     m_mytool = Factory::find_tn<IMyTool>(mytool_tn);
     if (!m_mytool) {
	 THROW(ValueError() << errmsg{"Failed to find IMyTool: " + mytool_tn});
     }
}

Notes:

1. All exceptions that WCT code may throw are defined in the header file included in the example.
2. Instead of the low-level C++ throw() use the provided CPP macro THROW() which will give the user extra info in the case that an exception is not caught.
3. The value returned may have an errmsg streamed to it which will add some dynamic description of the error in the form of a string. If a complex string needs to be build see WireCell::String::format().
4. No special C++ is needed to catch exceptions.