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How to Retrieve Type Names in Template Metaprogramming for Informative Error Messages?
How to Retrieve Type Names in Template Metaprogramming for Informative Error Messages?
Nov 23, 2024 pm 09:03 PM
Template Metaprogramming: Retrieving Type Names
When working with generic template classes, it often becomes necessary to extract the name of the type being templated. This information can be valuable for providing informative error messages, such as those related to parsing data files.
The Challenge
In the case of parsing text data files, a common requirement is to furnish users with detailed error messages that include the type of data expected. For instance, an error message might read:
Error parsing example.txt. Value ("notaninteger") of [MySectiom]Key is not a valid int
The template function provided above (GetValue) retrieves the file, section, and key names from arguments passed to the template function and member variables in the class. However, determining the type of data expected is proving problematic.
A Compile-Time Solution
To address this issue, a compile-time solution is desired. This eliminates any runtime overhead during creation of the template function, which is crucial since the function is called frequently and load times have already become somewhat protracted.
Using typeid(T).name()
The solution is to leverage the typeid(T).name() expression, where typeid(T) returns a std::type_info object. This provided the name of the type as a constant character array. The updated code segment looks like this:
{
std::map<std::wstring, std::wstring>::iterator it = map[section].find(key);
if(it == map[section].end())
throw ItemDoesNotExist(file, section, key)
else
{
try{return boost::lexical_cast<T>(it->second);}
// throw error with the typename provided
catch(...)throw ParseError(file, section, key, it->second, typeid(T).name());
}
}
This solution effectively resolves the challenge of retrieving the name of the type being templated, enabling the provision of informative error messages during data file parsing.
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