How to store variant data in C++

Question

I'm in the process of creating a class that stores metadata about a particular data source. The metadata is structured in a tree, very similar to how XML is structured. The metadata values can be integer, decimal, or string values.

I'm curious if there is a good way in C++ to store variant data for a situation like this. I'd like for the variant to use standard libraries, so I'm avoiding the COM, Ole, and SQL VARIANT types that are available.

My current solution looks something like this:

enum MetaValueType {     MetaChar,     MetaString,     MetaShort,     MetaInt,     MetaFloat,     MetaDouble };  union MetaUnion {     char cValue;     short sValue;     int iValue;     float fValue;     double dValue; };  class MetaValue { ... private:     MetaValueType ValueType;     std::string StringValue;     MetaUnion VariantValue; }; 

The MetaValue class has various Get functions for obtaining the currently stored variant value, but it ends up making every query for a value a big block of if/else if statements to figure out which value I'm looking for.

I've also explored storing the value as only a string, and performing conversions to get different variant types out, but as far as I've seen this leads to a bunch of internal string parsing and error handling which isn't pretty, opens up a big old can of precision and data loss issues with floating point values, and still doesn't eliminate the query if/else if issue stated above.

Has anybody implemented or seen something that's cleaner to use for a C++ variant data type using standard libraries?

Answer 1

As of C++17, there’s std::variant.

If you can’t use that yet, you might want Boost.Variant. A similar, but distinct, type for modelling polymorphism is provided by std::any (and, pre-C++17, Boost.Any).

Just as an additional pointer, you can look for “type erasure”.