Proto expression types are PODs (Plain Old Data), and do not have constructors. They are brace-initialized, as follows:

terminal<int>::type const _i = {1};

The reason is so that expression objects like _i above can be statically initialized. Why is static initialization important? The terminals of many domain- specific embedded languages are likely to be global const objects, like _1 and _2 from the Boost Lambda Library. Were these object to require run-time initialization, it might be possible to use these objects before they are initialized. That would be bad. Statically initialized objects cannot be misused that way.