What are "separately compiled C++ templates"?

Once I saw a statement that "separately compiled C++ templates" is a standard feature that none of available C++ compilers support.

What are those开发者_Python百科 "separately compiled templates" and why are they ignored?

C++98 introduced the export keyword which allowed you to have the definition of a function template in another translation unit, with only its declaration needed to compile code that uses it. (See here if you are hazy on what's a definition vs. a declaration. Basically, you could have the function templates implementation in another translation unit.) That's just as it is with other functions.

However, only compilers using EDG's compiler front end ever supported it, and not all of them even did officially. In fact, the only compiler I know that officially supported it was Comeau C++. That's why the keyword, unfortunately, got removed from C++11.

I think it's safe to say that it is expected that a proper module system would cure C++ from many of its shortcomings that surround the whole compilation model, but, again unfortunately, a module system was not considered something that could be tackled in a reasonable amount of time for C++11. We will have to hope for the next version of the standard.

Separately compiled templates is where you can bring in template definitions from another translation unit instead of having to define them in every TU (normally in the header).

Basically, they're ignored because they're virtually impossible to implement in terms of complexity and bring a number of unfortunate side effects.