Templates compilation: gcc vs VS2010

Question

From the book - C++ Templates: The Complete Guide by David, Nicolai

Thus, templates are compiled twice:

1. Without instantiation, the

Answer 1

The book you're looking at seems to reflect (mostly) that author's observations about how compilers really work, not the requirement(s) of the standard. The standard doesn't really say much to give extra leniency to a compiler about ill-formed code inside a template, just because it's not instantiated.

At the same time, the book is correct that there are some things a compiler really can't do much about checking until it's instantiated. For example, you might use a dependent name as the name of a function (or invoke it like a function, anyway -- if it's functor instead that would be fine too). If you instantiate that template over a class where it is a function, fine and well. If you instantiate it over a class where it's really an int, attempting to call it will undoubtedly fail. Until you've instantiated it, the compiler can't tell which is which though.

This is a large part of what concepts were really intended to add to C++. You could directly specify (for example) that template X will invoke T::y like a function. Then the compiler could compare the content of the template with the declarations in the concept, and determine whether the body of the template fit with the declaration in the concept or not. In the other direction, the compiler only needed to compare a class (or whatever) to the concept to determine whether instantiating that template would work. If it wasn't going to work, it could report the error directly as a violation of the relevant concept (as it is now, it tries to instantiate the template, and you frequently get some strange error message that indicates the real problem poorly, if at all).