C++ Templates: The Complete Guide

That's because the function a<int>::b() is not instantiated, due to the fact that it is a template. When you try to instantiate it, i.e. call it like var.b();, the compiler will spit an error. You have to understand that templates are instantiated "on demand", i.e. when the compiler needs the instantiation. Otherwise only minimal syntactic verifications take place. The details regarding instantiations/name lookups in templates is a rather complicated subject, I highly recommend this book: C++ Templates: The Complete Guide by David Vandevoode and Nicolai Josuttis.

That's not the case with the first code snippet: the function has to be valid from the very beginning.

According to the book C++ Templates: The Complete Guide Appendix B.1,

At a very high level, a call to a named function can be processed in the following way:

1. The name is looked up to form an initial overload set.

2. If necessary, this set is tweaked in various ways (for example, template deduction occurs).

3. Any candidate that doesn't match the call at all (even after considering implicit conversions and default arguments) is eliminated from the overload set. This results in a set of so-called viable function candidates.

4. Overload resolution is performed to find a best candidate. If there is one, it is selected; otherwise, the call is ambiguous.

5. The selected candidate is checked. For example, if it is an inaccessible private member, a diagnostic is issued.

As you can see, the access authority is checked at last, so the call to fn() is ambiguous will be reported firstly at step #4.

Beginner

Introductory, no previous programming experience

• C++ Primer * (Stanley Lippman, Josée Lajoie, and Barbara E. Moo) (updated for C++11) Coming at 1k pages, this is a very thorough introduction into C++ that covers just about everything in the language in a very accessible format and in great detail. The fifth edition (released August 16, 2012) covers C++11. [Review]

• Programming: Principles and Practice Using C++ (Bjarne Stroustrup) (updated for C++11/C++14) An introduction to programming using C++ by the creator of the language. A good read, that assumes no previous programming experience, but is not only for beginners.

_{* Not to be confused with C++ Primer Plus (Stephen Prata), with a significantly less favorable review.}

Introductory, with previous programming experience

• A Tour of C++ (Bjarne Stroustrup) (EBOOK) The “tour” is a quick (about 180 pages and 14 chapters) tutorial overview of all of standard C++ (language and standard library, and using C++11) at a moderately high level for people who already know C++ or at least are experienced programmers. This book is an extended version of the material that constitutes Chapters 2-5 of The C++ Programming Language, 4th edition.

• Accelerated C++ (Andrew Koenig and Barbara Moo) This basically covers the same ground as the C++ Primer, but does so on a fourth of its space. This is largely because it does not attempt to be an introduction to programming, but an introduction to C++ for people who've previously programmed in some other language. It has a steeper learning curve, but, for those who can cope with this, it is a very compact introduction to the language. (Historically, it broke new ground by being the first beginner's book to use a modern approach to teaching the language.) Despite this, the C++ it teaches is purely C++98. [Review]

Best practices

• Effective C++ (Scott Meyers) This was written with the aim of being the best second book C++ programmers should read, and it succeeded. Earlier editions were aimed at programmers coming from C, the third edition changes this and targets programmers coming from languages like Java. It presents ~50 easy-to-remember rules of thumb along with their rationale in a very accessible (and enjoyable) style. For C++11 and C++14 the examples and a few issues are outdated and Effective Modern C++ should be preferred. [Review]

• Effective Modern C++ (Scott Meyers) This is basically the new version of Effective C++, aimed at C++ programmers making the transition from C++03 to C++11 and C++14.

• Effective STL (Scott Meyers) This aims to do the same to the part of the standard library coming from the STL what Effective C++ did to the language as a whole: It presents rules of thumb along with their rationale. [Review]

Intermediate

• More Effective C++ (Scott Meyers) Even more rules of thumb than Effective C++. Not as important as the ones in the first book, but still good to know.

• Exceptional C++ (Herb Sutter) Presented as a set of puzzles, this has one of the best and thorough discussions of the proper resource management and exception safety in C++ through Resource Acquisition is Initialization (RAII) in addition to in-depth coverage of a variety of other topics including the pimpl idiom, name lookup, good class design, and the C++ memory model. [Review]

• More Exceptional C++ (Herb Sutter) Covers additional exception safety topics not covered in Exceptional C++, in addition to discussion of effective object-oriented programming in C++ and correct use of the STL. [Review]

• Exceptional C++ Style (Herb Sutter) Discusses generic programming, optimization, and resource management; this book also has an excellent exposition of how to write modular code in C++ by using non-member functions and the single responsibility principle. [Review]

• C++ Coding Standards (Herb Sutter and Andrei Alexandrescu) “Coding standards” here doesn't mean “how many spaces should I indent my code?” This book contains 101 best practices, idioms, and common pitfalls that can help you to write correct, understandable, and efficient C++ code. [Review]

• C++ Templates: The Complete Guide (David Vandevoorde and Nicolai M. Josuttis) This is the book about templates as they existed before C++11. It covers everything from the very basics to some of the most advanced template metaprogramming and explains every detail of how templates work (both conceptually and at how they are implemented) and discusses many common pitfalls. Has excellent summaries of the One Definition Rule (ODR) and overload resolution in the appendices. A second edition covering C++11, C++14 and C++17 has been already published . [Review]

Advanced

• Modern C++ Design (Andrei Alexandrescu) A groundbreaking book on advanced generic programming techniques. Introduces policy-based design, type lists, and fundamental generic programming idioms then explains how many useful design patterns (including small object allocators, functors, factories, visitors, and multi-methods) can be implemented efficiently, modularly, and cleanly using generic programming. [Review]

• C++ Template Metaprogramming (David Abrahams and Aleksey Gurtovoy)

• C++ Concurrency In Action (Anthony Williams) A book covering C++11 concurrency support including the thread library, the atomics library, the C++ memory model, locks and mutexes, as well as issues of designing and debugging multithreaded applications.

• Advanced C++ Metaprogramming (Davide Di Gennaro) A pre-C++11 manual of TMP techniques, focused more on practice than theory. There are a ton of snippets in this book, some of which are made obsolete by type traits, but the techniques, are nonetheless useful to know. If you can put up with the quirky formatting/editing, it is easier to read than Alexandrescu, and arguably, more rewarding. For more experienced developers, there is a good chance that you may pick up something about a dark corner of C++ (a quirk) that usually only comes about through extensive experience.

Reference Style - All Levels

• The C++ Programming Language (Bjarne Stroustrup) (updated for C++11) The classic introduction to C++ by its creator. Written to parallel the classic K&R, this indeed reads very much like it and covers just about everything from the core language to the standard library, to programming paradigms to the language's philosophy. [Review]

• C++ Standard Library Tutorial and Reference (Nicolai Josuttis) (updated for C++11) The introduction and reference for the C++ Standard Library. The second edition (released on April 9, 2012) covers C++11. [Review]

• The C++ IO Streams and Locales (Angelika Langer and Klaus Kreft) There's very little to say about this book except that, if you want to know anything about streams and locales, then this is the one place to find definitive answers. [Review]

C++11/14 References:

• The C++ Standard (INCITS/ISO/IEC 14882-2011) This, of course, is the final arbiter of all that is or isn't C++. Be aware, however, that it is intended purely as a reference for experienced users willing to devote considerable time and effort to its understanding. As usual, the first release was quite expensive ($300+ US), but it has now been released in electronic form for $60US.

• The C++14 standard is available, but seemingly not in an economical form – directly from the ISO it costs 198 Swiss Francs (about $200 US). For most people, the final draft before standardization is more than adequate (and free). Many will prefer an even newer draft, documenting new features that are likely to be included in C++17.

• Overview of the New C++ (C++11/14) (PDF only) (Scott Meyers) (updated for C++1y/C++14) These are the presentation materials (slides and some lecture notes) of a three-day training course offered by Scott Meyers, who's a highly respected author on C++. Even though the list of items is short, the quality is high.

• The C++ Core Guidelines (C++11/14/17/…) (edited by Bjarne Stroustrup and Herb Sutter) is an evolving online document consisting of a set of guidelines for using modern C++ well. The guidelines are focused on relatively higher-level issues, such as interfaces, resource management, memory management and concurrency affecting application architecture and library design. The project was announced at CppCon'15 by Bjarne Stroustrup and others and welcomes contributions from the community. Most guidelines are supplemented with a rationale and examples as well as discussions of possible tool support. Many rules are designed specifically to be automatically checkable by static analysis tools.

• The C++ Super-FAQ (Marshall Cline, Bjarne Stroustrup and others) is an effort by the Standard C++ Foundation to unify the C++ FAQs previously maintained individually by Marshall Cline and Bjarne Stroustrup and also incorporating new contributions. The items mostly address issues at an intermediate level and are often written with a humorous tone. Not all items might be fully up to date with the latest edition of the C++ standard yet.

• cppreference.com (C++03/11/14/17/…) (initiated by Nate Kohl) is a wiki that summarizes the basic core-language features and has extensive documentation of the C++ standard library. The documentation is very precise but is easier to read than the official standard document and provides better navigation due to its wiki nature. The project documents all versions of the C++ standard and the site allows filtering the display for a specific version. The project was presented by Nate Kohl at CppCon'14.

Classics / Older

Note: Some information contained within these books may not be up-to-date or no longer considered best practice.

• The Design and Evolution of C++ (Bjarne Stroustrup) If you want to know why the language is the way it is, this book is where you find answers. This covers everything before the standardization of C++.

• Ruminations on C++ - (Andrew Koenig and Barbara Moo) [Review]

• Advanced C++ Programming Styles and Idioms (James Coplien) A predecessor of the pattern movement, it describes many C++-specific “idioms”. It's certainly a very good book and might still be worth a read if you can spare the time, but quite old and not up-to-date with current C++.

• Large Scale C++ Software Design (John Lakos) Lakos explains techniques to manage very big C++ software projects. Certainly, a good read, if it only was up to date. It was written long before C++ 98 and misses on many features (e.g. namespaces) important for large-scale projects. If you need to work in a big C++ software project, you might want to read it, although you need to take more than a grain of salt with it. The first volume of a new edition is expected in 2018.

• Inside the C++ Object Model (Stanley Lippman) If you want to know how virtual member functions are commonly implemented and how base objects are commonly laid out in memory in a multi-inheritance scenario, and how all this affects performance, this is where you will find thorough discussions of such topics.

• The Annotated C++ Reference Manual (Bjarne Stroustrup, Margaret A. Ellis) This book is quite outdated in the fact that it explores the 1989 C++ 2.0 version - Templates, exceptions, namespaces and new casts were not yet introduced. Saying that however, this book goes through the entire C++ standard of the time explaining the rationale, the possible implementations, and features of the language. This is not a book to learn programming principles and patterns on C++, but to understand every aspect of the C++ language.

• Thinking in C++ (Bruce Eckel) Two volumes; is a tutorial style free set of intro level books. Downloads: vol 1, vol 2. Unfortunately they’ re marred by a number of trivial errors (e.g. maintaining that temporaries are automatically const), with no official errata list. A partial 3^rd party errata list is available at (http://www.computersciencelab.com/Eckel.htm), but it’s apparently not maintained.

• Scientific and Engineering C++: An Introduction to Advanced Techniques and Examples (John Barton and Lee Nackman) It is a comprehensive and very detailed book that tried to explain and make use of all the features available in C++, in the context of numerical methods. It introduced at the time several new techniques, such as the Curiously Recurring Template Pattern (CRTP, also called Barton-Nackman trick). It pioneered several techniques such as dimensional analysis and automatic differentiation. It came with a lot of compilable and useful code, ranging from an expression parser to a Lapack wrapper. The code is still available here: http://www.informit.com/store/scientific-and-engineering-c-plus-plus-an-introduction-9780201533934. Unfortunately, the books have become somewhat outdated in the style and C++ features, however, it was an incredible tour-de-force at the time (1994, pre-STL). The chapters on dynamics inheritance are a bit complicated to understand and not very useful. An updated version of this classic book that includes move semantics and the lessons learned from the STL would be very nice.

C++ Templates: The Complete Guide is your best bet. You could also learn about the Standard Library which heavily uses templates.

With templates we simply don't have clean output facilities and there are no compilers i know of that allow you to directly view template instantiations. The closest i found regarding metaprogram debugging was a paper on Templight.

For now the best utilities seem to be:

• static asserts & concept checks (clearly assert your assumptions)
• the mentioned instantiation backtraces (e.g. by using static asserts)
• letting instantiations generate warnings (boost::mpl::print might do it)
• a tracer, a custom class that gets passed as a template argument and is used to emit runtime output (introduced by C++ Templates - The Complete Guide)

Short summary

This is just the way the current C++11 rules are: this->bar.baz<int>() introduces a dependent name not in the current instantiation context that requires disambiguation with the template keyword, even though it is very hard to come up with an actual example of a competing parse that change the semantics of the expression this->bar.baz<int>().

Parsing ambiguity from angle brackets

First: why in general is there a need for template?

When a C++ compiler encounters an expression f<g>(0), it can interpret this either as "call the function template f for template argument g and function argument 0 and evaluate the result" or it can mean "make the comparison (f<g)>(0) for names f and g and constant 0." Without further information it cannot make this decision. This is an unfortunate consequence of the choice of angle brackets for template arguments.

In many (most) cases, the compiler does have enough context to decide whether a template expression or a comparison is being parsed. However, when a so-called dependent name (essentially a name that is explicitly or implicitly dependent on a template parameter of the current scope) is encountered, another language subtlety comes into play.

Two-phase name lookup

Because a name dependent on a template could change its meaning (e.g. through specializations) when a template is being instantiated for a concrete type, name lookup of dependent names is done in two phases (quote from C++ Templates the Complete Guide):

During the first phase, nondependent names are looked up while the template is being parsed using both the ordinary lookup rules and, if applicable, the rules for argument-dependent lookup (ADL). Unqualified dependent names (which are dependent because they look like the name of a function in a function call with dependent arguments) are also looked up that way, but the result of the lookup is not considered complete until an additional lookup is performed when the template is instantiated.

During the second phase, which occurs when templates are instantiated at a point called the point of instantiation (POI), dependent qualified names are looked up (with the template parameters replaced with the template arguments for that specific instantiation), and an additional ADL is performed for the unqualified dependent names.

Why this-> makes your code different

Using this-> inside a class template introduces a dependent name and triggers two-phase name lookup. In that case, the rule cited by @40two comes into play. The point is that the ADL at the 2nd phase can bring in new names from explicit specializations that redefine the meaning of your bar and baz and it could conceivably change the meaning of this->bar.baz<int>(0) to a comparison rather than a function template call.

Granted, for non-type template arguments such as this->bar.another_baz<0>() this would be more likely than for a type template parameter. In this related Q&A a similar discussion arose whether one could find a syntactic valid form that changes the meaning of this->f<int>() vs this->template f<int>(0), without a clear conclusion.

Note that C++11 already relaxes the rule for template disambiguation compared to C++98. Under the current rules this->f<int>() for a template<class> f() inside Foo would not require template because it is in the so-called current instantiation. See this answer from the canonical Q&A for this topic for more details

What are C++ Expression Templates in simple terms?

Expression templates are a category of C++ template meta programming which delays evaluation of subexpressions until the full expression is known, so that optimizations (especially the elimination of temporaries) can be applied.

Are there books around that discuss numerical methods/computations using C++ Expression Templates?

I believe ET's were invented by Todd Veldhuizen who published a paper on it 15 years ago. (It seems that many older links to it are dead by now, but currently here is a version of it.) Some material about it is in David Vandevoorde's and Nicolai Josuttis' C++ Templates: The Complete Guide.

In what way, C++ Expression Templates are better than using pure C?

They allow you to write your code in an expressive high level way without losing performance. For example,

void f(const my_array<double> a1, const my_array<double> a2) 
{ 
  my_array<double> a3 = 1.2 * a1 + a1 * a2; 
  // ..
}

can be optimized all the way down to

for( my_array<double>::size_type idx=0; idx<a1.size(); ++idx ) 
  a3[idx] = 1.2*a1[idx] + a1[idx]*a2[idx]; 

which is faster, but harder to understand.

This question is also addressed specifically in the definitive book on C++ Templates ("C++ Templates: The Definitive Guide", by Vandvoorde and Josuttis), Section 7.1, p.87 (at least in my edition):

7.1 "Class Template" or "Template Class"?

[snip]

There is some confusion about how a class that is a template is called:

The term class template states that the class is a template. That is, it is a parameterized description of a family of classes.

The term template class on the other hand has been used

• as a synonym for class template.

• to refer to classes generated from templates.

• to refer to classes with a name that is a template-id.

The difference between the second and third meaning is somewhat subtle and unimportant for the remainder of the text.

Because of this imprecision, we avoid the term template class in this book.

Similarly, we use function template and member function template, but avoid template function and template member function.

(Anyone, I mean anyone, working with C++ templates should have a copy of this book, by the way.)

Templates are a very powerful mechanism which can simplify many things. However to use them properly requires much time and experience - in order to decide when their usage is appropriate.

For me the most important advantages are:

• reducing the repetition of code (generic containers, algorithms)
• reducing the repetition of code advanced (MPL and Fusion)
• static polymorphism (=performance) and other compile time calculations
• policy based design (flexibility, reusability, easier changes, etc)
• increasing safety at no cost (i.e. dimension analysis via Boost Units, static assertions, concept checks)
• functional programming (Phoenix), lazy evaluation, expression templates (we can create Domain-specific embedded languages in C++, we have great Proto library, we have Blitz++)
• other less spectacular tools and tricks used in everyday life:
  • STL and the algorithms (what's the difference between for and for_each)
  • bind, lambda (or Phoenix) ( write clearer code, simplify things)
  • Boost Function (makes writing callbacks easier)
  • tuples (how to genericly hash a tuple? Use Fusion for example...)
  • TBB (parallel_for and other STL like algorithms and containers)
• Can you imagine C++ without templates? Yes I can, in the early times you couldn't use them because of compiler limitations.
• Would you write in C++ without templates? No, as I would lose many of the advantages mentioned above.

Downsides:

• Compilation time (for example throw in Sprit, Phoenix, MPL and some Fusion and you can go for a coffee)
• People who can use and understand templates are not that common (and these people are useful)
• People who think that they can use and understand templates are quite common (and these people are dangerous, as they can make a hell out of your code. However most of them after some education/mentoring will join the group mentioned in the previous point)
• template export support (lack of)
• error messages could be less cryptic (after some learning you can find what you need, but still...)

I highly recommend the following books:

• C++ Templates: The Complete Guide by David Vandevoorde and Nicolai Josuttis (thorough introduction to the subject of templates)
• Modern C++ Design. Generic Programming and Design Patterns Applied by Andrei Alexandrescu (what is the less known way of using templates to simplify your code, make development easier and result in code robust to changes)
• C++ Template Metaprogramming by David Abrahms and Aleksey Gutov (again - different way of using the templates)
• More C++ Idioms from Wikibooks presents some nice ideas.