How do I force a particular instance of a C++ template to instantiate. Blog Archive » The Curiously Recurring Template Pattern in C++ C++ provides pretty good support for polymorphism by means of virtual functions. This is dynamic polymorphism (or runtime polymorphism), since the actual function to be called is resolved at runtime. It’s usually implemented by adding a hidden pointer in every object of a class with virtual functions. The pointer will point for any given object at the actual functions to call for it, so even when the compiler only knows this object through a pointer to a base class, it can generate correct code. The problem with dynamic polymorphism is its runtime cost. Extra indirection (pointer dereference) for each call to a virtual method.Virtual methods usually can’t be inlined, which may be a significant cost hit for some small methods.Additional pointer per object. Although in general dynamic polymorphism is a great tool, due to the aforementioned costs some applications prefer not to use it, at least for some performance-critical classes.
And a simple derived class: Related posts: Blog Archive » C++ template syntax patterns. I’ve used templates in my C++ code, but never in a very "hard-core" way. I don’t think I ever used partial specialization or template members, for instance. Definitely not metaprogramming. I guess these techniques just aren’t in my 20% subset of C++, and I feel good about it, since they’re not in most people’s 20% subset of C++. However, I do sometimes run into complex template constructs in code I want to understand, and not even grokking the syntax of what I’m seeing is kinda humiliating. So this article will fix this. This is not one of my usual articles – I don’t intend to teach anything here. Class templates and function templates For the sake of completeness, the basics of templates: template <typename T> class Array { ... // blah blah int len() const; }; This is a class template. Template <typename T> int Array<T>::len() const { ... } When instantiated, the template parameter must be made explicit: Array<int> int_arr; Array<Person*> people; The following is a function template: Take:
C++ - template class, friend operator << overload. A Description of the C++ typename keyword. The purpose of this document is to describe the reasoning behind the inclusion of the typename keyword in standard C++, and explain where, when, and how it can and can't be used. Note: This page is correct (AFAIK) for C++98/03. The rules have been loosened in C++11. Table of contents A secondary use There is a use of typename that is entirely distinct from the main focus of this discussion. I will present it first because it is easy.
It seems to me that someone said "hey, since we're adding typename anyway, why not make it do this" and people said "that's a good idea. " Most older C++ books, when discussing templates, use syntax such as the following: template <class T> ... I know when I was starting to learn templates, at first I was a little thrown by the fact that T was prefaced by class, and yet it was possible to instantiate that template with primitive types such as int. Template <typename T> ... This means exactly the same thing as the previous instance. The real reason for typename. Template metaprogramming. Template metaprogramming was, in a sense, discovered accidentally: see History of TMP. [edit] Template metaprogramming is generally Turing-complete, meaning that any computation expressible by a computer program can be computed, in some form, by a template metaprogram.
[edit] Though the syntax of template metaprogramming is usually very different from the programming language it is used with, it has practical uses. Some common reasons to use templates are to implement generic programming (avoiding sections of code which are similar except for some minor variations) or to perform automatic compile-time optimization such as doing something once at compile time rather than every time the program is run — for instance, by having the compiler unroll loops to eliminate jumps and loop count decrements whenever the program is executed. Compile-time class generation[edit] The code above will execute at run time to determine the factorial value of the literals 4 and 0. Static polymorphism[edit]
Warning - declares a non-template functi. [C++] Templates for setters and getters. C++ - Template Class – Symbols not found. Template, Forward Class Declaration and Typedef. Now that I'm back to the C++ wonder land, it's time to re-familiarize myself with the basic. In C++, ideally, you should "forward declare" a class and only include its full declaration when necessary, since this would minimize the dependency relationship between files and cut down build time: class A; ... void foo(const A& a); But things can quickly get problematic when there is typedef involved, for example in the previous case, if A is later typedefed as: typedef B A; Compiler will complaint that A is defined in two places with different type. Class B; //forward declaration of B first typedef B A; //now typedef A ... void foo(const A& a); But what if B is a template, something like: typedef vector<B> A; //now typedef A Unfortunately, I have yet find a way to make this work...
Forward declaration of a template class?