Understanding and Using Angle Brackets in C: A Comprehensive Guide

Angle brackets, denoted as , are a fundamental aspect of the C programming language, particularly when it comes to template specialization and handling various template-related features. This article delves into the significance and usage of angle brackets in C, addressing questions related to template specializations and providing detailed examples and explanations to help you master this concept.

Introduction to Angle Brackets in C

In the context of C programming, angle brackets () are often associated with templates and type parameters. However, it's important to note that angle brackets are not a single token but rather represent a textual construct formed by the juxtaposition of two tokens, typically a template name followed by its arguments.

Explicit Template Specialization

One of the primary uses of angle brackets is in explicit template specialization. Template specialization allows you to provide a definition that supersedes the generic definition of a template for a particular set of template arguments. This is achieved using the keyword, which is often written without an intervening space: .

Example of Explicit Template Specialization

Consider the following example:

template typename T struct One {  // Primary template    T const get const { return element; }    T get { return element; }private:    T element;}template  struct Onevoid {  // Explicit specialization    void get const {}}

In this example, the primary template does not work when T void. However, by using explicit specialization, the void case is covered. This construct adheres to the pattern you mentioned with angle brackets surrounding the template name and arguments.

Empty Template Argument Lists

Angle brackets can also be used to represent an empty list of template arguments. This is possible for several reasons:

Default Arguments

Consider a template defined with default arguments:

template typename T  void struct One { … }

You can then write:

One ov  // Equivalent to Onevoid ov

This construct also follows the pattern you mentioned with angle brackets surrounding an empty list.

Variadic Parameters

Since C11, templates can have an arbitrary number of arguments, which can be represented using parameter packs:

template typename ... Ts struct TypeSeq {}

A zero-length sequence of type arguments can be substituted:

TypeSeq  empty  // Okay.

This example also adheres to the pattern you mentioned with angle brackets around an empty list.

Deduction

Template arguments can be deduced from the context of use, such as in a function call:

template typename T T f() { … }double d  f(0.0);  // Calls with T  double.

However, you can still provide an empty set of angle brackets, which is useful for ensuring a specific template is chosen when multiple templates are available:

template typename T int f(T);int i1  f(42);  // Calls non-template  i2  f(42);  // Calls with T  int.

This construct again follows the pattern you mentioned with angle brackets around an empty list.

Exotic Uses of Angle Brackets

There are various other ways to use angle brackets in C, such as in contexts where angle brackets might seem unexpected. Below are a couple of examples:

Pointer-to-Member as Template Argument

struct S {    bool operatorS(S) const { return *this  *arg; }}templatebool S::S struct X {};X S::operatorS();

This construct uses a pointer-to-member as a non-type template argument immediately followed by the closing angle bracket.

Overloaded Operator with Angle Brackets

struct S {    S operatorS() const { return *this; }    bool operatorbool(S) const { return true; }}bool operatorbool(S::S) S;bool r  S::operatorbool();

This example again uses a pointer-to-member as an operand for an overloaded operator, adhering to the pattern you mentioned with angle brackets surrounding an empty list.

Conclusion

Understanding and utilizing angle brackets effectively in C is crucial for working with templates. Whether it's for explicit specialization, empty template argument lists, or exotic uses, angle brackets give you the flexibility and power to create flexible and efficient code.