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Function

本文介绍C++中function的概念，以及与此相关的内容。C++是复杂的，这体现在它的function上，我们将看到C++中function的内容是非常繁杂的。

Classification of function

本节描述C++ function的分类:

1) 基于programming paradigm

上面的分类方法是采用的在C++\Language-reference\Basic-concept\index.md#Extend to OOP中总结的思路:

首先描述Non-OOP，然后描述OOP

类别 章节
OOP Member functions C++\Language-reference\Classes\Members\Non-static-member\Function-member
Non-OOP free function

2) 是否有名

类别 章节
named function
unnamed function lambda-expressions C++\Language-reference\Functions\Lambda-expression

3) 其他

Friend function

cppreference Functions

Functions are C++ entities that associate a sequence of statements (a function body) with a name and a list of zero or more function parameters.

NOTE: 有一点需要注意的是，function的value category是lvalue，这在cppreference Value categories中是由特殊说明的，这是因为function满足: i特性，即"has identity"，参见C++\Language-reference\Expressions\Value-categories，但是"Functions are not objects"。

Termination

NOTE: 函数终止，参见./Function-termination章节。

A function can terminate by returning or by throwing an exception.

Coroutine is a kind of function

NOTE: 符合uniform function model，参见 C++\Language-reference\Basic-concept\index

暂时还没有学习coroutine。

Function declaration and function definition

NOTE: 参见C++\Language-reference\Functions\Function-declaration。

将对能够用于function declaration的所有specifier进行说明。

Functions are not objects

NOTE: 在C++\Language-reference\Basic-concept\index的"Thoughts"段中也进行了说明。

Functions are not objects: there are no arrays of functions and functions cannot be passed by value or returned from other functions. Pointers and references to functions are allowed, and may be used where functions themselves cannot.

NOTE: 由于function is not object，因此在C++中，function is not first class citizen，但是C++中，提供了walkround让可以间接地实现: function is not first class citizen，这将在后面进行详细说明。

Implicit conversions#Function to pointer

Function overloading

NOTE: 这是C++的重要特性，参见C++\Language-reference\Functions\Function-overload

Function type

NOTE: 函数也是有类型的

Each function has a type, which consists of the function's return type, the types of all parameters (after array-to-pointer and function-to-pointer transformations, see parameter list) , whether the function is noexcept or not (since C++17), and, for non-static member functions, cv-qualification and ref-qualification (since C++11).

NOTE: 上面这段话的意思是，function type是由如下组成的:

  • return type
  • types of all parameters
  • whether the function is noexcept or not (since C++17)
  • for non-static member functions, cv-qualification and ref-qualification (since C++11) (free function是没有cv-qualification和ref-qualification的)

需要注意的是: function overload是不考虑return type的，这和function type不同。

Function types also have language linkage.

NOTE: 关于language linkage，参见C-and-C++\From-source-code-to-exec\Link\Linkage\Language-linkage。

There are no cv-qualified function types (not to be confused with the types of cv-qualified functions such as int f() const; or functions returning cv-qualified types, such as std::string const f();). Any cv-qualifier is ignored if it is added to an alias for a function type.

NOTE: 对于function而言，它的CV是完全没有语义的，我们需要从"Function and data model"来进行理解，function位于memory中，它们对应的是instruction，显然，修改instruction是没有任何意义的。

Function objects

Besides function lvalues, the function call expression supports pointers to functions, and any value of class type that overloads the function-call operator(functor) or is convertible to function pointer (including lambda-expressions) (since C++11). Together, these types are known as *FunctionObject*s, and they are used ubiquitously(无所不在) through the C++ standard library, see for example, usages of BinaryPredicate and Compare.

NOTE:

在C++\Language-reference\Functions\First-class-function\Function-objects章节中对function object进行了专门讨论。

在C++\Language-reference\Functions\First-class-function\Function-objects\Functor章节中对functor进行了专门讨论。

Function call expression

本节讨论C++中支持function call expression的，它包括:

类别 explanation 章节
function lvalue function is not object，就是我们平时所说的function
reference to function reference不是object，所以它无法归入function object中
function object cppreference FunctionObject，上面这段中列出的:
- pointers to functions
- functor		 Function objects

Function call expression and Invokation expression

上面总结了C++ function call expression支持的类型，虽然它们都能够用于function call expression，但是它们的syntax并不相同，即不是uniform(主要是pointer to member function)，因此这就造成了generic programming的时候，需要由programmer进行abstraction和implementation，造成programmer的负担；C++17提出的callable named requirement解决了这个问题，在其中将uniform invokation interface，称为Invokation expression；参见C++\Language-reference\Functions\Generic-programming\Callable章节。

Function lvalue and reference to function and function object

它们的相同点是: 都能够用于 Function call expression，不同点，在 cppreference C++ named requirements: FunctionObject 中进行了描述:

Functions and references to functions are not function object types, but can be used where function object types are expected due to function-to-pointer implicit conversion.