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Initialization

思路

C++的initialization不仅种类繁多而且规则冗杂，下面是比较好的整理思路:

首先需要区分每种initialization的syntax，因为compiler根据syntax来决定采用何种initialization；

其次是需要搞清楚每种initialization的**effect**，对effect的讨论主要集中在对下面三种type的effect:

type 注解
built-in type int、char等
OOP class type class
aggregate type array

各种initialization的差异重要体现在它们的**effect**上。

cppreference Initialization

Initialization of a variable provides its initial value at the time of construction.

NOTE: initialization发生在object construction阶段，关于object的lifetime，参见C++\Language-reference\Basic-concept\Data-model\Object\Object-storage-duration-and-lifetime。

Initializer

The initial value may be provided in the initializer section of a declarator or a new expression.

Terminology: initializer

"initializer"是cppreference中用来表示**初始值**的术语，在下面文章中使用了这个术语:

For each declarator, the initializer may be one of the following:

initializer explanation 注解
( expression-list ) comma-separated list of arbitrary expressions and braced-init-lists in parentheses 需要注意的是，必须是 braced-init-lists in parentheses
= expression
{ initializer-list } braced-init-list: possibly empty, comma-separated list of expressions and other braced-init-lists

Depending on context, the initializer may invoke:

initialization example 注解
Value initialization std::string s{};
Direct initialization std::string s("hello");
Copy initialization std::string s = "hello";
List initialization std::string s{'a', 'b', 'c'};
Aggregate initialization char a[3] = {'a', 'b'};
Reference initialization char& c = a[0];

If no initializer is provided, the rules of default initialization apply.

Non-local variables

NOTE: 参见C++\Language-reference\Basic-concept\Object\Lifetime-and-storage-duration\Static-storage-duration\Non-local-object

Static local variables

NOTE: 参见C++\Language-reference\Basic-concept\Object\Lifetime-and-storage-duration\Static-storage-duration\Static-local-object

Class members

Non-static data members can be initialized with member initializer list or with a default member initializer.

其他initialization

下面是一些其他initialization:

Zero initialization Sets the initial value of an object to zero.
Default initialization This is the initialization performed when a variable is constructed with no initializer. no initializer
Constant initialization Sets the initial values of the static variables to a compile-time constant.

Draft

先看看learncpp的1.4 — Variable assignment and initialization中关于initialization的介绍，因为其中结合了具体的示例，更加容易理解，然后再来看cppreference中对Initialization的更加标准化的定义。

learncpp 1.4 — Variable assignment and initialization

One downside of assignment is that it requires at least two statements: one to define the variable, and one to assign the value.

These two steps can be combined. When a variable is defined, you can also provide an initial value for the variable at the same time. This is called initialization.

C++ supports three basic ways to initialize a variable. First, we can do copy initialization by using an equals sign:

int width = 5; // copy initialization of value 5 into variable width

Much like copy assignment, this copies the value on the right-hand side of the equals to the variable being created on the left-hand side.

Second, we can do direct initialization by using parenthesis.

int width( 5 ); // direct initialization of value 5 into variable width

For simple data types (like integers), copy and direct initialization are essentially the same. But for some advanced types, direct initialization can perform better than copy initialization. Prior to C++11, direct initialization was recommended over copy initialization in most cases because of the performance boost.

TO READ

learncpp 1.6 — Uninitialized variables and undefined behavior

learncpp 8.5b — Non-static member initialization

learncpp 8.5a — Constructor member initializer lists

learncpp 11.4 — Constructors and initialization of derived classes

learncpp 10.7 — std::initializer_list

learncpp 9.12 — Copy initialization