今天跟朋友谈到了JavaScript里面String和对象的问题，他们说String是primitive value，所以String存储的字符串是在stack上分配的。之后给我截了个图：

10.3 Execution Contexts

When control is transferred to ECMAScript executable code, control is entering an execution context. Active execution contexts logically form a stack. The top execution context on this logical stack is the running execution context. A new execution context is created whenever control is transferred from the executable code associated with the currently running execution context to executable code that is not associated with that execution context. The newly created execution context is pushed onto the stack and becomes the running execution context.

4.3.2 primitive value

member of one of the types Undefined, Null, Boolean, Number, or String as defined in Clause 8

NOTE A primitive value is a datum that is represented directly at the lowest level of the language implementation.

4.3.16 String value

primitive value that is a finite ordered sequence of zero or more 16-bit unsigned integer

NOTE A String value is a member of the String type. Each integer value in the sequence usually represents a single 16-bit unit of UTF-16 text. However, ECMAScript does not place any restrictions or requirements on the values except that they must be 16-bit unsigned integers.

4.3.17 String type

set of all possible String values

4.3.18 String object

member of the Object type that is an instance of the standard built-in String constructor

NOTE A String object is created by using the String constructor in a new expression, supplying a String value as an argument. The resulting object has an internal property whose value is the String value. A String object can be coerced to a String value by calling the String constructor as a function (15.5.1).

8 Types

Algorithms within this specification manipulate values each of which has an associated type. The possible value types are exactly those defined in this clause. Types are further subclassified into ECMAScript language types and specification types.

An ECMAScript language type corresponds to values that are directly manipulated by an ECMAScript programmer using the ECMAScript language. The ECMAScript language types are Undefined, Null, Boolean, String, Number, and Object.

...

8.4 The String Type

The String type is the set of all finite ordered sequences of zero or more 16-bit unsigned integer values (“elements”). The String type is generally used to represent textual data in a running ECMAScript program, in which case each element in the String is treated as a code unit value (see Clause 6). Each element is regarded as occupying a position within the sequence. These positions are indexed with nonnegative integers. The first element (if any) is at position 0, the next element (if any) at position 1, and so on. The length of a String is the number of elements (i.e., 16-bit values) within it. The empty String has length zero and therefore contains no elements.

When a String contains actual textual data, each element is considered to be a single UTF-16 code unit. Whether or not this is the actual storage format of a String, the characters within a String are numbered by their initial code unit element position as though they were represented using UTF-16. All operations on Strings (except as otherwise stated) treat them as sequences of undifferentiated 16-bit unsigned integers; they do not ensure the resulting String is in normalised form, nor do they ensure language-sensitive results.

NOTE: The rationale behind this design was to keep the implementation of Strings as simple and high-performing as possible. The intent is that textual data coming into the execution environment from outside (e.g., user input, text read from a file or received over the network, etc.) be converted to Unicode Normalised Form C before the running program sees it. Usually this would occur at the same time incoming text is converted from its original character encoding to Unicode (and would impose no additional overhead). Since it is recommended that ECMAScript source code be in Normalised Form C, string literals are guaranteed to be normalised (if source text is guaranteed to be normalised), as long as they do not contain any Unicode escape sequences.


11.9.6 The Strict Equality Comparison Algorithm

The comparison x === y, where x and y are values, produces true or false. Such a comparison is performed as follows:

If Type(x) is different from Type(y), return false.
If Type(x) is Undefined, return true.
If Type(x) is Null, return true.
If Type(x) is Number, then
If x is NaN, return false.
If y is NaN, return false.
If x is the same Number value as y, return true.
If x is +0 and y is −0, return true.
If x is −0 and y is +0, return true.
Return false.
If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions); otherwise, return false.
If Type(x) is Boolean, return true if x and y are both true or both false; otherwise, return false.
Return true if x and y refer to the same object. Otherwise, return false.
NOTE This algorithm differs from the SameValue Algorithm (9.12) in its treatment of signed zeroes and NaNs.

> s = "abc"
"abc"
> a = new String(s)
String {0: "a", 1: "b", 2: "c"}
> b = new String(s)
String {0: "a", 1: "b", 2: "c"}
> a === b
false

local a = "f".."oo"
local b = "fo".."o"
print(a == b)    -- prints: true

A string looks like this:

                                     [_Rep]
                                     _M_length
[basic_string<char_type>]            _M_capacity
_M_dataplus                          _M_refcount
_M_p ---------------->               unnamed array of char_type

#include <string>
#include <iostream>
using namespace std;

int main() {
  string foo("foo");   // (1)
  string goo = foo;    // (2)
  goo[0] = 'g';        // (3)
  cout << foo << endl;
  cout << goo << endl;
  return 0;
}