QStringLiteral(str)

When looking at the project code , You will always see the following

QString str = QStringLiteral("123rt");

QString use QStringLiteral(str) To initialize the , A little curious , Just check it out , Make a note of .

This is the use of QStringLiteral initialization QString Why ：

Use QStringLiteral Macros can convert constant strings in code at compile time str Directly constructed as QString object , therefore The runtime does not require additional construction overhead .

Here we add an implicit knowledge sharing ：

Implicit sharing runs automatically in the background , So we don't have to write any more code to make this optimization process happen .

Illustrate with examples ：

QString srr1 = "happy";

QString str2 = str1;

here QString Two objects of str1 and str2 All point to the same internal data structure in memory . With character data , The data structure holds a Reference count , To indicate how many QString Point to the same data structure . because str1 and str2 All point to the same data , So the value of the reference count is 2

str2[0] = 'D';

When modifying str2 when , It starts with Deep replication of data , In order to ensure that str1 and str2 Point to different data structures , Then the new value is applied to the data he copies . here str1 and str2 The reference count of all becomes 1 了 , The reference count becomes 1 Indicates that the data is not shared .

str.truncate(4);

If you modify it again str2, because str2 Of Data reference count is 1, Data replication will not occur ,truncate() Function directly to str2 Data to operate , String becomes Dappy, The reference count remains 1.

str1 = str2;

When will str2 Assign to str1 when ,str1 Of The data reference count drops to 0, This means that no one QString Still in use “happy” data . This frees the data from memory . Two QString All point to “Dappy”, Now its reference count is 2 了 .

Due to race in reference count , Data sharing is usually used as an option in multithreaded programs without attention .