如何将QString初始化为null? [英] how to initialize a QString to null?

问题描述

QString :: number(0)和((const char *)0)有什么区别?

我想将 phoneNumber 的 QString 初始化为 null . phoneNumber(QString :: number(0))和 phoneNumber((const char *)0)都可以吗?

解决方案

要创建一个空的 QString ，只需默认对其进行初始化:

  QString phoneNumber;//，或者如果您已经有一个QString变量并想清除"它:phoneNumber = QString(); 

请注意， QString :: number(0)绝对为 not null-它会创建一个值为"0" 的QString./p>

您还可以使用 NULL 指针初始化 QString ，但是除非您传递一个指针，无论它是否为 NULL，否则我不建议这样做(不是)(即有时可能指向C字符串)，因为它是不必要的.

您还应该了解遵循Qt文档:

空字符串与空字符串的区别

出于历史原因， QString 区分空字符串和一个空字符串.空字符串是已初始化的字符串使用 QString 的默认构造函数或通过传递(const char *)0 到构造函数.空字符串是任何大小为0的字符串.null字符串始终为空，但空字符串不一定为null:

  QString().isNull();//返回trueQString().isEmpty();//返回trueQString(").isNull();//返回falseQString(").isEmpty();//返回trueQString("abc").isNull();//返回falseQString("abc").isEmpty();//返回false 

除 isNull()之外的所有函数都将空字符串视为空字符串.例如， toAscii().constData()返回一个指向'\ 0'字符表示空字符串(不是空指针)，并且 QString()比较等于 QString(").我们建议您始终使用 isEmpty()函数，并避免 isNull().

What is the difference between QString::number(0) and ((const char*) 0)?

I want to initialize a QString say phoneNumber to null. Will phoneNumber(QString::number(0)) and phoneNumber((const char*) 0) both work?

解决方案

To create a null QString just default initialize it:

QString phoneNumber;

// or if you already have a QString variable and want to 'clear' it:

phoneNumber = QString();

Note that QString::number(0) is decidedly not null - it creates a QString with the value "0".

You could also initialize the QString with a NULL pointer, but I wouldn't recommend it unless you're passing a pointer regardless of whether it's NULL or not (i.e., it could sometimes point to a C string) since it's unnecessary.

You should also understand the following Qt docs:

Distinction Between Null and Empty Strings

For historical reasons, QString distinguishes between a null string and an empty string. A null string is a string that is initialized using QString's default constructor or by passing (const char *)0 to the constructor. An empty string is any string with size 0. A null string is always empty, but an empty string isn't necessarily null:

QString().isNull();               // returns true
QString().isEmpty();              // returns true

QString("").isNull();             // returns false
QString("").isEmpty();            // returns true

QString("abc").isNull();          // returns false
QString("abc").isEmpty();         // returns false

All functions except isNull() treat null strings the same as empty strings. For example, toAscii().constData() returns a pointer to a '\0' character for a null string (not a null pointer), and QString() compares equal to QString(""). We recommend that you always use the isEmpty() function and avoid isNull().

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