C++ 如何将 QString 初始化为 null？
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how to initialize a QString to null?
提问by user1065969
What is the difference between QString::number(0)and ((const char*) 0)?
QString::number(0)和 和有((const char*) 0)什么区别？
I want to initialize a QStringsay phoneNumberto null. Will phoneNumber(QString::number(0))and phoneNumber((const char*) 0)both work?
我想初始化一个QString说phoneNumber来null。会phoneNumber(QString::number(0))和phoneNumber((const char*) 0)两者都有效吗？
回答by Michael Burr
To create a null QStringjust default initialize it:
要创建一个空值，QString只需默认初始化它：
QString phoneNumber;
// or if you already have a QString variable and want to 'clear' it:
phoneNumber = QString();
Note that QString::number(0)is decidedly notnull - it creates a QString with the value "0".
请注意，QString::number(0)它绝对不是null - 它创建了一个 QString 的值"0"。
You could also initialize the QStringwith a NULLpointer, but I wouldn't recommend it unless you're passing a pointer regardless of whether it's NULLor not (i.e., it could sometimes point to a C string) since it's unnecessary.
你也可以QString用一个NULL指针来初始化它，但我不会推荐它，除非你传递一个指针，不管它是否是NULL（即，它有时可能指向一个 C 字符串），因为它是不必要的。
You should also understand the following Qt docs:
您还应该了解以下 Qt 文档：
Distinction Between Null and Empty Strings
For historical reasons,
QStringdistinguishes between a null string and an empty string. A null string is a string that is initialized usingQString's default constructor or by passing(const char *)0to 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 falseAll 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), andQString()compares equal toQString(""). We recommend that you always use theisEmpty()function and avoidisNull().
空字符串和空字符串的区别
由于历史原因，
QString区分空字符串和空字符串。空字符串是使用QString的默认构造函数或通过传递(const char *)0给构造函数初始化的字符串。空字符串是大小为 0 的任何字符串。空字符串始终为空，但空字符串不一定为空：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除了
isNull()将空字符串视为空字符串之外的所有函数。例如，toAscii().constData()返回指向'\0'空字符串（不是空指针）的字符的指针，并QString()比较等于QString("")。我们建议您始终使用该isEmpty()功能并避免使用isNull().
回答by immerhart
#include <QCoreApplication>
#include <QString>
#include <QDebug>
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
QString name(QString::null);
// or QString name = QString::null;
// or QString name;
qDebug() << name.isNull();
qDebug() << name;
return a.exec();
}
Output:
输出：
true
""
Michael Burr`s solution is of course also correct... But I like the QString::nullmore.
Michael Burr 的解决方案当然也是正确的……但我QString::null更喜欢。
