为什么我可以将字符串文字初始化为const char *和QString而不是QString *？

Question

I am relatively new to qt and C++, and I am having trouble understanding why I can declare the following in a header file:

const char* CONSIGNMENT_TAG_DONATE = "DTag";

or

QString CONSIGNMENT_TAG_DONATE = "DTag";

or

const QString CONSIGNMENT_TAG_DONATE = "DTag";

But when I try to use QString*

QString* CONSIGNMENT_TAG_DONATE = "DTag";

or

const QString* CONSIGNMENT_TAG_DONATE = "DTag";

I get an error saying cannot initialize a member subobject of type 'QString*' with an lvalue of type 'const char[5]' . It's my understanding that string literals are of a type const char* so I get why that works. But why is assigning it to QString valid but QString* not valid? I'm sure this is a fairly basic concept of C++, I am just trying to understand it.

Answer 1

A QString* is a pointer to QString . That means it can hold the address of a QString object. You cannot (or at least are not supposed to most of the time) store addresses of non- QString in a QString* .

const char* CONSIGNMENT_TAG_DONATE = "DTag";

works because the "DTag" (which is of type const char[5] ) decays to const char* , thus the assignment makes sense.

QString CONSIGNMENT_TAG_DONATE = "DTag";

works because QString s have a non-explicit constructor taking const char* , so the literal can again decay to const char* which is then used to construct the QString .

QString* CONSIGNMENT_TAG_DONATE = "DTag";

is just nonsense, see first paragraph.

Using a QString* instead of a plain QString is most likely not helpful for your usecase.

Answer 2

C++ doesn't know how to convert a char array to a pointer to QString .

const char* CONSIGNMENT_TAG_DONATE = "DTag";

Constant char array to const char * - permitted

QString CONSIGNMENT_TAG_DONATE = "DTag";

Implicit conversion using the QString::QString(const char *str) constructor

QString* CONSIGNMENT_TAG_DONATE = "DTag";

Constant char array to QString * - how does this work? There are no rules to make such a conversion. It may be simple enough conceptually, but without the actual code to enable it, it will not work.