c++ - 类型转换 char* -> QString，可读性还是清晰度？ (C++/Qt)

Question

我正在尝试将 testclasses 添加到现有的 Qt 项目中，我遇到了这个问题:

void doSomething (QString str, int i) {..}

int main () {
    //Do I use
    doSomething("string", 0);
    //Or
    doSomething(QString("string"), 0);
}

如果我没记错的话，它在内部做同样的事情，因为它使用 QString(char* c) 构造函数隐式转换 char*。

但哪种方式更受欢迎？

我个人喜欢隐式转换，因为它比构造函数调用更容易阅读，但在我阅读的 C++ 书中，应尽可能避免隐式转换。

Answer 1

实际上，如果您使用的是 Qt5，更好的方法是 QStringLiteral()。它不仅表明了意图(编译时常量 QString)，而且(略微)提高了效率，因为不需要从某些多字节编码到 UTF-16 的运行时转换。

如果需要使用Qt4，那么有条件的自己定义:

#ifndef QStringLiteral
#define QStringLiteral(x) (QString::fromUtf8(x))
#endif

或者按照 Kuba Ober 的评论，添加

lessThan(QT_MAJOR_VERSION, 5): DEFINES += QStringLiteral=QString::fromUtf8

到您的 .pro 文件。

引自Qt文档:

The macro generates the data for a QString out of str at compile time if the compiler supports it. Creating a QString from it is free in this case, and the generated string data is stored in the read-only segment of the compiled object file.

For compilers not supporting the creation of compile time strings, QStringLiteral will fall back to QString::fromUtf8().

If you have code looking like:

 if (node.hasAttribute("http-contents-length")) //... 

One temporary QString will be created to be passed as the hasAttribute function parameter. This can be quite expensive, as it involves a memory allocation and the copy and the conversion of the data into QString's internal encoding.

This can be avoided by doing

if (node.hasAttribute(QStringLiteral("http-contents-length"))) //...

Then the QString's internal data will be generated at compile time and no conversion or allocation will occur at runtime

Using QStringLiteral instead of a double quoted ascii literal can significantly speed up creation of QString's from data known at compile time.

If the compiler is C++11 enabled the string str can actually contain unicode data.