Question

In learning c++, I first use Qt library instead of the standard C++, STL and all that (Ok, so I’m new with c++ and spoiled by Qt). On Qt, QString used implicit sharing, thus enabling me to just copy assign it to another variable like:

QString var1=QString("Hi there!");
QString var2=var1

And that would do nicely without much overhead. But now, i’m trying std::string so, should I do

std::string var1=std::string()

or

std::string* var1=new std::string()

And also, how about QVector and std::vector. And If I do have to use the pointer… any tips?

Answer 1

Whether std::string uses copy-on-write depends on the implementation (i.e. your standard library vendor decides that). However, most std::string implementations will not use COW, largely due to the fact that most if not all read operations force a copy — operator[] returns a reference, c_str() and data() return a pointer. Compare this to QString::operator[], which returns a proxy object.

In spite of the above, don’t use pointers to std::string, unless you determine (by measuring) that string copies are the bottleneck in your application.

Also, beware that QString stores UTF-16 strings, whereas std::string stores a sequence of chars — QByteArray would be the Qt equivalent.