Should I use pointer to std::string

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?

Solution 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.

Solution 2

std::string var1("Hi there!");
std::string var2=var1; 

std::string class has an = operator defined as:

string& operator= ( const string& str );

Solution 3

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

Don't do that. Just pass it by reference wherever possible:

void f(const std::string& s); // no copying

If you really need to share the string, use:

std::shared_ptr<std::string> var1 = std::make_shared<std::string>();

Solution 4

If you are going to return std::string from function then don't use pointer - return by value. In this case most likely Return Value Optimization will be applied and string data will not be copied.

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