CODEWITHSUNDEEP
stringqtpointerspass-by-reference
yan bellavance
yan bellavance

Reputation: 4840

Whats the best way to send QStrings in a function call?

I would like to know what is the most efficient and practical way of sending a Qstring as a parameter to a function, in QT more specifically. I want to use a reference. The problem is I also want to instantiate that string in the function itself like so for example:

this is the function prototype: 

void myFunction(QString & theMsg);

this is the function call: 

myFunction(tr("Hello StringWorld"));

now the function tr() returns a QString but it doesn't work with a reference(I can see why).

I have to do this:

QString theQstr("Hello StringWorld"); 
myFunction(theQstr);

Is there a simpler way to do this while still using references or could I just change the function parameter to use a QString and it would still be efficient?

Upvotes: 23

Views: 17177

Answers (3)

Philippe
Philippe

Reputation: 442

When you pass a QString, the call site has to call QString copy ctor then a dtor: both implies an atomic reference counting operation (not neglectable), and some more generated assembly code. Hence slower and bigger code (I don't mention here the less common std::move scenario).

On the other hand, when you pass a const QString&, on the called site, there is a double indirection to access the characters: a pointer to a pointer. Hence this is slower than passing a QString, especially if the QString parameter is much used.

I would recommend to always pass a const QString&, and if you need maximum speed on the called side, make a QString copy there, and access this local copy to avoid a double-indirection (faster, less generated code).

Upvotes: 2

rpg
rpg

Reputation: 7787

The most efficient and practical way is using a const reference. The QString COW will be slower than pass by reference but faster than a regular copy.

Upvotes: 10

Morten Fjeldstad
Morten Fjeldstad

Reputation: 903

QString uses COW (Copy On Write) behind the scenes, so the actual string isn't copied even if you use a signature like this:

void myFunction(QString theMsg)

(until you modify it that is).

If you absolutely want a reference I would use a const& unless you plan to modify the input argument.

void myFunction(QString const& theMsg)

Upvotes: 16

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