CODEWITHSUNDEEP
qtqt5
ilya1725
ilya1725

Reputation: 4938

QString assignment releases allocation

I have a simple QT code that is called very often. It has to process some data and then format it into a QString that is sen sent to a QwtText object. Right now the function every time creates a QString object with all the dynamic memory allocation work. Then this object is destroyed and memory deallocated after the function is done.

I'm trying to optimize the code by creating a local class QString variable that would hold this formatted string. The idea is to prevent repeated malloc/free calls. However, right after the first string assignment it appears that the QString object frees and allocates the memory again, judging by the number returned by int QString::capacity() link.

m_valuesLabelText.clear();
// Capacity is 1011
m_valuesLabelText += "<table width=50>";
// Capacity is 16

Is there any way to prevent this re-allocation and convince QString to reuse the old buffer?

Thank you.

Upvotes: 1

Views: 584

Answers (2)

NP Rooski Z
NP Rooski Z

Reputation: 3657

QString & oftenCalledFunction(BS *bs, QString &p)
{
   p += "assignNew";
}

void func()
{
   QString p(MAX_SIZE); //set MAX_SIZE appropriately.
   BS bs;
   p = oftenCalledFunction(&bs, p);
}

This way you allocate on stack, avoiding malloc calls. Does this help? Malloc is not very bad though, it doesnt straightaway release the memory back to the system. Most free store library internally implement some kind of pool allocation strategy.

Upvotes: 0

user9088793
user9088793

Reputation:

QString::clear deallocates, as you can see by reading the source, e.g. here.

QString::resize does not deallocate, so it can be a solution to your problem: yourString.resize(0).

Use QString::reserve to allocate a suitable buffer.

Upvotes: 2

Related Questions