Recurring C++ and Qt anti-patterns

J.Hilk

@aha_1980
well, I would give the creator here the benefit of the doubt and they that's refactored code and used to be p->deleteLater() :-)

aha_1980

Hi @J.Hilk,

Yeah, but in that case it was pure C++ without Qt. Otherwise you would be right.

fcarney

@aha_1980 said in Recurring C++ and Qt anti-patterns:

because delete does the check anyway

Wait, what? Since when does C++ check if the pointer is nullptr before delete? I didn't know this was a thing now.

Also, is deleteLater not a way to delete Qt objects? Its morning and I haven't had my coffee.

fcarney

@fcarney said in Recurring C++ and Qt anti-patterns:

Wait, what? Since when does C++ check if the pointer is nullptr before delete?

Wow, since 2003? Lol, keep this thread going! I am learning a lot.

fcarney

Okay, I am just confusing myself. If you delete a pointer you must immediately set it to null. Otherwise you risk double delete, which is bad. But its okay to delete something set to null. Got it.

Edit:
Why doesn't delete set the pointer to null then? That seems like it may be an antipattern in and of itself.

aha_1980

@fcarney said in Recurring C++ and Qt anti-patterns:

Why doesn't delete set the pointer to null then? That seems like it may be an antipattern in and of itself.

I have indeed asked that myself. If someone has the correct answer for that, I'm all ears.

fcarney

@aha_1980
Apparently the standard allows for it:
https://stackoverflow.com/questions/704466/why-doesnt-delete-set-the-pointer-to-null

The creator himself wonders why it isn't so. Its like C++ is this beautiful, amazing, and now, WILD animal roaming free in cyberspace... Yeah, maybe the analogy isn't all that great, but it does conjure up a cool picture.

ODБOï

@fcarney said in Recurring C++ and Qt anti-patterns:

Why doesn't delete set the pointer to null

likely because it would bring more problems than solutions

aha_1980

@LeLev

likely because it would bring more problems than solutions

That would mean, that this pointer shows to an invalid memory region after the delete. Can you think of an example where you still want to use that pointer afterwards? (That is a real question - because for now I have no idea).

fcarney

@LeLev said in Recurring C++ and Qt anti-patterns:

likely because it would bring more problems than solutions

I could see a case where a program is deleting thousands of pointers and there might actually be overhead in a mov instruction for each delete. I have no idea if this overhead would be significant over the delete operation, but it would still be overhead. It would not be that hard to test such a scenario. I should try it!

ODБOï

@aha_1980 said in Recurring C++ and Qt anti-patterns:

Can you think of an example where you still want to use that pointer afterwards?

not a real world application

fcarney

@fcarney said in Recurring C++ and Qt anti-patterns:

deleting thousands of pointers and there might actually be overhead

I cannot actually tell if the overhead in this code is the indexing of the array, or if the movement of data is significant. I tried doing a dummy no op index, but I am guessing it is being optimized out:

#include <QCoreApplication>
#include <QElapsedTimer>
#include <QDebug>

#define MEM_SEG_LEN 8
#define MEM_SEGS 100000000

char** createMemoryList(){
    char** list = new char*[MEM_SEGS];
    for(int index=0; index<MEM_SEGS; index++){
        list[index]=new char[MEM_SEG_LEN];
    }

    return list;
}

void deleteMemoryList(char** list){
    for(int index=0; index<MEM_SEGS; index++){
        delete list[index];
        list[index]; // can you force an index to occur?
    }
    delete list;
}

void deleteMemoryListNull(char** list){
    for(int index=0; index<MEM_SEGS; index++){
        delete list[index];
        list[index] = nullptr;
    }
    delete list;
    list = nullptr;
}

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QElapsedTimer timer1;

    char** list1 = createMemoryList();
    timer1.start();
    deleteMemoryList(list1);
    qInfo() << timer1.elapsed();

    QElapsedTimer timer2;

    char** list2 = createMemoryList();
    timer2.start();
    deleteMemoryListNull(list2);
    qInfo() << timer2.elapsed();

    return a.exec();
}

I get the following output:

813
1301

I doubt that is the overhead of the movement of null into the pointer. My guess is the the index overhead is in there too.

fcarney

I eliminated the extra index (probably compiler already did this before):

void deleteMemoryList(char** list){
    for(int index=0; index<MEM_SEGS; index++){
        char* tmp = list[index];
        delete tmp;
    }
    delete list;
}

void deleteMemoryListNull(char** list){
    for(int index=0; index<MEM_SEGS; index++){
        char* tmp = list[index];
        delete tmp;
        tmp = nullptr;
    }
    delete list;
    list = nullptr;
}

Results:

877
1369

Edit: Real world usage? I really highly doubt it. That is a LOT of iterations of delete. So I would say the extra cycles are negligible.

Edit2:
Pointer math:

void deleteMemoryList(char** list){
    for(int index=0; index<MEM_SEGS; index++){
        char** tmp = &(list[index]);
        delete *tmp;
    }
    delete list;
}

void deleteMemoryListNull(char** list){
    for(int index=0; index<MEM_SEGS; index++){
        char** tmp = &(list[index]);
        delete *tmp;
        *tmp = nullptr;
    }
    delete list;
    list = nullptr;
}

Results:

853
1307

Sometimes apples and apples is hard.

JonB

@fcarney
Since this is the lounge... Surprised by your findings (in earlier examples). What exactly is the difference in the assembly between the two versions? What is being generated for your tmp = nullptr;? (Not the later *tmp = nullptr;, that's different.)

fcarney

@JonB said in Recurring C++ and Qt anti-patterns:

tmp = nullptr;

I changed it to not update a local variable. char* tmp is local, so setting it to null is just setting a local variable to null. So it was setting the wrong area of memory to null. That is why I took the address of where that pointer is stored.

The assembler for tmp = nullptr in previous incarnation:

movq   $0x0,-0x8(%rbp)

The assembler for *tmp = nullptr in latest incarnation:

mov    -0x8(%rbp),%rax
movq   $0x0,(%rax)

But you are right, the tmp = nullptr is more representative.
The timing is not much different.

fcarney

Okay, I think I am done, but here is my last incarnation:

#include <QCoreApplication>
#include <QElapsedTimer>
#include <QDebug>

#define MEM_SEG_LEN 8
#define MEM_SEGS 100000000 // 846 1315 // assignment
#define MEM_SEGS 100000000 // 885 1349 // correct assignment
//#define MEM_SEGS 100

char** createMemoryList(){
    char** list = new char*[MEM_SEGS];
    for(int index=0; index<MEM_SEGS; index++){
        list[index]=new char[MEM_SEG_LEN];
    }

    return list;
}

void deleteMemoryList(char** list){
    for(int index=0; index<MEM_SEGS; index++){
        char** tmp = &(list[index]);
        delete *tmp;
    }
    delete list;
}

void deleteMemoryListNull(char** list){
    for(int index=0; index<MEM_SEGS; index++){
        char** tmp = &(list[index]);
        delete *tmp;
        *tmp = nullptr;
//        char* tmp = (list[index]);
//        delete tmp;
//        tmp = nullptr;
    }
    delete list;
    list = nullptr;
}

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    double time1, time2;

    qInfo() << QString(R"(Starting memory test 1 of not setting pointer to nullptr: %1 cycles)").arg(MEM_SEGS);

    QElapsedTimer timer1;

    char** list1 = createMemoryList();
    timer1.start();
    deleteMemoryList(list1);
    time1 = timer1.elapsed();
    qInfo() << time1/1000.0 << QString().number((time1/1000.0)/double(MEM_SEGS),10,12);

    qInfo() << QString(R"(Starting memory test 2 of setting pointer to nullptr: %1 cycles)").arg(MEM_SEGS);

    QElapsedTimer timer2;

    char** list2 = createMemoryList();
    timer2.start();
    deleteMemoryListNull(list2);
    time2 = timer2.elapsed();
    qInfo() << time2/1000.0 << QString().number((time2/1000.0)/double(MEM_SEGS),10,12);

    qInfo() << "Difference:" << QString().number((time2/1000.0)/double(MEM_SEGS)-(time1/1000.0)/double(MEM_SEGS),10,12);

    return a.exec();
}

So, 5 nanoseconds difference for a delete operation of dereffed pointer assignment:

"Starting memory test 1 of not setting pointer to nullptr: 100000000 cycles"
0.848 "0.000000008480"
"Starting memory test 2 of setting pointer to nullptr: 100000000 cycles"
1.321 "0.000000013210"
Difference: "0.000000004730"

Edit:
Math was wrong.

JonB

@fcarney
You're talking about a single movq. Just how big is MEM_SEGS? Your timings, are they in milliseconds?? And I assume list is filled with zeroes? I'm talking about your earlier barebones example, where your timings were
877
1369

Oh now I see more code in other examples. If your list contained 10 million news and you are deleteing them, common-sense should tell you the cost of whether or not you set one local variable to nullptr with just a mov instruction must be negligible, compared to whatever is involved in freeing memory, no?

fcarney

@JonB said in Recurring C++ and Qt anti-patterns:

one local variable to nullptr with just a mov instruction must be negligible, compared to whatever is involved in freeing memory, no?

Correct. My math was wrong. It is 5 nanoseconds. At least for dereffed pointer, but timing was nearly the same for local variable as well.

JonB

@fcarney
Ahhh, that would make much more sense! :)
A nanosecond doesn't sound too long, I don't think I could get much done in it could I?

aha_1980

One more example for the hall of shame:

QByteArray ba = "Hello World";
QString s = QString::fromStdString(ba.toStdString());

We have two problems here:

The conversion to and from std::string is unneeded
This only works for ASCII characters. QString::fromUtf8(ba); would most often be the correct choice, sometimes also QString::fromLocal8Bit(ba);