CODEWITHSUNDEEP
c++qtvector
Vahagn Avagyan
Vahagn Avagyan

Reputation: 776

QVector spends different times when adding an element

I have a very simple code, I noticed that if I add values ​​to QVector constants, then I spend a lot of time than if I add sequential knowledge

code example:

QVector<int> intVector;

QTime myTimer;
myTimer.start();
for (int i = 0; i < 8500000; ++i) {
    intVector.push_back(i);
}
qDebug()<< myTimer.elapsed(); // output: ~297 ms

but if i add a constant value i get this

code example:

QVector<int> intVector;
    
QTime myTimer;
myTimer.start();
for (int i = 0; i < 8500000; ++i) {
    intVector.push_back(1); 
}
qDebug()<< myTimer.elapsed(); // output: ~344 ms

tested on Qt_5_15_0_MSVC2019_32bit.

What explains this?

Upvotes: 1

Views: 58

Answers (1)

Botje
Botje

Reputation: 30840

Don't use QTimer to do microbenchmarks, use the google-benchmark library instead.

On my system (M1 mac) both loops are equal to within 0,04%:

------------------------------------------------------
Benchmark            Time             CPU   Iterations
------------------------------------------------------
BM_constant    1211287 ns      1211286 ns          510
BM_i           1211735 ns      1211229 ns          573

Code:

#include <benchmark/benchmark.h>
#include <QtCore/QVector>

static void BM_constant(benchmark::State& state) {
    for (auto _ : state) {
        QVector<int> intVector;
        for (int i = 0; i < 1000000; ++i) {
            intVector.push_back(1);
        }
    }
}
BENCHMARK(BM_constant);

static void BM_i(benchmark::State& state) {
    for (auto _ : state) {
        QVector<int> intVector;
        for (int i = 0; i < 1000000; ++i) {
            intVector.push_back(i);
        }
    }
}
BENCHMARK(BM_i);

BENCHMARK_MAIN();

Upvotes: 1

Related Questions