Is there a performance difference between i++ and ++i in C++?

Question

We have the question is there a performance difference between i++ and ++i in C?

What\'s the answer for C++?

Answer 1

Here's a benchmark for the case when increment operators are in different translation units. Compiler with g++ 4.5.

Ignore the style issues for now

// a.cc
#include 
#include 
class Something {
public:
    Something& operator++();
    Something operator++(int);
private:
    std::array data;
};

int main () {
    Something s;

    for (int i=0; i<1024*1024*30; ++i) ++s; // warm up
    std::clock_t a = clock();
    for (int i=0; i<1024*1024*30; ++i) ++s;
    a = clock() - a;

    for (int i=0; i<1024*1024*30; ++i) s++; // warm up
    std::clock_t b = clock();
    for (int i=0; i<1024*1024*30; ++i) s++;
    b = clock() - b;

    std::cout << "a=" << (a/double(CLOCKS_PER_SEC))
              << ", b=" << (b/double(CLOCKS_PER_SEC)) << '\n';
    return 0;
}

---

O(n) increment

Test

// b.cc
#include 
class Something {
public:
    Something& operator++();
    Something operator++(int);
private:
    std::array data;
};


Something& Something::operator++()
{
    for (auto it=data.begin(), end=data.end(); it!=end; ++it)
        ++*it;
    return *this;
}

Something Something::operator++(int)
{
    Something ret = *this;
    ++*this;
    return ret;
}

Results

Results (timings are in seconds) with g++ 4.5 on a virtual machine:

Flags (--std=c++0x)       ++i   i++
-DPACKET_SIZE=50 -O1      1.70  2.39
-DPACKET_SIZE=50 -O3      0.59  1.00
-DPACKET_SIZE=500 -O1    10.51 13.28
-DPACKET_SIZE=500 -O3     4.28  6.82

---

O(1) increment

Test

Let us now take the following file:

// c.cc
#include 
class Something {
public:
    Something& operator++();
    Something operator++(int);
private:
    std::array data;
};


Something& Something::operator++()
{
    return *this;
}

Something Something::operator++(int)
{
    Something ret = *this;
    ++*this;
    return ret;
}

It does nothing in the incrementation. This simulates the case when incrementation has constant complexity.

Results

Results now vary extremely:

Flags (--std=c++0x)       ++i   i++
-DPACKET_SIZE=50 -O1      0.05   0.74
-DPACKET_SIZE=50 -O3      0.08   0.97
-DPACKET_SIZE=500 -O1     0.05   2.79
-DPACKET_SIZE=500 -O3     0.08   2.18
-DPACKET_SIZE=5000 -O3    0.07  21.90

---

Conclusion

Performance-wise

If you do not need the previous value, make it a habit to use pre-increment. Be consistent even with builtin types, you'll get used to it and do not run risk of suffering unecessary performance loss if you ever replace a builtin type with a custom type.

Semantic-wise

• i++ says increment i, I am interested in the previous value, though.
• ++i says increment i, I am interested in the current value or increment i, no interest in the previous value. Again, you'll get used to it, even if you are not right now.

Knuth.

Premature optimization is the root of all evil. As is premature pessimization.