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

[Executive Summary: Use ++i if you don't have a specific reason to use i++.]

For C++, the answer is a bit more complicated.

If i is a simple type (not an instance of a C++ class), then the answer given for C ("No there is no performance difference") holds, since the compiler is generating the code.

However, if i is an instance of a C++ class, then i++ and ++i are making calls to one of the operator++ functions. Here's a standard pair of these functions:

Foo& Foo::operator++()   // called for ++i
{
    this->data += 1;
    return *this;
}

Foo Foo::operator++(int ignored_dummy_value)   // called for i++
{
    Foo tmp(*this);   // variable "tmp" cannot be optimized away by the compiler
    ++(*this);
    return tmp;
}

Since the compiler isn't generating code, but just calling an operator++ function, there is no way to optimize away the tmp variable and its associated copy constructor. If the copy constructor is expensive, then this can have a significant performance impact.

Yes. There is.

The ++ operator may or may not be defined as a function. For primitive types (int, double, ...) the operators are built in, so the compiler will probably be able to optimize your code. But in the case of an object that defines the ++ operator things are different.

The operator++(int) function must create a copy. That is because postfix ++ is expected to return a different value than what it holds: it must hold its value in a temp variable, increment its value and return the temp. In the case of operator++(), prefix ++, there is no need to create a copy: the object can increment itself and then simply return itself.

Here is an illustration of the point:

struct C
{
    C& operator++();      // prefix
    C  operator++(int);   // postfix

private:

    int i_;
};

C& C::operator++()
{
    ++i_;
    return *this;   // self, no copy created
}

C C::operator++(int ignored_dummy_value)
{
    C t(*this);
    ++(*this);
    return t;   // return a copy
}

Every time you call operator++(int) you must create a copy, and the compiler can't do anything about it. When given the choice, use operator++(); this way you don't save a copy. It might be significant in the case of many increments (large loop?) and/or large objects.

It's not entirely correct to say that the compiler can't optimize away the temporary variable copy in the postfix case. A quick test with VC shows that it, at least, can do that in certain cases.

In the following example, the code generated is identical for prefix and postfix, for instance:

#include <stdio.h>

class Foo
{
public:

    Foo() { myData=0; }
    Foo(const Foo &rhs) { myData=rhs.myData; }

    const Foo& operator++()
    {
        this->myData++;
        return *this;
    }

    const Foo operator++(int)
    {
        Foo tmp(*this);
        this->myData++;
        return tmp;
    }

    int GetData() { return myData; }

private:

    int myData;
};

int main(int argc, char* argv[])
{
    Foo testFoo;

    int count;
    printf("Enter loop count: ");
    scanf("%d", &count);

    for(int i=0; i<count; i++)
    {
        testFoo++;
    }

    printf("Value: %d\n", testFoo.GetData());
}

Whether you do ++testFoo or testFoo++, you'll still get the same resulting code. In fact, without reading the count in from the user, the optimizer got the whole thing down to a constant. So this:

for(int i=0; i<10; i++)
{
    testFoo++;
}

printf("Value: %d\n", testFoo.GetData());

Resulted in the following:

00401000  push        0Ah  
00401002  push        offset string "Value: %d\n" (402104h) 
00401007  call        dword ptr [__imp__printf (4020A0h)] 

So while it's certainly the case that the postfix version could be slower, it may well be that the optimizer will be good enough to get rid of the temporary copy if you're not using it.

The Google C++ Style Guide says:

Preincrement and Predecrement

Use prefix form (++i) of the increment and decrement operators with iterators and other template objects.

Definition: When a variable is incremented (++i or i++) or decremented (--i or i--) and the value of the expression is not used, one must decide whether to preincrement (decrement) or postincrement (decrement).

Pros: When the return value is ignored, the "pre" form (++i) is never less efficient than the "post" form (i++), and is often more efficient. This is because post-increment (or decrement) requires a copy of i to be made, which is the value of the expression. If i is an iterator or other non-scalar type, copying i could be expensive. Since the two types of increment behave the same when the value is ignored, why not just always pre-increment?

Cons: The tradition developed, in C, of using post-increment when the expression value is not used, especially in for loops. Some find post-increment easier to read, since the "subject" (i) precedes the "verb" (++), just like in English.

Decision: For simple scalar (non-object) values there is no reason to prefer one form and we allow either. For iterators and other template types, use pre-increment.

I would like to point out an excellent post by Andrew Koenig on Code Talk very recently.

http://dobbscodetalk.com/index.php?option=com_myblog&show=Efficiency-versus-intent.html&Itemid=29

At our company also we use convention of ++iter for consistency and performance where applicable. But Andrew raises over-looked detail regarding intent vs performance. There are times when we want to use iter++ instead of ++iter.

So, first decide your intent and if pre or post does not matter then go with pre as it will have some performance benefit by avoiding creation of extra object and throwing it.

@Ketan

...raises over-looked detail regarding intent vs performance. There are times when we want to use iter++ instead of ++iter.

Obviously post and pre-increment have different semantics and I'm sure everyone agrees that when the result is used you should use the appropriate operator. I think the question is what should one do when the result is discarded (as in for loops). The answer to this question (IMHO) is that, since the performance considerations are negligible at best, you should do what is more natural. For myself ++i is more natural but my experience tells me that I'm in a minority and using i++ will cause less metal overhead for most people reading your code.

After all that's the reason the language is not called "++C".[*]

[*] Insert obligatory discussion about ++C being a more logical name.

Mark: Just wanted to point out that operator++'s are good candidates to be inlined, and if the compiler elects to do so, the redundant copy will be eliminated in most cases. (e.g. POD types, which iterators usually are.)

That said, it's still better style to use ++iter in most cases. :-)

@wilhelmtell

The compiler can elide the temporary. Verbatim from the other thread:

The C++ compiler is allowed to eliminate stack based temporaries even if doing so changes program behavior. MSDN link for VC 8:

http://msdn.microsoft.com/en-us/library/ms364057(VS.80).aspx

An the reason why you ought to use ++i even on built-in types where there's no performance advantage is to create a good habit for yourself.

The intended question was about when the result is unused (that's clear from the question for C). Can somebody fix this since the question is "community wiki"?

About premature optimizations, Knuth is often quoted. That's right. but Donald Knuth would never defend with that the horrible code which you can see in these days. Ever seen a = b + c among Java Integers (not int)? That amounts to 3 boxing/unboxing conversions. Avoiding stuff like that is important. And uselessly writing i++ instead of ++i is the same mistake. EDIT: As phresnel nicely puts it in a comment, this can be summed up as "premature optimization is evil, as is premature pessimization".

Even the fact that people are more used to i++ is an unfortunate C legacy, caused by a conceptual mistake by K&R (if you follow the intent argument, that's a logical conclusion; and defending K&R because they're K&R is meaningless, they're great, but they aren't great as language designers; countless mistakes in the C design exist, ranging from gets() to strcpy(), to the strncpy() API (it should have had the strlcpy() API since day 1)).

Btw, I'm one of those not used enough to C++ to find ++i annoying to read. Still, I use that since I acknowledge that it's right.

Since you asked for C++ too, here is a benchmark for java (made with jmh) :

private static final int LIMIT = 100000;

@Benchmark
public void postIncrement() {
    long a = 0;
    long b = 0;
    for (int i = 0; i < LIMIT; i++) {
        b = 3;
        a += i * (b++);
    }
    doNothing(a, b);
}

@Benchmark
public void preIncrement() {
    long a = 0;
    long b = 0;
    for (int i = 0; i < LIMIT; i++) {
        b = 3;
        a += i * (++b);
    }
    doNothing(a, b);
}  

The result shows, even when the value of the incremented variable (b) is actually used in some computation, forcing the need to store an additional value in case of post-increment, the time per operation is exactly the same :

Benchmark                         Mode  Cnt  Score   Error  Units
IncrementBenchmark.postIncrement  avgt   10  0,039   0,001  ms/op
IncrementBenchmark.preIncrement   avgt   10  0,039   0,001  ms/op

++i is faster than i = i +1 because in i = i + 1 two operation are taking place, first increment and second assigning it to a variable. But in i++ only increment operation is taking place.