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

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 performance difference between ++i and i++ will be more apparent when you think of operators as value-returning functions and how they are implemented. To make it easier to understand what's happening, the following code examples will use int as if it were a struct.

++i increments the variable, then returns the result. This can be done in-place and with minimal CPU time, requiring only one line of code in many cases:

int& int::operator++() { 
     return *this += 1;
}

But the same cannot be said of i++.

Post-incrementing, i++, is often seen as returning the original value before incrementing. However, a function can only return a result when it is finished. As a result, it becomes necessary to create a copy of the variable containing the original value, increment the variable, then return the copy holding the original value:

int int::operator++(int& _Val) {
    int _Original = _Val;
    _Val += 1;
    return _Original;
}

When there is no functional difference between pre-increment and post-increment, the compiler can perform optimization such that there is no performance difference between the two. However, if a composite data type such as a struct or class is involved, the copy constructor will be called on post-increment, and it will not be possible to perform this optimization if a deep copy is needed. As such, pre-increment generally is faster and requires less memory than post-increment.

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.

Both are as fast ;) If you want it is the same calculation for the processor, it's just the order in which it is done that differ.

For example, the following code :

#include <stdio.h>

int main()
{
    int a = 0;
    a++;
    int b = 0;
    ++b;
    return 0;
}

Produce the following assembly :

 0x0000000100000f24 <main+0>: push   %rbp
 0x0000000100000f25 <main+1>: mov    %rsp,%rbp
 0x0000000100000f28 <main+4>: movl   $0x0,-0x4(%rbp)
 0x0000000100000f2f <main+11>:    incl   -0x4(%rbp)
 0x0000000100000f32 <main+14>:    movl   $0x0,-0x8(%rbp)
 0x0000000100000f39 <main+21>:    incl   -0x8(%rbp)
 0x0000000100000f3c <main+24>:    mov    $0x0,%eax
 0x0000000100000f41 <main+29>:    leaveq 
 0x0000000100000f42 <main+30>:    retq

You see that for a++ and b++ it's an incl mnemonic, so it's the same operation ;)