为什么在语句中的其他地方没有使用值的情况下使用++ i而不是i ++？

Question

I'm well aware that in C++

int someValue = i++;
array[i++] = otherValue;

has different effect compared to

int someValue = ++i;
array[++i] = otherValue;

but every once in a while I see statements with prefix increment in for-loops or just by their own:

for( int i = 0; i < count; ++i ) {
     //do stuff
}

or

for( int i = 0; i < count; ) {
    //do some stuff;
    if( condition ) {
        ++i;
    } else {
        i += 4;
    }
}

In the latter two cases the ++i looks like an attempt to produce smarty-looking code. Am I overseeing something? Is there a reason to use ++i instead of i++ in the latter two cases?

Answer 1

Look at possible implementations of the two operators in own code:

// Pre-increment
T*& operator ++() {
    // Perform increment operation.
    return *this;
}

// Post-increment
T operator ++(int) {
    T copy = *this;
    ++*this;
    return copy;
}

The postfix operator invokes the prefix operator to perform its own operation: by design and in principle the prefix version will always be faster than the postfix version, although the compiler can optimize this in many cases (and especially for builtin types).

The preference for the prefix operator is therefore natural; it's the other that needs explanation: why are so many people intrigued by the use of the prefix operator in situations where it doesn't matter – yet nobody is ever astonished by the use of the postfix operator.

Answer 2

If we ignore force of habit, '++i' is a simpler operation conceptually : It simply adds one to the value of i, and then uses it.

i++ on the other hand, is "take the original value of i , store it as a temporary, add one to i , and then return the temporary". It requires us to keep the old value around even after i has been updated.

And as Konrad Rudolph showed, there can be performance costs to using i++ with user-defined types.

So the question is, why not always just default to ++i ?

If you have no reason to use `i++´, why do it? Why would you default to the operation which is more complicated to reason about, and may be slower to execute?

Answer 3

As you noted - it does not matter to the result.

There is a performance consideration for non-primitive types.

Also semantically using pre-increment is usually clearer in showing the intention of a test when the return value is used, so its better to use it habitually than post-increment to avoid accidentally testing the old value.

Answer 4

There is one reason, and it has to do with overloaded operators. In an overloaded postincrement function, the function must remember the previous value of the object, increment it, and then return the previous value. In a preincrement function, the function can simply increment the object and then return a reference to itself (its new value).

In the case of an integer, the above probably won't apply because the compiler knows the context in which the increment is being done, and will generate appropriate increment code in either case.

Answer 5

Yes, for performance reasons. In case of post increment a copy of the variable i needs to be made before incrementing so that the old value can be returned (eventhough you are not using the return value). In case of pre-increment this copy is not required.

Answer 6

There is little reason to favour pre-increment over post-increment when you are talking about natural types like integers. The compiler is typically able to generate the same code in both cases anyway, assuming you don't use the return value. This is not true for more complex types, such as iterators.

Answer 7

The C++ FAQ Lite has a nice discussion of i++ vs. ++i here:

[13.15] Which is more efficient: i++ or ++i?

In particular, with respect to which form to use within a for loop, the FAQ expresses a preference for ++i . Here's the text:

So if you're writing i++ as a statement rather than as part of a larger expression, why not just write ++i instead? You never lose anything, and you sometimes gain something. Old line C programmers are used to writing i++ instead of ++i . Eg, they'll say, for (i = 0; i <10; i++) .... Since this uses i++ as a statement, not as a part of a larger expression, then you might want to use ++i instead. For symmetry, I personally advocate that style even when it doesn't improve speed, eg, for intrinsic types and for class types with postfix operators that return void.