递增迭代器:++it 比 it++ 更有效吗?

问题描述

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

I am writing a program where an iterator is used to loop through a std::vector. Somebody told me that doing ++it in the for statement leads to more efficient code. In other words, they are saying that:

for ( vector<string>::iterator it=my_vector.begin(); it != my_vector.end(); ++it )

runs faster than

for ( vector<string>::iterator it=my_vector.begin(); it != my_vector.end(); it++ )

Is this true? If it is, what is the reason behind the efficiency improvement? All it++/++it does is move the iterator to the next item in the vector, isn't it?

The reason behind the preincrement being faster is that post-increment has to make a copy of the old value to return. As GotW #2 put it, "Preincrement is more efficient than postincrement, because for postincrement the object must increment itself and then return a temporary containing its old value. Note that this is true even for builtins like int."

GotW #55 provides the canonical form of postincrement, which shows that it has to do preincrement plus some more work:

T T::operator++(int)
{
  T old( *this ); // remember our original value
  ++*this;        // always implement postincrement
                  //  in terms of preincrement
  return old;     // return our original value
}

As others have noted, it's possible for some compiler to optimize this away in some cases, but if you're not using the return value it's a good idea not to rely on this optimization. Also, the performance difference is likely to be very small for types which have trivial copy constructors, though I think using preincrement is a good habit in C++.

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