c++ what does "iter = ++iter" do? is it valid?

I have a question on c++ std iterators.

suppose iter is std::set<SomeType>::iterator type.

Is:

iter = ++iter 

the same as:

++iter 

or are they different?

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edit:

I found a code std::set<UserDefinedClass*>::iterator being used that way. (pointer)

I wonder if that can cause the program I'm debugging开发者_C百科 malfunction.

I'm reading up the answers but it's hard to judge which answer is correct.

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There's no definitive answer to this question. The answer depends on the nature of std::set<SomeType>::iterator type. If it is a user-defined type (i.e. a class with overloaded operators), then the behavior of iter = ++iter is defined ans is indeed equivalent to a mere ++iter. However, if std::set<SomeType>::iterator is a built-in type, then iter = ++iter produces undefined behavior, since it modifies the same object twice in one expression without an intervening sequence point (violation of the requirements presented in 5/4 of the language standard).

So, theoretically in general case one should avoid doing something like that since in general case the behavior is undefined. In practice though std::set<SomeType>::iterator will normally be a user-defined type and iter = ++iter will work. Nevertheless this is not a reason to use such expressions in the code.

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Yes they are the same thing - but why do this?

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Yes, same thing. A simpler case, same thing:

int i = 10;   //i = 10
int j = i++;  //j = 10
int k = ++i;  //k = 12

However

i = ++i; 

is unnecessary as it increments i and then assigns i to i