About ostream of c ++

map<string,int> m;
string str;
while (cin >> str){
    ++m[str];
}
map<string, int>::const_iterator iter = m.begin();

while (iter != m.end()){
    cout << iter->first << (iter++)->second << endl;
}
return 0;

above is the code,but compiler give me a error about invalid dereference,so I change last line like that:

cout<<(iter++)->first<<iter->second<<endl;

and I got right answer,my question is what cout have done make me have to write like this,Is it read string from back to front?how does it implement? or am I wrong other place?

Given an expression of the form a << b, a and b must both be evaluated, but the order they are evaluated is unspecified, which means any order is permitted (a first, b first, both concurrently somehow).

The same goes when multiple such expressions are chained, such as a << b << c.

Practically, overloaded operators are implemented as function calls. For example

std::cout << iter->first << (iter++)->second;   // drop  << endl for simplicity of discussion

may be expanded to something like

operator<<(operator<<(std::cout << iter->first), (iter++)->second);

The order of evaluation of function arguments is also unspecified. So the order of evaluating operator<<(std::cout << iter->first) and (iter++)->second) before passing them to the outermost call of operator<<().

It just so happens with your compiler that (iter++)->second) is evaluated before operator<<(std::cout << iter->first), even though you were expecting the opposite.

The solution is not to mix too many operations with side effects in one statement, because that is a good way to trip over such ordering concerns. In your case, do the output, and increment iter in the next statement.

cout << iter->first << iter->second << endl;
++iter;