Question about BOOST_FOREACH

Suppose I have a std::vector of double called myVec, and a std::map of type <int, double> called myMap. Why is it that the following works

BOOST_FOREACH(double x, myVec) x = 10.0;

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and the following gives an error

BOOST_FOREACH(pair<int,double> y, myMap) 
cout << y.first << "\t" << y.second << endl;

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so that I need to use

pair<int,double> y;
BOOST_FOREACH(y, myMap) cout << y.first << "\t" << y.second << endl;

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instead?