for ( decl : coll ) { statement }
where decl is the declaration of each element of the passed collection coll and for which the statements specified are called.
1. using the initializer list for ( int i : { 2, 3, 5, 7, 9, 13, 17, 19 } ) { std::cout << i << std::endl; } 2. normal container std::vector<double> vec; ... for ( auto& elem : vec ) { elem *= 3; }
Declaring elem as a reference is important because otherwise the statements in the body of the for loop act on a local copy of the elements in the vector.
To avoid calling the copy constructor and the destructor for each element, you should usually declare the current element to be a constant reference.
template <typename T> void printElements (const T& coll) { for (const auto& elem : coll) { std::cout << elem << std::endl; } } //which is equivalent to the following: { for (auto _pos=coll.begin(); _pos != coll.end(); ++_pos ) { const auto& elem = *_pos; std::cout << elem << std::endl; } }
Which violate the rules introduced in "the philosophy behind of the design of the STL"
//perfectly correct one template<T> printElements(iterator<T> begin, iterator<T> end) { for(;begin < end && begin != end; begin ++) { const auto& element = *begin; std::cout << element << std::endl;
}
}
Range-Based for Loops,布布扣,bubuko.com
原文：http://www.cnblogs.com/Cmpl/p/3785419.html