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Range-Based for Loops

2024-07-08 03:10:08   217人阅读

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 listfor ( int i : { 2, 3, 5, 7, 9, 13, 17, 19 } ) {    std::cout << i << std::endl;}2. normal containerstd::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 onetemplate<T>printElements(iterator<T> begin, iterator<T> end){    for(;begin < end && begin != end; begin ++)    {        const auto& element = *begin;        std::cout << element << std::endl;
 　　} 
}

 

 

 


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