一些C++11语言新特性 - Range-Based for Loops

1. Range-Based for Loops

for ( decl : coll ) {
 statement
}

 

eg:

for ( int i : { 2, 3, 5, 7, 9, 13, 17, 19 } ) {
    std::cout << i << std::endl;
}

 

std::vector<double> vec;
...
for ( auto& elem : vec ) {
    elem *= 3;
}

Here, 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 (which sometimes also might be useful).

This means that to avoid calling the copy constructor and the destructor for each element, you should usually declare the current element to be a constant reference. Thus, a generic function to print all elements of a collection should be implemented as follows:

template <typename T>
void printElements (const T& coll)
{
    for (const auto& elem : coll) {
        std::cout << elem << std::endl;
    }
}

那段range-based for loops代码等价于如下:

for (auto _pos=coll.begin(); _pos != coll.end(); ++_pos ) {
    const auto& elem = *_pos;
    std::cout << elem << std::endl;
}

 

int array[] = { 1, 2, 3, 4, 5 };
long sum=0; // process sum of all elements
for (int x : array) {
    sum += x;
}
for (auto elem : { sum, sum*2, sum*4 } ) { // print 15 30 60
    std::cout << elem << std::endl;
}