Range-based for Statement (C++)
Executes statement repeatedly and sequentially for each element in expression.
for ( for-range-declaration : expression )
statement
Remarks
Use the range-based for statement to construct loops that must execute through a "range", which is defined as anything that you can iterate through—for example, std::vector, or any other STL sequence whose range is defined by a begin() and end(). The name that is declared in the for-range-declaration portion is local to the for statement and cannot be re-declared in expression or statement. Note that the auto keyword is preferred in the for-range-declaration portion of the statement.
This code shows how to use ranged for loops to iterate through an array and a vector:
// range-based-for.cpp
// compile by using: cl /EHsc /nologo /W4
#include <iostream>
#include <vector>
using namespace std;
int main()
{
// Basic 10-element integer array.
int x[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
// Range-based for loop to iterate through the array.
for( int y : x ) { // Access by value using a copy declared as a specific type.
// Not preferred.
cout << y << " ";
}
cout << endl;
// The auto keyword causes type inference to be used. Preferred.
for( auto y : x ) { // Copy of 'x', almost always undesirable
cout << y << " ";
}
cout << endl;
for( auto &y : x ) { // Type inference by reference.
// Observes and/or modifies in-place. Preferred when modify is needed.
cout << y << " ";
}
cout << endl;
for( const auto &y : x ) { // Type inference by reference.
// Observes in-place. Preferred when no modify is needed.
cout << y << " ";
}
cout << endl;
cout << "end of integer array test" << endl;
cout << endl;
// Create a vector object that contains 10 elements.
vector<double> v;
for (int i = 0; i < 10; ++i) {
v.push_back(i + 0.14159);
}
// Range-based for loop to iterate through the vector, observing in-place.
for( const auto &j : v ) {
cout << j << " ";
}
cout << endl;
cout << "end of vector test" << endl;
}
Here is the output:
1 2 3 4 5 6 7 8 9 10
1 2 3 4 5 6 7 8 9 10
1 2 3 4 5 6 7 8 9 10
1 2 3 4 5 6 7 8 9 10
end of integer array test
0.14159 1.14159 2.14159 3.14159 4.14159 5.14159 6.14159 7.14159 8.14159 9.14159
end of vector test
A range-based for loop terminates when one of these in statement is executed: a break, return, or goto to a labeled statement outside the range-based for loop. A continue statement in a range-based for loop terminates only the current iteration.
Keep in mind these facts about range-based for:
Automatically recognizes arrays.
Recognizes containers that have .begin() and .end().
Uses argument-dependent lookup begin() and end() for anything else.