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Important This document may not represent best practices for current development, links to downloads and other resources may no longer be valid. Current recommended version can be found here.

How to: Iterate Over STL Collection with for each

In addition to .NEt Framework collections, the for each keyword can also be used to iterate over Standard C++ Library (STL) collections. An STL collection is also known as a container. For more information, see STL Containers.

Example

This sample uses for each in a <map>.

// for_each_stl.cpp
// compile with: /EHsc
#include <map>
#include <iostream>
using namespace std;

int main() {
   int retval  = 0;
   map<const char*, int> months;
   
   months["january"] = 31;
   months["february"] = 28;
   months["march"] = 31;
   months["april"] = 30;
   months["may"] = 31;
   months["june"] = 30;
   months["july"] = 31;
   months["august"] = 31;
   months["september"] = 30;
   months["october"] = 31;
   months["november"] = 30;
   months["december"] = 31;
   
   map<const char*, int> months_30;
   
   for each( pair<const char*, int> c in months )
      if ( c.second == 30 )
         months_30[c.first] = c.second;

   for each( pair<const char*, int> c in months_30 )
      retval++;
   
   cout << "Months with 30 days = " << retval << endl;
}

Output

Months with 30 days = 4

This sample uses const& for an iteration variable with STL containers. You can use an & as an iteration variable on any collection of a type that can be declared as a T&.

// for_each_stl_2.cpp
// compile with: /EHsc
#include <vector>
#include <iostream>
using namespace std;

int main() {
   int retval = 0;
      
   vector<int> col(3);
   col[0] = 10;
   col[1] = 20;
   col[2] = 30;
   
   for each( const int& c in col )
      retval += c;

   cout << "retval: " << retval << endl;
}

Output

retval: 60

See Also

Reference

for each, in

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