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How to use map::max_size, clear, erase, size STL functions in Visual C++

Article ID:157593
Last Review:August 11, 2005
Revision:6.0
This article was previously published under Q157593
Note Microsoft Visual C++ NET (2002) supported both the managed code model that is provided by the .NET Framework and the unmanaged native Windows code model. The information in this article applies to unmanaged Visual C++ code only.
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SUMMARY

The sample code below illustrates how to use the map::max_size, map::clear, map::empty, map::erase, map::size, map::operator[], map::end, map::begin, map::find, map::iterator, map::value_type STL symbols in Visual C++.

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MORE INFORMATION

Required header

   <map>

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Prototypes

   size_type max_size() const;
   void clear() const;
   bool empty() const;
   iterator erase(iterator first, iterator last);
   size_type size() const;
   A::reference operator[](const Key& key);    // A is the allocator
   iterator map::begin();
   iterator map::end();
   iterator map::find(const Key& key);
Note The class/parameter names in the prototypes may not match the version in the header file. Some have been modified to improve readability.

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Description

The following sample creates a map of strings to ints and fills it first with a map of month names to month numbers, then empties and refills it with a map of week-day names to corresponding ints.

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Sample code

////////////////////////////////////////////////////////////////////// 
// 
// Compile options needed: None
// 
// <filename> :  main.cpp
// 
// Functions:    iterator map::max_size();
//               void clear() const;
//               bool empty() const;
//               iterator erase(iterator first, iterator last);
//               size_type size() const;
//               A::reference operator[](const Key& key);
//               iterator map::begin();
//               iterator map::end();
//               iterator map::find(const Key& key);
// 
// Written by Rick Troemel
// of Microsoft Product Support Services,
// Copyright (c) 1996 Microsoft Corporation. All rights reserved.
////////////////////////////////////////////////////////////////////// 

#pragma warning(disable:4786)

#include <iostream>
#include <string>
#include <map>

#if _MSC_VER > 1020   // if VC++ version is > 4.2
   using namespace std;  // std c++ libs implemented in std
   #endif

typedef map<string, int, less<string>, allocator<int> > STRING2INT;

void main()
{
    STRING2INT MyMap;
    STRING2INT::iterator MyIterator;
    string MyBuffer;

    // print the maximum number of <key,data> pairs that MyMap can hold
    cout << "MyMap is capable of holding " << MyMap.max_size()
         << " <string,int> pairs" << endl;

    if(!MyMap.empty())
        cout << "MyMap has " << MyMap.size() << " entries" << endl;
    else
        cout << "MyMap is empty" << endl;
        cout << "Entering new entries in MyMap" << endl;
    // Fill in MyMap with the months of the year, mapped to their number
    // January - 1, February - 2, etc. using operator[].
    MyMap["January"] = 1;
    MyMap["February"] = 2;
    MyMap["March"] = 3;
    MyMap["April"] = 4;
    MyMap["May"] = 5;
    MyMap["June"] = 6;
    MyMap["July"] = 7;
    MyMap["August"] = 8;
    MyMap["September"] = 9;
    MyMap["October"] = 10;
    MyMap["November"] = 11;
    MyMap["December"] = 12;

    if(!MyMap.empty())
        cout << "MyMap has " << MyMap.size() << " entries" << endl;
    else
        cout << "MyMap is empty" << endl;

    // Ask the user for a month of the year and print the number
    // that corresponds to the month entered
   MyIterator = MyMap.end();
    while(MyIterator == MyMap.end()){
        cout << "Enter a Month :";
        cin >> MyBuffer;
        if((MyIterator = MyMap.find(MyBuffer)) != MyMap.end())
            cout << (*MyIterator).first << " is Month Number "
                 << (*MyIterator).second << endl;
        else
            cout << "Enter a Valid Month (example: March)" << endl;
    }

    // empty MyMap - note that clear simply calls erase(begin(),end());
    MyMap.clear();

    if(!MyMap.empty())
        cout << "MyMap has " << MyMap.size() << " entries" << endl;
    else
        cout << "MyMap is empty" << endl;
        cout << "Entering new entries in MyMap" << endl;
    // Fill MyMap with the days of the week, each mapped to an int
    MyMap["Monday"] = 1;
    MyMap["Tuesday"] = 2;
    MyMap["Wednesday"] = 3;
    MyMap["Thursday"] = 4;
    MyMap["Friday"] = 5;
    MyMap["Saturday"] = 6;
    MyMap["Sunday"] = 7;

    if(!MyMap.empty())
        cout << "MyMap has " << MyMap.size() << " entries" << endl;
    else
        cout << "MyMap is empty" << endl;

    // Ask the user for a day of the week and print the number
    // that corresponds to the day entered
    MyIterator = MyMap.end();
    while(MyIterator == MyMap.end()){
        cout << "Enter a Day of the Week:";
        cin >> MyBuffer;
        if((MyIterator = MyMap.find(MyBuffer)) != MyMap.end())
            cout << (*MyIterator).first << " is Day Number "
                 << (*MyIterator).second << endl;
        else
            cout <<"Enter a Valid Day of the Week(example: Monday)"<< endl;
    }
// Now clear MyMap again - this time using erase instead of clear
    MyMap.erase(MyMap.begin(), MyMap.end());

    if(!MyMap.empty())
        cout << "MyMap has " << MyMap.size() << " entries" << endl;
    else
        cout << "MyMap is empty" << endl;

}
Program Output is (User input is shown inside []):
MyMap is capable of holding 1073741823 <string,int> pairs
MyMap is empty
Entering new entries in MyMap
MyMap has 12 entries
Enter a Month :[April]
April is Month Number 4
MyMap is empty
Entering new entries in MyMap
MyMap has 7 entries
Enter a Day of the Week:[Friday]
Enter a Valid Day of the Week(example: Monday)
Enter a Day of the Week:[Friday]
Friday is Day Number 5
MyMap is empty

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REFERENCES

For the same information about map::max_size, clear, erase, size, visit the following MSDN Web site:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vcstdlib/html/vcsampsamplemapccmaxsizeclearerasesizestlsample.asp (http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vcstdlib/html/vcsampsamplemapccmaxsizeclearerasesizestlsample.asp)

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APPLIES TO
The Standard C++ Library, when used with:
  Microsoft Visual C++ 4.2 Enterprise Edition
  Microsoft Visual C++ 5.0 Enterprise Edition
  Microsoft Visual C++ 6.0 Enterprise Edition
  Microsoft Visual C++ 4.2 Professional Edition
  Microsoft Visual C++ 5.0 Professional Edition
  Microsoft Visual C++ 6.0 Professional Edition
  Microsoft Visual C++ 6.0 Standard Edition
  Microsoft Visual C++ .NET 2002 Standard Edition
  Microsoft Visual C++ .NET 2003 Standard Edition

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Keywords: 
kbhowto kbinfo kbcode KB157593

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