Microsoft KB Archive/158199

= How to use the string::operator== Standard Template Library (STL) function in Visual C++ =

Article ID: 158199

Article Last Modified on 8/11/2005

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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</li></ul>

 Microsoft Visual C++ 5.0 Professional Edition</li></ul>

 Microsoft Visual C++ 6.0 Professional Edition</li></ul>

 Microsoft Visual C++ 6.0 Standard Edition</li></ul>

 Microsoft Visual C++ .NET 2002 Standard Edition</li></ul>

 Microsoft Visual C++ .NET 2003 Standard Edition</li></ul> </li></ul>

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This article was previously published under Q158199

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Note Microsoft Visual C++ .NET (2002) supports both the managed code model that is provided by the Microsoft .NET Framework and the unmanaged native Microsoft Windows code model. The information in this article applies only to unmanaged Visual C++ code.

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SUMMARY
The sample code below illustrates how to use the string::operator== STL function in Visual C++.

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Prototype
Function 1: template<class _E, class _TYPE, class _A> inline bool operator==(const basic_string<_E, _TYPE, _A>& LString,          const _E *RCharArray);

Function 2: template<class _E, class _TYPE, class _A> inline bool operator==(const _E * LCharArray,          const basic_string<_E, _TYPE, _A>& RString);

Function 3: template<class _E, class _TYPE, class _A> inline bool operator==(const basic_string<_E, _TYPE, _A>& LString,          const basic_string<_E, _TYPE, _A>& RString); NOTE: The class/parameter names in the prototype may not match the version in the header file. Some have been modified to improve readability.

Description
There are three versions of the string::operator== function. The first two functions compare a null-terminated character array against a basic_string. They perform this operation by using the basic_string's string::compare function and returning true if the compare function returns a 0 value. The third function compares two basic_string variables, using the string::compare function of the first basic_string variable. For more information on the compare function, please see the string::compare function.

Note that this operator does not work with NULL pointer for the character array. You will need to make sure that the character array is not NULL before you pass it to the operator.

Sample Code
////////////////////////////////////////////////////////////////////// // // Compile options needed: /GX // // StringEqual.cpp: Illustrates how to use the operator== to test for //                 equality of a basic_string variable and a //                  null-terminated string. It also illustrates how to //                 use the operator== to test for equality of two //                 basic_string variables. // // Functions: // //   operator==   returns true if the basic_string and the null- //                terminated string are equal. //   operator==   returns true if both basic_strings are equal. // // Written by Derek Jamison // of Microsoft Product Support Services, // Copyright (c) 1996 Microsoft Corporation. All rights reserved. //////////////////////////////////////////////////////////////////////


 * 1) pragma warning(disable:4786)

using namespace std;
 * 1) include
 * 2) include

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

void trueFalse(int x)

{ cout << (x? "True": "False") << endl; }

void main

{

string S1="ABC"; string S2="ABC"; string S3="DEF"; string S4; //This specifies an empty initial-controlled sequence. char CP1[]="abc"; char CP2[]="DEF"; char *CP3 = NULL;

cout << "S1 is " << S1 << endl; cout << "S2 is " << S2 << endl; cout << "S3 is " << S3 << endl; cout << "S4 is " << S4 << endl; cout << "CP1 is " << CP1 << endl; cout << "CP2 is " << CP2 << endl;

cout << "S1==CP1 returned "; trueFalse(S1==CP1); // False    (calls function 1)

cout << "S1==CP2 returned "; trueFalse(S1==CP2); // False   (calls function 1)

cout << "CP1==S1 returned "; trueFalse(CP1==S1); // False    (calls function 2)

cout << "CP2==S1 returned "; trueFalse(CP2==S1); // False   (calls function 2)

cout << "S1==S2 returned "; trueFalse(S1==S2);  // True    (calls function 3)

cout << "S1==S3 returned "; trueFalse(S1==S3);  // False   (calls function 3)

cout << "S1==S4 returned "; trueFalse(S1==S4);  // False   (calls function 3)

// Following use of the operator will cause the program to  // crash since CP3 is NULL. // cout << "S1==CP3 returned "; // trueFalse(S1==CP3);

}

Program Output
S1 is ABC S2 is ABC S3 is DEF S4 is CP1 is abc CP2 is DEF S1==CP1 returned False S1==CP2 returned False CP1==S1 returned False CP2==S1 returned False S1==S2 returned True S1==S3 returned False S1==S4 returned False

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