basic_string::operator

Artigo
07/29/2015

Provides a reference to the character with a specified index in a string.

const_reference operator[](
   size_type _Off
) const;
reference operator[](
   size_type _Off
);

Parameters

_Off
The index of the position of the element to be referenced.

Return Value

A reference to the character of the string at the position specified by the parameter index.

Remarks

The first element of the string has an index of zero, and the following elements are indexed consecutively by the positive integers, so that a string of length n has an nth element indexed by the number n - 1.

operator[] is faster than the member function at for providing read and write access to the elements of a string.

operator[] does not check whether the index passed as a parameter is valid, but the member function at does and so should be used in the validity is not certain. An invalid index (an index less that zero or greater than or equal to the size of the string) passed to the member function at throws an out_of_range Class exception. An invalid index passed to operator[] results in undefined behavior, but the index equal to the length of the string is a valid index for const strings and the operator returns the null character when passed this index.

The reference returned may be invalidated by string reallocations or modifications for the non-const strings.

When compiling with _SECURE_SCL 1, a runtime error will occur if you attempt to access an element outside the bounds of the string. See Checked Iterators for more information.

Example

// basic_string_op_ref.cpp
// compile with: /EHsc
#include <string>
#include <iostream>

int main( )
{
   using namespace std;
   string str1 ( "Hello world" ), str2 ( "Goodbye world" );
   const string cstr1 ( "Hello there" ), cstr2 ( "Goodbye now" );
   cout << "The original string str1 is: " << str1 << endl;
   cout << "The original string str2 is: " << str2 << endl;

   // Element access to the non-const strings
   basic_string <char>::reference refStr1 = str1 [6];
   basic_string <char>::reference refStr2 = str2.at ( 3 );

   cout << "The character with an index of 6 in string str1 is: "
        << refStr1 << "." << endl;
   cout << "The character with an index of 3 in string str2 is: "
        << refStr2 << "." << endl;

   // Element access to the const strings
   basic_string <char>::const_reference crefStr1 = cstr1 [ cstr1.length ( ) ];
   basic_string <char>::const_reference crefStr2 = cstr2.at ( 8 );

   if ( crefStr1 == '\0' )
      cout << "The null character is returned as a valid reference."
           << endl;
   else
      cout << "The null character is not returned." << endl;
   cout << "The character with index of 8 in the const string cstr2 is: "
        << crefStr2 << "." << endl;
}

Output

The original string str1 is: Hello world
The original string str2 is: Goodbye world
The character with an index of 6 in string str1 is: w.
The character with an index of 3 in string str2 is: d.
The null character is returned as a valid reference.
The character with index of 8 in the const string cstr2 is: n.

Requirements

Header: <string>

Namespace: std

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