What does string npos mean in this code

Question

What does the phrase std  string  npos mean in the following snippet of code   found   str find str2    if  found    std  string  npos      std  cout  lt  lt   first  needle  found at     lt  lt  int found   lt  lt  std  endl

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std  string  npos is implementation defined index that is always out of bounds of any std  string instance  Various std  string functions return it or accept it to signal beyond the end of the string situation  It is usually of some unsigned integer type and its value is usually std  numeric limits lt std  string  size type gt   max    which is  thanks to the standard integer promotions  usually comparable to -1

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Value of string  npos is 18446744073709551615   Its a value returned if there is no string found

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The document for string  npos says   npos is a static member constant value with the greatest possible value for an element of type size t  As a return value it is usually used to indicate failure  This constant is actually defined with a value of -1  for any trait   which because size t is an unsigned integral type  becomes the largest possible representable value for this type

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An answer for these days of C  17  when we have std  optional   If you squint a bit and pretend std  string  find   returns an std  optional lt std  string  size type gt   which it sort of should     - then the condition becomes   auto position   str find str2    if   position has value           std  cout  lt  lt   first  needle  found at     lt  lt  found value    lt  lt  std  endl

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string  npos is a constant  probably -1  representing a non-position   It s returned by method find when the pattern was not found

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21 4 -  static const size type npos   -1     It is returned by string functions indicating error not found etc

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static const size t npos   -1   Maximum value for size t  npos is a static member constant value with the greatest possible value for an element of type size t   This value  when used as the value for a len  or sublen  parameter in string s member functions  means  until the end of the string    As a return value  it is usually used to indicate no matches   This constant is defined with a value of -1  which because size t is an unsigned integral type  it is the largest possible representable value for this type

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size t is an unsigned variable  thus  unsigned value   - 1  automatically makes it the largest possible value for size t  18446744073709551615

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It means not found   It is usually defined like so   static const size t npos   -1    It is better to compare to npos instead of -1 because the code is more legible

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found will be npos in case of failure to find the substring in the search string

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npos is just a token value that tells you that find   did not find anything  probably -1 or something like that   find   checks for the first occurence of the parameter  and returns the index at which the parameter begins  For Example      string name    asad txt     int i   name find   txt        i holds the value 4 now  that s the index at which   txt  starts   if  i  string  npos    if   txt  was NOT found - in this case it was  so  this condition is false     name append   txt

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we have to use string  size type for the return type of the find function otherwise the comparison with string  npos might not work  size type  which is defined by the allocator of the string  must be an unsigned integral type  The default allocator  allocator  uses type size t as size type  Because -1 is converted into an unsigned integral type  npos is the maximum unsigned value of its type  However  the exact value depends on the exact definition of type size type  Unfortunately  these maximum values differ  In fact   unsigned long -1 differs from  unsigned short -1 if the size of the types differs  Thus  the comparison  idx    std  string  npos   might yield false if idx has the value -1 and idx and string  npos have different types   std  string s      int idx   s find  not found       assume it returns npos if  idx    std  string  npos       ERROR  comparison might not work         One way to avoid this error is to check whether the search fails directly   if  s find  hi      std  string  npos            However  often you need the index of the matching character position  Thus  another simple solution is to define your own signed value for npos   const int NPOS   -1    Now the comparison looks a bit different and even more convenient   if  idx    NPOS       works almost always

[c++] What does string::npos mean in this code?

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