Differences between C string and compare

Question

I just read some recommendations on using   std  string s   get string    std  string t   another string     if   s compare t         instead of   if  s    t       I m almost always using the last one because I m used to it and it feels natural  more readable  I didn t even know that there was a separate comparison function  To be more precise  I thought    would call compare     What are the differences  In which contexts should one way be favored to the other   I m considering only the cases where I need to know if a string is the same value as another string

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std  string  compare   returns an int    equal to zero if s and t are equal  less than zero if s is less than t  greater than zero if s is greater than t    If you want your first code snippet to be equivalent to the second one  it should actually read   if   s compare t             s  and  t  are equal      The equality operator only tests for equality  hence its name  and returns a bool   To elaborate on the use cases  compare   can be useful if you re interested in how the two strings relate to one another  less or greater  when they happen to be different  PlasmaHH rightfully mentions trees  and it could also be  say  a string insertion algorithm that aims to keep the container sorted  a dichotomic search algorithm for the aforementioned container  and so on   EDIT  As Steve Jessop points out in the comments  compare   is most useful for quick sort and binary search algorithms  Natural sorts and dichotomic searches can be implemented with only std  less

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One thing that is not covered here is that it depends if we compare string to c string  c string to string or string to string   A major difference is that for comparing two strings size equality is checked before doing the compare and that makes the    operator faster than a compare   here is the compare as i see it on g   Debian 7     operator                 brief  Test equivalence of two strings         param   lhs  First string         param   rhs  Second string         return  True if  a   lhs compare  a   rhs     0   False otherwise          template lt typename  CharT  typename  Traits  typename  Alloc gt      inline bool     operator   const basic string lt  CharT   Traits   Alloc gt  amp    lhs             const basic string lt  CharT   Traits   Alloc gt  amp    rhs        return   lhs compare   rhs     0       template lt typename  CharT gt      inline     typename   gnu cxx    enable if lt   is char lt  CharT gt     value  bool gt     type     operator   const basic string lt  CharT gt  amp    lhs             const basic string lt  CharT gt  amp    rhs        return    lhs size        rhs size              amp  amp   std  char traits lt  CharT gt   compare   lhs data      rhs data                                  lhs size                      brief  Test equivalence of C string and string         param   lhs  C string         param   rhs  String         return  True if  a   rhs compare  a   lhs     0   False otherwise          template lt typename  CharT  typename  Traits  typename  Alloc gt      inline bool     operator   const  CharT    lhs             const basic string lt  CharT   Traits   Alloc gt  amp    rhs        return   rhs compare   lhs     0                  brief  Test equivalence of string and C string         param   lhs  String         param   rhs  C string         return  True if  a   lhs compare  a   rhs     0   False otherwise          template lt typename  CharT  typename  Traits  typename  Alloc gt      inline bool     operator   const basic string lt  CharT   Traits   Alloc gt  amp    lhs             const  CharT    rhs        return   lhs compare   rhs     0

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This is what the standard has to say about operator       21 4 8 2 operator    template lt class charT  class traits  class Allocator gt  bool operator   const basic string lt charT traits Allocator gt  amp  lhs                  const basic string lt charT traits Allocator gt  amp  rhs  noexcept        Returns  lhs compare rhs     0    Seems like there isn t much of a difference

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compare   is equivalent to strcmp       is simple equality checking  compare   therefore returns an int     is a boolean

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If you just want to check string equality  use the    operator  Determining whether two strings are equal is simpler than finding an ordering  which is what compare   gives   so it might be better performance-wise in your case to use the equality operator   Longer answer  The API provides a method to check for string equality and a method to check string ordering  You want string equality  so use the equality operator  so that your expectations and those of the library implementors align   If performance is important then you might like to test both methods and find the fastest

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compare   will return false  well  0  if the strings are equal   So don t take exchanging one for the other lightly   Use whichever makes the code more readable

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compare has overloads for comparing substrings  If you re comparing whole strings you should just use    operator  and whether it calls compare or not is pretty much irrelevant

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Suppose consider two string s and t  Give them some values  When you compare them using  s  t  it returns a boolean value true or false   1 or 0   But when you compare using s compare t   the expression returns a value  i     0 - if s and t are equal  ii    lt 0 - either if the value of the first unmatched character in s is less than that of t or the length of s is less than that of t   iii   0 - either if the value of the first unmatched character in t is less than that of s or the length of t is less than that of s

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Internally  string  operator     is using string  compare    Please refer to  CPlusPlus - string  operator       I wrote a small application to compare the performance  and apparently if you compile and run your code on debug environment the string  compare   is slightly faster than string  operator      However if you compile and run your code in Release environment  both are pretty much the same    FYI  I ran 1 000 000 iteration in order to come up with such conclusion   In order to prove why in debug environment the string  compare is faster  I went to the assembly and here is the code   DEBUG BUILD  string  operator              if  str1    str2  00D42A34  lea         eax  str2    00D42A37  push        eax   00D42A38  lea         ecx  str1    00D42A3B  push        ecx   00D42A3C  call        std  operator   lt char std  char traits lt char gt  std  allocator lt char gt   gt   0D23EECh    00D42A41  add         esp 8   00D42A44  movzx       edx al   00D42A47  test        edx edx   00D42A49  je          Algorithm  PerformanceTest  stringComparison usingEqualOperator1 0C4h  0D42A54h      string  compare                if  str1 compare str2     0  00D424D4  lea         eax  str2    00D424D7  push        eax   00D424D8  lea         ecx  str1    00D424DB  call        std  basic string lt char std  char traits lt char gt  std  allocator lt char gt   gt   compare  0D23582h    00D424E0  test        eax eax   00D424E2  jne         Algorithm  PerformanceTest  stringComparison usingCompare1 0BDh  0D424EDh    You can see that in string  operator      it has to perform extra operations  add esp  8 and movzx edx al     RELEASE BUILD  string  operator              if  str1    str2  008533F0  cmp         dword ptr  ebp-14h  10h   008533F4  lea         eax  str2    008533F7  push        dword ptr  ebp-18h    008533FA  cmovae      eax dword ptr  str2    008533FE  push        eax   008533FF  push        dword ptr  ebp-30h    00853402  push        ecx   00853403  lea         ecx  str1    00853406  call        std  basic string lt char std  char traits lt char gt  std  allocator lt char gt   gt   compare  0853B80h      string  compare                if  str1 compare str2     0      00853830  cmp         dword ptr  ebp-14h  10h       00853834  lea         eax  str2        00853837  push        dword ptr  ebp-18h        0085383A  cmovae      eax dword ptr  str2        0085383E  push        eax       0085383F  push        dword ptr  ebp-30h        00853842  push        ecx   00853843  lea         ecx  str1    00853846  call        std  basic string lt char std  char traits lt char gt  std  allocator lt char gt   gt   compare  0853B80h    Both assembly code are very similar as the compiler perform optimization   Finally  in my opinion  the performance gain is negligible  hence I would really leave it to the developer to decide on which one is the preferred one as both achieve the same outcome  especially when it is release build

[c++] Differences between C++ string == and compare()?

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