How to convert a const char to std string

Question

What is the correct best simplest way to convert a c-style string to a std  string   The conversion should accept a max length  and terminate the string at the first  0 char  if this occur before max length charter

User · Answer

What you want is this constructor  std  string   const string amp  str  size t pos  size t n   npos    passing pos as 0  Your const char  c-style string will get implicitly cast to const string for the first parameter   const char  c style    012abd   std  string cpp style   new std  string c style  0  10

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There is a constructor accepting two pointer parameters  so the code is simply   std  string cppstr cstr  cstr   min max length  strlen cstr       this is also going to be as efficient as std  string cppstr cstr  if the length is smaller than max length

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std  string str c str  strnlen c str  max length        At Christian Rau s request   strnlen is specified in POSIX 1-2008 and available in GNU s glibc and the Microsoft run-time library   It is not yet found in some other systems  you may fall back to Gnulib s substitute

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This is actually trickier than it looks  because you can t call strlen unless the string is actually nul terminated   In fact  without some additional constraints  the problem practically requires inventing a new function  a version of strlen which never goes beyond the a certain length   However   If the buffer containing the c-style string is guaranteed to be at least max length char s  although perhaps with a   0  before the end    then you can use the address-length constructor of std  string  and trim afterwards   std  string result  c string  max length    result erase  std  find  result begin    result end      0     result end        and if you know that c string is a nul terminated string  but perhaps longer than max length  you can use strlen   std  string result  c string  std  min  strlen  c string    max length

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std  string the string c string   if the string size    gt  max length      the string resize max length

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This page on string  string gives two potential constructors that would do what you want   string   const char   s  size t n    string   const string amp  str  size t pos  size t n   npos      Example    include lt cstdlib gt   include lt cstring gt   include lt string gt   include lt iostream gt  using namespace std   int main         char  p   char  calloc 30  sizeof char        strcpy p   Hello world         string s p  15       cout  lt  lt  s size    lt  lt        lt  lt  s  lt  lt       lt  lt  endl      string t p  0  15       cout  lt  lt  t size    lt  lt        lt  lt  t  lt  lt       lt  lt  endl       free p       return 0      Output   15  Hello world  11  Hello world    The first form considers p to be a simple array  and so will create  in our case  a string of length 15  which however prints as a 11-character null-terminated string with cout  lt  lt       Probably not what you re looking for   The second form will implicitly convert the char  to a string  and then keep the maximum between its length and the n you specify  I think this is the simplest solution  in terms of what you have to write

[c++] How to convert a const char * to std::string

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