const char concatenation

Question

I need to concatenate two const chars like these   const char  one    Hello    const char  two    World     How might I go about doing that   I am passed these char s from a third-party library with a C interface so I can t simply use std  string instead

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Connecting two constant char pointer without using strcpy command in the dynamic allocation of memory   const char  one    Hello    const char  two    World     char  three   new char strlen one    strlen two    1     0     strcat s three  strlen one    1  one   strcat s three  strlen one    strlen two    1  two    cout  lt  lt  three  lt  lt  endl   delete   three  three   nullptr

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const char  one    one   const char  two    two   char result 40   sprintf result    s s   one  two

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One more example      calculate the required buffer size  also accounting for the null terminator   int bufferSize   strlen one    strlen two    1      allocate enough memory for the concatenated string  char  concatString   new char  bufferSize        copy strings one and two over to the new buffer  strcpy  concatString  one    strcat  concatString  two             delete buffer  delete   concatString    But unless you specifically don t want or can t use the C   standard library  using std  string is probably safer

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const char  one    Hello    const char  two    World    string total  string one    two        to use the concatenation as const char   use  total c str     Updated  changed      string total   string one    string two   to string total  string one    two    for performance reasons  avoids construction of string two and temporary string total      string total move move string one     two        even faster

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First of all  you have to create some dynamic memory space   Then you can just strcat the two strings into it   Or you can use the c    string  class   The old-school C way     char  catString   malloc strlen one  strlen two  1     strcpy catString  one     strcat catString  two        use the string then delete it when you re done    free catString     New C   way    std  string three one     three    two

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The C way   char buf 100   strcpy buf  one   strcat buf  two     The C   way   std  string buf one   buf append two     The compile-time way    define one  hello    define two  world   define concat first  second  first second  const char  buf   concat one  two

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In your example one and two are char pointers  pointing to char constants  You cannot change the char constants pointed to by these pointers  So anything like   strcat one two      append string two to string one    will not work  Instead you should have a separate variable char array  to hold the result  Something like this   char result 100        array to hold the result   strcpy result one      copy string one into the result  strcat result two      append string two to the result

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If you are using C    why don t you use std  string instead of C-style strings   std  string one  Hello   std  string two  World    std  string three  one two    If you need to pass this string to a C-function  simply pass three c str

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It seems like you re using C   with a C library and therefore you need to work with const char     I suggest wrapping those const char   into std  string   const char  a    hello     const char  b    world    std  string c   a   std  string d   b   cout  lt  lt  c   d

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You can use strstream  It s formally deprecated  but it s still a great tool if you need to work with C strings  i think   char result 100      max size 100 std  ostrstream s result  sizeof result - 1    s  lt  lt  one  lt  lt  two  lt  lt  std  ends  result 99      0     This will write one and then two into the stream  and append a terminating  0 using std  ends  In case both strings could end up writing exactly 99 characters - so no space would be left writing  0 - we write one manually at the last position

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Using std  string    include  lt string gt   std  string result   std  string one    std  string two

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If you don t know the size of the strings  you can do something like this    include  lt stdio h gt   include  lt string h gt   include  lt stdlib h gt   int main        const char  q1    First String       const char  q2     Second String        char   qq    char   malloc  strlen q1   strlen q2   sizeof char        strcpy qq q1       strcat qq q2        printf   s n  qq        return 0

[c++] const char* concatenation

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