C deprecated conversion from string constant to char

Question

I have a class with a private char str 256    and for it I have an explicit constructor   explicit myClass const char  func        strcpy str func       I call it as   myClass obj  example      When I compile this I get the following warning        deprecated conversion from string constant to  char     Why is this happening

User · Answer

As answer no  2 by fnieto - Fernando Nieto clearly and correctly describes that this warning is given because somewhere in your code you are doing  not in the code you posted  something like   void foo char  str   foo  hello      However  if you want to keep your code warning-free as well then just make respective change in your code   void foo char  str   foo  char    hello      That is  simply cast the string constant to  char

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A reason for this problem  which is even harder to detect than the issue with char  str    some string  - which others have explained  is when you are using constexpr   constexpr char  str    some string     It seems that it would behave similar to const char  str  and so would not cause a warning  as it occurs before char   but it instead behaves as char  const str   Details  Constant pointer  and pointer to a constant  The difference between const char  str  and char  const str can be explained as follows    const char  str   Declare str to be a pointer to a const char  This means that the data to which this pointer is pointing to it constant  The pointer can be modified  but any attempt to modify the data would throw a compilation error    str           VALID  We are modifying the pointer  and not the data being pointed to   str    a     INVALID  We are trying to modify the data being pointed to   char  const str   Declare str to be a const pointer to char  This means that point is now constant  but the data being pointed too is not  The pointer cannot be modified but we can modify the data using the pointer    str           INVALID  We are trying to modify the pointer variable  which is a constant   str    a     VALID  We are trying to modify the data being pointed to  In our case this will not cause a compilation error  but will cause a runtime error  as the string will most probably will go into a read only section of the compiled binary  This statement would make sense if we had dynamically allocated memory  eg  char  const str   new char 5     const char  const str   Declare str to be a const pointer to a const char  In this case we can neither modify the pointer  nor the data being pointed to    str           INVALID  We are trying to modify the pointer variable  which is a constant   str    a     INVALID  We are trying to modify the data pointed by this pointer  which is also constant     In my case the issue was that I was expecting constexpr char  str to behave as const char  str  and not char  const str  since visually it seems closer to the former   Also  the warning generated for constexpr char  str    some string  is slightly different from char  str    some string     Compiler warning for constexpr char  str    some string   ISO C  11 does not allow conversion from string literal to  char  const  Compiler warning for char  str    some string   ISO C  11 does not allow conversion from string literal to  char       Tip  You can use C gibberish   English converter to convert C declarations to easily understandable English statements  and vice versa  This is a C only tool  and thus wont support things  like constexpr  which are exclusive to C

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This is an error message you see whenever you have a situation like the following   char  pointer to nonconst    string literal     Why  Well  C and C   differ in the type of the string literal  In C the type is array of char and in C   it is constant array of char  In any case  you are not allowed to change the characters of the string literal  so the const in C   is not really a restriction but more of a type safety thing  A conversion from const char  to char  is generally not possible without an explicit cast for safety reasons  But for backwards compatibility with C the language C   still allows assigning a string literal to a char  and gives you a warning about this conversion being deprecated   So  somewhere you are missing one or more consts in your program for const correctness  But the code you showed to us is not the problem as it does not do this kind of deprecated conversion  The warning must have come from some other place

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For what its worth  I find this simple wrapper class to be helpful for converting C   strings to char     class StringWrapper       std  vector lt char gt  vec  public      StringWrapper const std  string  amp str    vec str begin    str end                 char  getChars             return  amp vec 0

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The warning      deprecated conversion from string constant to  char     is given because you are doing somewhere  not in the code you posted  something like   void foo char  str   foo  hello      The problem is that you are trying to convert a string literal  with type const char    to char    You can convert a const char   to const char  because the array decays to the pointer  but what you are doing is making a mutable a constant   This conversion is probably allowed for C compatibility and just gives you the warning mentioned

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I solve this problem by adding this macro in the beginning of the code  somewhere  Or add it in  lt iostream gt   hehe     define C TEXT  text     char  std  string  text   c str

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Maybe you can try this   void foo const char  str            Do something    foo  Hello     It works for me

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There are 3 solutions   Solution 1   const char  x    foo bar     Solution 2    char  x    char    foo bar     Solution 3   char  x    char   malloc strlen  foo bar   1       1 for the terminator strcpy x  foo bar      Arrays also can be used instead of pointers because an array is already a constant pointer

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The following illustrates the solution  assign your string to a variable pointer to a constant array of char  a string is a constant pointer to a constant array of char - plus length info     include  lt iostream gt   void Swap const char    amp  left  const char    amp  right        const char  const temp   left      left   right      right   temp     int main         const char   x    Hello      These works because you are making a variable     const char   y    World      pointer to a constant string     std  cout  lt  lt   x      lt  lt  x  lt  lt     y      lt  lt  y  lt  lt    n       Swap x  y       std  cout  lt  lt   x      lt  lt  x  lt  lt     y      lt  lt  y  lt  lt    n

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In fact a string constant literal is neither a const char   nor a char  but a char    Its quite strange but written down in the c   specifications  If you modify it the behavior is undefined because the compiler may store it in the code segment

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I also got the same problem  And what I simple did is just adding const char  instead of char   And the problem solved  As others have mentioned above it is a compatible error  C treats strings as char arrays while C   treat them as const char arrays

[c++] C++ deprecated conversion from string constant to 'char*'

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