How to check if a std string is set or not

Question

If using a char   I can initialize it to NULL and later check if it is set by doing a comparison  How to do the same thing for a std  string  How to check if the string is set or not   EDIT      What if the string I set to is also empty  Do I have to use an additional flag to check if the std  string is set or not

User · Answer

You can t  at least not the same way you can test whether a pointer is NULL     A std  string object is always initialized and always contains a string  its contents by default are an empty string          You can test for emptiness  using s size      0 or s empty

User · Answer

As several answers pointed out  std  string has no concept of  nullness  for its value   If using the empty string as such a value isn t good enough  ie   you need to distinguish between a string that has no characters and a string that has no value   you can use a std  string  and set it to NULL or to a valid std  string instance as appropriate   You may want to use some sort of smart pointer type  boost  scoped ptr or something  to help manage the lifetime of any std  string object that you set the pointer to

User · Answer

The default constructor for std  string always returns an object that is set to a null string

User · Answer

Use empty     std  string s   if  s empty           nothing in s

User · Answer

I don t think you can tell with the std  string class   However  if you really need this information  you could always derive a class from std  string and give the derived class the ability to tell if it had been changed since construction  or some other arbitrary time    Or better yet  just write a new class that wraps std  string since deriving from std  string may not be a good idea given the lack of a base class virtual destructor   That s probably more work  but more work tends to be needed for an optimal solution   Of course  you can always just assume if it contains something other than    then it has been  set   this won t detect it manually getting set to    though

User · Answer

There is no  unset  state for std  string  it is always set to something

[c++] How to check if a std::string is set or not?

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