C++: is string.empty() always equivalent to string == ""?

Can I make an assumption that given

std::string str;
... // do something to str

Is the following statement is always true?

(str.empty() == (str == ""))

Solution 1:

Answer

Yes. Here is the relevant implementation from bits/basic_string.h, the code for basic_string<_CharT, _Traits, _Alloc>:

  /**
   *  Returns true if the %string is empty.  Equivalent to *this == "".
   */
  bool
  empty() const
  { return this->size() == 0; }

Discussion

Even though the two forms are equivalent for std::string, you may wish to use .empty() because it is more general.

Indeed, J.F. Sebastian comments that if you switch to using std::wstring instead of std::string, then =="" won't even compile, because you can't compare a string of wchar_t with one of char. This, however, is not directly relevant to your original question, and I am 99% sure you will not switch to std::wstring.

Solution 2:

It should be. The ANSI/ISO standard states in 21.3.3 basic_string capacity:

size_type size() const;

Returns: a count of char-like objects currently in the string.

bool empty() const;

Returns: size() == 0

However, in clause 18 of 21.3.1 basic_string constructors it states that the character-type assignment operator uses traits::length() to establish the length of the controlled sequence so you could end up with something strange if you are using a different specialization of std::basic_string<>.

I think that the 100% correct statement is that

(str.empty() == (str == std::string()))

or something like that. If you haven't done anything strange, then std::string("") and std::string() should be equivalent

They are logically similar but they are testing for different things. str.empty() is checking if the string is empty where the other is checking for equality against a C-style empty string. I would use whichever is more appropriate for what you are trying to do. If you want to know if a string is empty, then use str.empty().

Solution 3:

str.empty() is never slower, but might be faster than str == "". This depends on implementation. So you should use str.empty() just in case.

This is a bit like using ++i instead of i++ to increase a counter (assuming you do not need the result of the increment operator itself). Your compiler might optimise, but you lose nothing using ++i, and might win something, so you are better off using ++i.

Apart from performance issues, the answer to your question is yes; both expressions are logically equivalent.