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

Question

Can I make an assumption that given

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

Is the following statement is always true?

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

Answer 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.

Answer 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().