Which is faster C++ String length() or size()?

Question

length() returns the number of characters in the string and size() returns a size_t which is also the same but used to make it consistent with other STL containers.

For computing length(), the string iterates through all the characters and counts the length. So, O(n) time.

Is size() also the same ?

Or can size of a variable be computed directly in O(1) time ?

So, my question is, are they the same in terms of speed (as in how they are calculated) or is size computed in O(1) time ?

Answer 1

Both have the same complexity: Constant.

From the N4431 working draft, §21.4.4

size_type size() const noexcept;

Returns: A count of the number of char-like objects currently in the string. Complexity: Constant time.

And

size_type length() const noexcept;

Returns: size().

---

[...] iterates through all the characters and counts the length [...]

That's C strings you're thinking of.

Answer 2

If you take a look at documentation here it says that length and size are the same.

Both string::size and string::length are synonyms and return the same value.

Also if you take a look at the code, length is cached, so the complexity is O(1). (Code from MS implementation but I'm sure other libraries are done the same way.)

size_type length() const _NOEXCEPT
    {   // return length of sequence
    return (this->_Mysize);
    }

size_type size() const _NOEXCEPT
    {   // return length of sequence
    return (this->_Mysize);
    }