size of the input string

Question

I am wondering how come the sizeof function returned 8 no matter the length of my input

int main(){
   string input;
   getline(cin,input);
   cout << "size of input is  " << sizeof(input) << endl; //I am guessing 
//it returns the size of a pointer because my OS is 64 bits.
   return 0;
}

So my question is that where the implicit conversion happened? here is the declaration of getline,

istream& getline ( istream& is, string& str );

Also, this sort of conversion always happen, i.e from whatever to a pointer type, is there a general case for that? Thank you.

Uh, input.size()... sizeof(input) is the size of any std::string object, not that std::string's contained data. — ildjarn
– ildjarn, Commented May 25, 2012 at 23:27
I would, I just wanna know if there was a conversion to pointer type. Thanks — Cong Hui
– Cong Hui, Commented May 25, 2012 at 23:28
Why would there be? Where would there be? The question is nonsensical -- sizeof(input) is certainly larger than sizeof(void*) or any other pointer type. — ildjarn
– ildjarn, Commented May 25, 2012 at 23:29
@ildjarn, I'd personally throw in a possibly explicit operator const char *() to my String class. You can get that using c_str() of course here though. — Qaz
– Qaz, Commented May 25, 2012 at 23:30
@chris : That wouldn't have any effect on sizeof's behavior though. — ildjarn
– ildjarn, Commented May 25, 2012 at 23:30

Qaz · Accepted Answer · 2012-05-25 23:34:50Z

13

sizeof gives you the size in bytes. std::string contains a pointer to the actual data. The size of the single pointer stays the same no matter the size of what it's pointing to. This, combined with the other factors gives you your total size of 8.

You're looking for either std::string::size or std::string::length for the actual length of the string. If you're looking for a function to retrieve the size of any null-terminated C-String, use strlen().

edited May 25, 2012 at 23:34

answered May 25, 2012 at 23:29

Qaz

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2 Comments

Donal Fellows Over a year ago

Don't those functions give the number of characters in the string? That's (in some senses at least) subtly different from the size of the string, particularly when the string isn't over char as the basic character type, though there are probably other (constant) factors too, and also the difference between the space used and the space reserved...

Qaz Over a year ago

@DonalFellows, that's exactly what I meant. If the OP wanted the actual number of characters in the string, length() would be the right one. I'm not 100% sure if size() is the same, but for consistency with other containers, but it's always worked that way for me. I guess if you have a wide string it might make a difference, but I haven't tested/looked at it.

lezebulon · Accepted Answer · 2012-05-25 23:28:22Z

3

There is no conversion done here, it's simply that sizeof will return the actual byte size of the object. Obviously std::string uses heap-allocated memory to store its actual content, so you have no real way to know exactly how many bytes in total are used for a specific string

answered May 25, 2012 at 23:28

lezebulon

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5 Comments

Cong Hui Over a year ago

Thanks, that makes much more sense, I always thought there was a conversion to a pointer type, so if the input gets long enough, the return of sizeof could be bigger than 8 in theory, right? Thanks

lezebulon Over a year ago

the result of sizeof will always be the same for every string you use, for a given compiler version. It's totally unrelated to the size of the input

Qaz Over a year ago

@ClintHui, sizeof (yourStringObject) will never change. As far as it's concerned, the only thing there is the pointer. How it uses the pointer is unrelated. The actual string you input is stored in a separate location in memory that is irrelevant to your object except for the fact that the object knows where it is and that it is responsible for it.

Cong Hui Over a year ago

the thing that the string contains an internal pointer cleared my confusion. Thank you all.

abarnert Over a year ago

It isn't necessarily true (much less "obvious") that std::string uses a heap pointer. Google "small string optimization" for a counter-example.

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