size of pointer to array vs string in C++

Question

string a= "Stack Overflow";
char b[]= "Stack Overflow";
cout<<sizeof(a)<<","<<sizeof(b)<<endl;

Output of above code is 4,15
Since 'a' points to the string, it has size 4 that of a string on my machine.

'b' is also pointer to string, but why it has size of 15 (i.e. of sizeof("Stack Overflow")) ?

A simple std::string can be implemented containing a pointer to a dynamically buffer containing the characters in the string and a bit of bookkeeping for the length and whatnot, but there are many ways to implement a std::string This one happens to pack everything it needs into 4 bytes. — user4581301
– user4581301, Commented Dec 14, 2017 at 18:22
@ArunAS Not really - that's a question about how string happens to be implemented, this one is more fundamental. — Barry
– Barry, Commented Dec 14, 2017 at 18:25
@Barry I know, that's why I posted this so OP can know how it is implemented. That itself should help understanding this more — Arun A S
– Arun A S, Commented Dec 14, 2017 at 18:27
Actually, std::string typically has a size of 16 or so. It seems string is an alias for char const* in this case. — Dietmar Kühl
– Dietmar Kühl, Commented Dec 14, 2017 at 18:48

R Sahu · Accepted Answer · 2017-12-14 18:19:05Z

6

Since 'a' points to the string, it has size 4 that of a string on my machine.

Not exactly.

a IS A string. It is not a pointer and, hence, does not point to a string. The implementation of string on your setup is such that sizeof(string) is 4.

'b' is also pointer to string, but why it has size of 15 (i.e. of sizeof("Stack Overflow")) ?

Not true.

b is not a pointer to a string. It is an array of char. The line

char b[]= "Stack Overflow";

is equivalent to:

char b[15]= "Stack Overflow";

The compiler deduces the size of the array and creates an array of the right size.

answered Dec 14, 2017 at 18:19

R Sahu

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2 Comments

IR-x86 Over a year ago

Then how does a store large strings in only 4 bytes?

R Sahu Over a year ago

@Shivam, it contains a pointer that points to dynamically allocated data.

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