Explain me this code please. Why in case of string sizeof () is giving wrong answer?

Question

https://code.sololearn.com/cYjTde5GDEl6/?ref=app

Answer 1

Thanks to Shadow I don't need to explain the string part. But its size will depend on the machine architecture/implementation, for example in my 32 bit machine the space allocated to the string was 12 Bytes.

Now the array part. As I think you know, all C/C++ char arrays has a hidden additional character, the termination character '\0', so when you made "char b[] = "hello"", the compiler set the size of array b to fit the word hello plus the \0 character, so this means 6 characters in total, that is exactly what you are getting as reply.

Any doubt remaining?

Answer 2

std::size() is a function included in the header <iterator>, however, it is a quite new feature (C++ 17), so not all compilers might support it yet. Reference:
https://en.cppreference.com/w/cpp/iterator/size

Now for the question.
First of all, sizeof() does not give a wrong answer! String "a" is actually 32 byte. This is due to how modern strings are implemented in C++. They are not simple arrays, but actually closer to vectors. What's so special about vectors is that they can grow in size, unlike arrays. And exactly for these grow-operations, vectors always allocate a bit more memory space than they actually need. So string "a" allocated space for 32 characters, but only 5 are used at the moment. That's were the difference comes from.
I'd suggest you to search more about vectors to understand how they work, etc.
I'll provide a few links you can start with:
http://www.cplusplus.com/reference/vector/vector/

https://en.cppreference.com/w/cpp/container/vector

https://www.geeksforgeeks.org/vector-in-cpp-stl/

Answer 3

I tried it first on VS 2015 and it worked correctly when i wrote size (a)...However now i understood that size () is function of string library and it should be used as a.size () but still my question is why does sizeof () is giving wrong answer?

Answer 4

Mark

Answer 5

Hi Iterator,

I change your code a bit, try this:
#include <iostream>
#include <string>
using namespace std;

int main() {
string a="hello";
char b[]="hello";
char *c;
char d[10];
char e;

cout<<sizeof(a)<<endl;
cout<<sizeof(b)<<endl;
cout<<sizeof(c)<<endl;
cout<<sizeof(d)<<endl;
cout<<sizeof(e)<<endl;
return 0;
}

Answer 6

Hi Iterator,

In my attempt to understand the behavior of your code I compiled it with other C++ compilers and both them give a compilation error due to the function size(), from which library this function comes from?

When experimenting with C++ code here in sololearn I recommend you to try true c++ compilers, either installed in your machine or online, you can find one in www.cpp.sh, I say this because the c++ compiler/interpreter in sololearn is simple shitty and buggy.

So, go ahead, fix your code to compile with a true compiler and I will try to explain why you are getting these differences. Spoiler: the reason is due to how a string is represented with an array of char and also the string c++ object contents.