sizeof char array problem

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Hi. Can someone explain me why this two line of codes doesnt output the same result ?

C++

char* str = "hello world";

cout << sizeof(str) << endl;  //output: 4
cout << sizeof("hello world") << endl;  //output: 12

edit: and how can I retrieve the size of a string using the pointer ?

Posted 18-May-14 1:01am

Mark

Updated 18-May-14 1:57am

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[no name] 18-May-14 7:12am

Because the sizeof of a char pointer is 4.

Kornfeld Eliyahu Peter 18-May-14 7:15am

This IS the answer, so post it that anyone should profit from this...

Mark 18-May-14 7:20am

So is there anyway to calculate the size of the string using pointers ?
sizeof(*str) doesnt work.

Kornfeld Eliyahu Peter 18-May-14 7:24am

Use strlen(str) for that...

Mark 18-May-14 7:26am

but strlen is for calc the length o_O
I want to calc the size not the length !

[no name] 18-May-14 7:31am

You might tell us what it is that you are actually trying to do. What is it that you think the difference is between the length and the size?

Mark 18-May-14 7:37am

length is the number of character the string contains..
size is the amount in bytes the string use.

[no name] 18-May-14 7:54am

Okay... and how is that any different than the information that you are getting? The length of "hello world" is 12 and the size is also 12.

Mark 18-May-14 7:55am

what if I use utf-8 characters ?

[no name] 18-May-14 8:06am

What if? Then you will most likely get a different result. What if you use UTF-16? or UTF-32?

Mark 18-May-14 8:08am

That's why I want to know how to get the size of a char* using sizeof() keyword..

Kornfeld Eliyahu Peter 18-May-14 8:16am

In C++ char type is a always a byte and can not be used to store Unicode strings easily. You may use wchar_t type for that...

Kornfeld Eliyahu Peter 18-May-14 7:41am

In the standard C++ libraries (std::string) there is a size() and a length() member. No difference whatsoever between the two...Both "Returns the length of the string, in terms of number of characters."...

Mark 18-May-14 7:50am

So u saying std::string::size == sizeof() ??

Kornfeld Eliyahu Peter 18-May-14 7:55am

No! I'm saying that string::size() === string::length().
sizeof() is a different beast - it used to get the size of a variable. In your case the size of a pointer, but not the size of the string the pointer point to...

Mark 18-May-14 7:59am

that's what I want to know.. How can I retrieve the size of a string using the pointer with keyword sizeof() ?

Kornfeld Eliyahu Peter 18-May-14 8:00am

strlen() or string::length()

Mark 18-May-14 8:10am

Oh dear.. strlen returns the length of the string.. not the size.. what if I use utf-8 characters in the string ? would strlen returns the size as if utf-8 characters doesn't consume more bytes ?

Richard MacCutchan 18-May-14 8:14am

You can't; sizeof is a compiler generated value not a run time function, so it only works for items or arrays that are defined at compile time. If you want to know the size of something at run time then you need to calculate it using strlen and the size of the characters in the string.

Kornfeld Eliyahu Peter 18-May-14 8:18am

In C++ char type is a always a byte and can not be used to store Unicode strings easily. You may use wchar_t type for that...

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Kornfeld Eliyahu Peter · Accepted Answer · 2014-05-18T02:28:00

Remember that the original C++ was defined before Unicode programming became a request.
The char type is always a byte size so a 12 letter long string will take up 12 bytes...
If you want to use Unicode you have to use wchar_t type and the corresponding functions like wcslen()...
But of course wcslen(0 returns the length in the units of the type so if you want to know the byte size you have to multiply it with sizeof(wchar_t)...

C++

wchar_t* str = L"hello world";
 
cout << wcslen(str) << endl;  // 12
cout <<  wcslen(str) * sizeof(wchar_t) << endl;  // probably 24, but depends on implementation