Difference between strlen() and sizeof()

[subhashish1213]

Hi Friends,

Can anyone tell me what is the difference between strlen() and sizeof()

Thanks in Advance!!!!

[g4j31a5]

The difference is:
strlen() is used to get the length of an array of chars / string. sizeof() is used to get the actual size of any type of data in bytes.

Consider there's this array of char:

const char* A = "Hello World!\0";

strlen will output 12 while sizeof 13. The NULL character at the end of the string won't be calculated in strlen.

[CornedBee]

That's not really accurate and can lead to a misunderstanding, especially since sizeof(A) will not give you 13.

sizeof() is a compile-time expression giving you the size of a type or a variable's type. It doesn't care about the value of the variable.

strlen() is a function that takes a pointer to a character, and walks the memory from this character on, looking for a NUL character. It counts the number of characters before it finds the NUL character. In other words, it gives you the length of a C-style NUL-terminated string.

The two are entirely different in purpose and have nearly nothing to do with each other. In C++, you shouldn't need either very much; strlen() is for C-style strings, which should be replaced by C++-style std::strings, whereas the primary application for sizeof() in C is as an argument to functions like malloc(), memcpy() or memset(), all of which you shouldn't use in C++ (use new, std::copy(), and std::fill() or constructors).

[iMalc]

The difference is:
strlen() is used to get the length of an array of chars / string. sizeof() is used to get the actual size of any type of data in bytes.

Consider there's this array of char:

const char* A = "Hello World!\0";

strlen will output 12 while sizeof 13. The NULL character at the end of the string won't be calculated in strlen.

No, sizeof this will give you 13:

const char A[] = "Hello World!";

And sizeof this would give you 14 since it's double-null-terminated:

const char A[] = "Hello World!\0";

But sizeof this will most likely give you 4 or 8 depending on the size of a pointer:

const char *A = "Hello World!\0";

[grumpy]

sizeof() is a compile-time expression giving you the size of a type or a variable's type. It doesn't care about the value of the variable.

Your comment here is inapplicable to string literals - the type (and therefore size) of a string literal is related to its value. However, the relationships between string literals, arrays of char, and pointers to char are technically an anomaly in C (and C++) - no other types are affected by this anomaly.

[CornedBee]

Your comment here is inapplicable to string literals - the type (and therefore size) of a string literal is related to its value.

The type derives from the value, which means that the size changes with the value. The sizeof() operator still operates strictly on the type, though.

[g4j31a5]

No, sizeof this will give you 13:

const char A[] = "Hello World!";

And sizeof this would give you 14 since it's double-null-terminated:

const char A[] = "Hello World!\0";

But sizeof this will most likely give you 4 or 8 depending on the size of a pointer:

const char *A = "Hello World!\0";

Umm... Yeah, you're right. LoL. What was in my head and what I had written is different.

Thanks for making it clear.

[subhashish1213]

No, sizeof this will give you 13:
Code:
const char A[] = "Hello World!";

What will be the output of strlen(A) ??

[Salem]

> What will be the output of strlen(A) ??
Try it for yourself!

[grumpy]

The type derives from the value, which means that the size changes with the value. The sizeof() operator still operates strictly on the type, though.

Agreed; we're on the same page now.

[carle]

Hey,

The ''sizeof" is a keyword of operator, while the strlen() is a function. The sizeof returns the length of a varible or data type, in other words, how many bytes this data block of type occupied. It can used on all dada types. The strlen() can only operate on a string and returns the number of characters before '\0' of a string.

For example:

char a[100] = "abc\0ddddd";

The result of sizeof(a) will be 100 while the strlen(a) will be 4.

Carle
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[subhashish1213]

Thanks A lot Carle...

Now I am much more clear...

[Sebastiani]

Hey,
For example:

char a[100] = "abc\0ddddd";

The result of sizeof(a) will be 100 while the strlen(a) will be 4.

Three, actually.

[subhashish1213]

Yaaaa Sebastiani...

you are correct....

Thanks...

[carle]

haha,

yes, you r right. I made a mistake~~~~~~~~~




carle
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