Difference between strlen() and sizeof()

This is a discussion on Difference between strlen() and sizeof() within the C++ Programming forums, part of the General Programming Boards category; Hi Friends, Can anyone tell me what is the difference between strlen() and sizeof() Thanks in Advance!!!!...

  1. #1
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    Question Difference between strlen() and sizeof()

    Hi Friends,

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

    Thanks in Advance!!!!

  2. #2
    In the Land of Diddly-Doo g4j31a5's Avatar
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    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:
    Code:
    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.
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    Cat without Hat CornedBee's Avatar
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    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).
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    Algorithm Dissector iMalc's Avatar
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    Quote Originally Posted by g4j31a5 View Post
    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:
    Code:
    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:
    Code:
    const char A[] = "Hello World!";
    And sizeof this would give you 14 since it's double-null-terminated:
    Code:
    const char A[] = "Hello World!\0";
    But sizeof this will most likely give you 4 or 8 depending on the size of a pointer:
    Code:
    const char *A = "Hello World!\0";
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    Quote Originally Posted by CornedBee View Post
    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.
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    Cat without Hat CornedBee's Avatar
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    Quote Originally Posted by grumpy View Post
    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.
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    In the Land of Diddly-Doo g4j31a5's Avatar
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    Quote Originally Posted by iMalc View Post
    No, sizeof this will give you 13:
    Code:
    const char A[] = "Hello World!";
    And sizeof this would give you 14 since it's double-null-terminated:
    Code:
    const char A[] = "Hello World!\0";
    But sizeof this will most likely give you 4 or 8 depending on the size of a pointer:
    Code:
    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.
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    Code:
    No, sizeof this will give you 13:
    Code:
    const char A[] = "Hello World!";
    What will be the output of strlen(A) ??

  9. #9
    and the hat of int overfl Salem's Avatar
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    > What will be the output of strlen(A) ??
    Try it for yourself!
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    Quote Originally Posted by CornedBee View Post
    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.
    Right 98% of the time, and don't care about the other 3%.

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    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.

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    Smile

    Thanks A lot Carle...

    Now I am much more clear...

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    Guest Sebastiani's Avatar
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    Quote Originally Posted by carle View Post
    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.

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    Yaaaa Sebastiani...

    you are correct....

    Thanks...
    Last edited by subhashish1213; 09-22-2009 at 12:51 AM.

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    Smile

    haha,

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




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