Why do we write sizeof(x) / sizeof(x[0]) instead of simply writing sizeof(x) to determine the size of the array in C++

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how does sizeof() operator calculates the size of array

What I have tried:

I wrote sizeof(x) to calculate the size of array but I didn't get the expected output.

Posted 9-Feb-24 1:03am

Charu Lata 2024

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PIEBALDconsult 9-Feb-24 10:36am

Having to calculate that kind of thing is a code smell. Why did you not store the number you allocated?

3 solutions

Solution 1

sizeof returns the number of bytes used by the parameter: for an array, that the size of each element multiplied by the number of elements.

Try this:

C

#include <stdio.h>

int main()
{
    int arr[10];
    printf("%lu:%lu\n", sizeof(arr), sizeof(arr[0]));

    return 0;
}

And you will get the result "40:4": the array total uses 40 bytes of memory, and each of the 10 elements in the array use 4 bytes.

Posted 9-Feb-24 1:39am

OriginalGriff

Comments

CPallini 9-Feb-24 8:36am

5.

Solution 3

You should rather ask yourself why you use a C array instead of a container in C++. You can use the size() method on containers and get the desired size, which you could also adjust at runtime. There are also many other advantages to using the possibilities of C++.

Posted 10-Feb-24 4:18am

merano99

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CPallini 11-Feb-24 4:28am

Indeed. 5.

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Rick York · Accepted Answer · 2024-02-09T04:57:00

One small addition: the expression you wrote,

C

sizeof( x ) / sizeof( x[0] )

is used to calculate the number of items in an array. This is the same as what the _countof macro does in VisualStudio C++.

To summarize the three expressions and their result for int x[8] :

sizeof(x)               size in bytes of x             : 32
sizeof(x[0])            size in bytes of one item of x : 4
sizeof(x)/sizeof(x[0])  number of items in x           : 8

Why do we write sizeof(x) / sizeof(x[0]) instead of simply writing sizeof(x) to determine the size of the array in C++

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