Data Types we cannot use to create array in C

In C programming, arrays can be created using most data types like int, char, float, double, etc. However, there are certain data types that cannot be used to create arrays. The most notable restriction is with the void data type.

Syntax

datatype array_name[size];  // Valid for most data types
void array_name[size];      // Invalid - compilation error

Data Types That Cannot Be Used for Arrays

1. Void Data Type

The void data type represents "no type" and has no size. Since arrays need to know the size of each element to allocate memory, void arrays are not permitted −

#include <stdio.h>

int main() {
    /* This will cause a compilation error */
    void arr1[4];
    printf("This will not compile<br>");
    return 0;
}

error: declaration of 'arr1' as array of voids
void arr1[4];

2. Incomplete Types

Arrays cannot be created with incomplete types (types whose size is not known at compile time) −

#include <stdio.h>

int main() {
    /* Valid array declarations */
    int numbers[5];
    char letters[10];
    float values[3];
    
    printf("Valid arrays created successfully<br>");
    printf("Size of int array: %zu bytes<br>", sizeof(numbers));
    printf("Size of char array: %zu bytes<br>", sizeof(letters));
    printf("Size of float array: %zu bytes<br>", sizeof(values));
    
    return 0;
}

Valid arrays created successfully
Size of int array: 20 bytes
Size of char array: 10 bytes
Size of float array: 12 bytes

Why Void Arrays Are Not Allowed

• No defined size: void has no specific size, making memory allocation impossible
• Type safety: Arrays need a specific type for element access and pointer arithmetic
• Memory layout: The compiler cannot determine how much memory each element requires

Conclusion

While C allows arrays of most data types, void and incomplete types cannot be used for array creation due to their undefined size. Always use complete data types with known sizes when declaring arrays.