What happens if I define a 0-size array in C/ C ++?

In C and C++, defining an array with a length of zero is invalid and results in undefined behavior. Specifically, when you attempt to define a zero-sized array, you may encounter compiler errors or warnings because an array must have at least one element.

C language case

In C99 or earlier standards, attempting to define a zero-length array directly results in a compilation error. For example:

c
int arr[0];  // In most C compilers, this results in a compilation error

C++ case

In C++, defining a zero-sized array is also invalid and results in a compilation error. For example:

cpp
int arr[0];  // In C++ compilers, this results in a compilation error

Possible warning or error messages

When attempting this, the compiler may provide the following error or warning messages:

• error: zero-size array 'arr'
• array size cannot be zero
• size of array must be greater than zero

Alternative for zero-length arrays

Although directly defining a zero-length array is not allowed, developers may use structures similar to zero-length arrays in certain cases, especially with dynamic memory allocation. For example, you can use dynamically allocated memory to simulate the behavior of a zero-length array:

c
int* arr = (int*)malloc(0);  // Allocate 0 bytes, but the returned pointer is typically not NULL
if (arr != NULL) {
    // You can perform operations with arr, but cannot dereference it because no actual memory is allocated
}
free(arr);  // Release the allocated memory

Although this approach compiles and runs, since no actual memory is allocated, any attempt to access array elements results in undefined behavior.

Conclusion

Defining a zero-sized array is not only a syntax error but also has no practical use case. If you need this behavior, it's better to use pointers and dynamic memory allocation to achieve similar functionality.