Why Can't I Directly Assign One Array to Another in C ?

Arrays in C : Understanding Assignability Restrictions

Arrays in C exhibit peculiar behavior, one of which is their restricted assignability. Let's investigate why assigning one array to another is forbidden, as illustrated by the following code:

int numbers[5] = {1, 2, 3};
int values[5] = {0, 0, 0, 0, 0};
values = numbers;  // Error: C2106

This code generates the error "error C2106: '=' : left operand must be l-value," signaling an attempt to assign to a non-modifiable object.

The error arises because arrays in C are considered non-modifiable or non-assignable, primarily due to backward compatibility with C. Assigning one array to another would require copying all its elements, which is highly inefficient and error-prone.

Instead, C encourages the use of containers from the Standard Template Library (STL), like std::array or std::vector, which provide efficient and type-safe array-like capabilities. For example, using std::array to achieve the desired functionality:

#include <array>

std::array<int, 5> numbers = {1, 2, 3};
std::array<int, 5> values = {};
values = numbers;  // No error!

In the case of primitive arrays (like the one in the original code), you can manually copy elements using a loop or leverage the std::copy function:

#include <algorithm>

int numbers[5] = {1, 2, 3};
int values[5] = {};
std::copy(numbers, numbers + 5, values);

Additionally, the provided code demonstrates a shortcut for initializing the values array with empty values, taking advantage of a C rule that unprovided initializers result in value initialization to zero. Therefore, the following two code snippets are equivalent:

int values[5] = {0, 0, 0, 0, 0};
int values[5] = {};

By understanding these nuances, you can avoid assignment errors and effectively work with arrays in C .

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