Why aren't pointers initialized with NULL by default?

In programming languages such as C++, the reasons why pointers are not initialized to NULL by default are as follows:

1. Performance Optimization: Automatically initializing pointers to NULL can introduce unnecessary performance overhead. In many cases, pointers are immediately assigned a valid address. If the compiler automatically initializes each uninitialized pointer to NULL and then immediately reassigns it a new address, this would result in redundant write operations, potentially impacting program efficiency.

2. Flexibility and Control: Programmers may desire greater control when declaring pointers. For instance, they might need to initialize pointers under more complex logical conditions or later in the program execution. Default initialization to NULL would limit this flexibility.

3. Dependence on Programmer Responsibility: C++ and other low-level programming languages typically prioritize providing more program control to programmers while also increasing their responsibility. Programmers must ensure that pointers are correctly initialized before use. This design philosophy assumes that programmers fully understand the behavior of their code and are responsible for managing memory, including pointer initialization.

4. Historical and Compatibility Reasons: In C++ and its predecessor C language, it has been a traditional practice not to automatically set uninitialized pointers to NULL. This practice also aims to maintain compatibility with earlier languages.

Example Illustration:

Suppose a function that internally needs to create a pointer to an integer and determine which integer the pointer should point to based on certain conditions. If the pointer is automatically initialized to NULL, but is later assigned a valid address after all conditional branches, this automatic initialization to NULL is redundant. Code example:

cpp
void exampleFunction() {
    int a = 10, b = 20;
    int* ptr;  // Uninitialized pointer
    if (condition) {
        ptr = &a;
    } else {
        ptr = &b;
    }
    cout << *ptr; // Using the pointer
}

In this example, the pointer ptr will ultimately point to either a or b. If ptr is default-initialized to NULL, the initial assignment to NULL is unnecessary because it is immediately reassigned later.

In summary, not automatically initializing pointers to NULL is done to optimize performance, provide greater programming flexibility, and align with C++'s design philosophy that places responsibility on the programmer.