choose endianness in cpp

Endianness refers to the ordering of bytes within a multi-byte data type in computer memory. There are two common types of endianness: little-endian and big-endian.

1. Little-endian: In little-endian systems, the least significant byte (LSB) is stored at the smallest memory address, while the most significant byte (MSB) is stored at the largest memory address. This means that when you read a multi-byte data type, you start with the LSB and move towards the MSB.

2. Big-endian: In big-endian systems, the most significant byte (MSB) is stored at the smallest memory address, while the least significant byte (LSB) is stored at the largest memory address. This means that when you read a multi-byte data type, you start with the MSB and move towards the LSB.

To determine the endianness of a system in C++, you can use a technique called "type-punning" which involves accessing the bytes of a type using a different type. Here's an example:

#include <iostream>

int main() {
    int value = 0x12345678;
    char bytePtr = reinterpret_cast<char>(&value);

    if (*bytePtr == 0x78) {
        std::cout << "Little-endian" << std::endl;
    } else if (*bytePtr == 0x12) {
        std::cout << "Big-endian" << std::endl;
    } else {
        std::cout << "Unknown endianness" << std::endl;
    }

    return 0;
}

In this example, we create an integer variable value and assign it the value 0x12345678. We then use a char* pointer bytePtr to access the individual bytes of value. Since char is 1 byte in size, we can use it to check the endianness.

By dereferencing bytePtr and comparing it with a known byte value, we can determine the endianness of the system. If the least significant byte is 0x78, it means the system is little-endian. If the most significant byte is 0x12, it means the system is big-endian. Otherwise, if neither condition is met, the endianness is unknown.

Note: It's important to mention that the endianness of a system is determined by the hardware architecture and is not something that can be changed or configured in C++. The above code is just a way to determine the endianness at runtime.