Note
As @user4581301 indicated in the comments, this depends on the implementation and will fail in many instances. And as @NathanOliver- Reinstate Monica and @ChrisMM pointed out this is Undefined Behavior and the result is not guaranteed.
According to the bitmap header format, the width of the bitmap in pixels is stored as a signed 32-bit integer beginning at byte offset 18. The syntax
int width = *(int*)&data[18];
reads bytes 19 through 22, inclusive (assuming a 32-bit int) and interprets the result as an integer.
How?
&data[18] gets the address of the unsigned char at index 18(int*) casts the address from unsigned char* to int* to avoid loss of precision on 64 bit architectures*(int*) dereferences the address to get the referred int value
So basically, it takes the address of data[18] and reads the bytes at that address as if they were an integer.
Why doesn't a simple cast to `int` work?
sizeof(data[18]) is 1, because unsigned char is one byte (0-255) but sizeof(&data[18]) is 4 if the system is 32-bit and 8 if it is 64-bit, this can be larger (or even smaller for 16-bit systems) but with the exception of 16-bit systems it should be at minimum 4 bytes. Obviously reading more than 4 bytes is not desired in this case, and the cast to (int*) and subsequent dereference to int yields 4 bytes, and indeed the 4 bytes between offsets 18 and 21, inclusive. A simple cast from unsigned char to int will also yield 4 bytes, but only one byte of the information from data. This is illustrated by the following example:
#include <iostream> #include <bitset> int main() { // Populate 18-21 with a recognizable pattern for demonstration std::bitset<8> _bits(std::string("10011010")); unsigned long bits = _bits.to_ulong(); for (int ii = 18; ii < 22; ii ++) { data[ii] = static_cast<unsigned char>(bits); } std::cout << "data[18] -> 1 byte " << std::bitset<32>(data[18]) << std::endl; std::cout << "*(unsigned short*)&data[18] -> 2 bytes " << std::bitset<32>(*(unsigned short*)&data[18]) << std::endl; std::cout << "*(int*)&data[18] -> 4 bytes " << std::bitset<32>(*(int*)&data[18]) << std::endl; }
data[18] -> 1 byte 00000000000000000000000010011010 *(unsigned short*)&data[18] -> 2 bytes 00000000000000001001101010011010 *(int*)&data[18] -> 4 bytes 10011010100110101001101010011010
data[18], treating it as a pointer to an integer, then dereferencing it. Basically, treating it as a number. This seems like UB though, sincedatais only size 1datais allocated as an array of 1unsigned char. I think it should have beennew unsigned char[size]*(int*)&data[18]will also fail on CPUs that require a 32 bit number to be aligned to a 32 bit address (Some CPUs will allow mis-aligned data, but access it much more slowly). Assuming thatdatais aligned to whatever size data the CPU prefers (usually 32 or 64 bits)data[18]will not be because 18 is not evenly divisible by 4 (32 bits in bytes). It will also fail if the CPU is big endian and the byte order is backwards.