I'm reading a book,and I have an exercise that gives me problems.Here's the code:
double aha [5] = {0,0,0,0,0};
unsigned char* ma = reinterpret_cast<unsigned char*>(&aha[0]);
memset(ma,1,sizeof(double));
function_to_cout_all_elements_of_aha(aha);
The problem is,it prints this:
7.784e-304
0
0
0
0
What's happening to the first element?! Why the other 4 are fine,and the first one got screwed up?
The pointer ma points to the beginning of the aha array. The memset function then writes "something" to the first double of ma, which is the first double of the aha array. That is why the first element of aha is borked, and the remaining four elements are not
memset(ma,1,sizeof(double)*5);
Still,the exercise asks: Why was not each element set to 1? I have no ideea how to explain it
Well you are pointing to the first element in the array, then modifying only sizeof(double) bytes.
Any memory after sizeof(double) bytes should be unaffected.
To modify all,
This has nothing to do with reinterpret_cast.memset(ma,1,sizeof(double)*5);
Wouldn't that just bork all of the array instead of just the first double?
Because you aren't setting each double to 1.0 -- you are setting each individual byte for the first double to 0x01. The layout for your double will be 0x0101010101010101, which is around 7.784e-304, according to IEEE-754.Still,the exercise asks: Why was not each element set to 1? I have no ideea how to explain it
double arr[4] = {};std::fill( arr, arr+4, 1. );
