I am learning C++ and right we are covering preprocessors but I am trying to solve a question from a quiz which I has confused me a bit or a lot.. I tried to worked out by my own before running the program.. and my output was..
System started...
Data at 2 is: 27 28 29 30
Data at 1 is: 23 24 25 26
The data is: 19
I checked the program in Xcode to see if my output is right but the right output is the next one:
System started...
Data at 1 is: 0 0 0 19
Data at 0 is: 7 0 0 0
The data is: 19 0 0 0
This is the code...
#include <iostream>
namespace test{
#define COMPILE_FAST
#define PRINT_SPLIT(v) std::cout << (int)*((char*)(v)) << ' ' << \
(int)*((char*)(v) + 1) << ' ' << (int)*((char*)(v) +2) << ' ' << \
(int)*((char*)(v) + 3) << std::endl
typedef unsigned long long uint;
namespace er{
typedef unsigned int uint;
}
void debug(void* data, int size = 0){
if(size==0){
std::cout << "The data is: ";
PRINT_SPLIT(data);
} else {
while(size--){
std::cout << "Data at " << size << " is: ";
char* a = (char*)data;
PRINT_SPLIT((a + (4+size)));
}
}
}
}// End of Test namespace...
int main(){
test::uint a = 19;
test::er::uint b[] = {256,7};
std::cout << "System started..." << std::endl;
test::debug(b,2);
test::debug(&a);
std::cout << "Test complete";
return 0;
}
My big doubt or what I actually don't understand is whats going on here in this preprocessor because clearly for what I did its totally wrong...
#define PRINT_SPLIT(v) std::cout << (int)*((char*)(v)) << ' ' << \
(int)*((char*)(v) + 1) << ' ' << (int)*((char*)(v) +2) << ' ' << \
(int)*((char*)(v) + 3) << std::endl
if someone can be so nice and give me a brief explanation I will extremely appreciate it.
The macro prints the values (as ints) of 4 consecutive bytes. It allows you to see how a 4 byte int is layed out in memory.
Memory contents, by byte, look like this (base10):
0x22abf0: 0 1 0 0 7 0 0 0
0x22abf8: 19 0 0 0 0 0 0 0
The sizeof(a) is different than the sizeof(b[0]) because there are 2 different typedefs for uint. Namely, test:uint and test::er::uint.
The address of a is greater than the address of b[] even though b is declared after a because the stack is growing downwards in memory.
Finally, I would say the output represents a defective program because the output would more reasonably be:
System started...
Data at 1 is: 7 0 0 0
Data at 0 is: 0 1 0 0
The data is: 19 0 0 0
To get that output the program needs to be changed as follows:
while(size--){
std::cout << "Data at " << size << " is: ";
int* a = (int*)data;
PRINT_SPLIT((a + (size)));
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