Gcc "overflow in expression" while equivalent equivalent expression works fine

Question

Here is my code

#include <iostream> static const unsigned long long int xx = (36 * 36 * 36 * 36) * (36 * 36 * 36 * 36); static const unsigned long long int y = 36 * 36 * 36 * 36; static const unsigned long long int yy = y * y; int main() { std::cout << xx << std::endl; std::cout << yy << std::endl; return 0; } 

This compilation conclusion

 # g++ -std=c++11 test.cpp -o test test.cpp:2:62: warning: integer overflow in expression [-Woverflow] static const unsigned long long int xx = (36 * 36 * 36 * 36) * (36 * 36 * 36 * 36); 

This is the execution output.

 # ./test 18446744073025945600 2821109907456 

Can you explain why I see this warning and different results? if 36 can fit in char, then 36 ^ 8 can fit in unsigned long long int, so I'm not sure if this is a problem, please report. (I am using gcc 4.9.2)

Answer 1

 static const unsigned long long int xx = (36 * 36 * 36 * 36) * (36 * 36 * 36 * 36); 

36 is of type int

∴ 36 * 36 is of type int

∴ (36 * 36 * 36 * 36) is of type int

∴ (36 * 36 * 36 * 36) * (36 * 36 * 36 * 36) is of type int and overflow, which is actually undefined for signed types.

You probably wanted

 static const unsigned long long int xx = (36ull * 36 * 36 * 36) * (36 * 36 * 36 * 36); 

Regarding the second case:

 static const unsigned long long int yy = y * y; 

y is of type unsigned long long

∴ y * y is of type unsigned long long , so there is no overflow.