Question

Why does valgrind reports on uninitialised bytes in the following code?

#include <valgrind/memcheck.h>

class Test {
public:
    Test() {}
};

int main(int argc, char* argv[]) {
    Test a;
    VALGRIND_CHECK_VALUE_IS_DEFINED(a);
    return 0;
}

If I add a member variable to test and initialise it, there is no output.

Answer 1

In your example, the size of ‘a’ will be 1:

printf("%ld\n",sizeof(a)); => 1

sizeof() only reports the data in a class, but since each unique class must have it’s own address, a padding byte is added. The reason valgrind complains is that you are accessing the padding byte, which the compiler has no obligation to initialize to any value.

See here for a comment on why the minimum size of a class is 1 and not 0.