Valgrind reports uninitialized bytes when using empty class

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.

phb · Accepted Answer · 2012-04-21 09:14:15Z

4

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.

answered Apr 21, 2012 at 9:14

phb

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