Debug why does size_t not work as a data member?

Can somebody tell me why the first program crashes, yet the second one doesn't? First one (crashes):

#include <cstdlib>
class test
{
public:
    test(const char *cstr)
    {开发者_Python百科
        size_t j=0;
        while(cstr[n++])
            ;
        //n = j;
    }
private:
    size_t n;
};

int main()
{
    test("Hello, world!\n");
    return 0;
}

Second one does not crash (use variable local to the constructor rather than data member to count):

   #include <cstdlib>
    class test
    {
    public:
        test(const char *cstr)
        {
            size_t j=0;
            while(cstr[j++])
                ;
            n = j;
        }
    private:
        size_t n;
    };

    int main()
    {
        test("Hello, world!\n");
        return 0;
    }

Running MinGW on windows. make: * [run] Error -1073741819

Quite simply because in your first example the constructor uses n before it's ever initialized (actually, n never gets initialized).

So the line

while(cstr[n++])

is undefined behavior.

Try:

   test(const char *cstr) : n(0)  // <-- initialize n
    {
        size_t j=0;
        while(cstr[n++])
            ;
        //j = n;
    }

In the first case, you're using uninitialized n which is why your program crashes which is one of the possibilities of undefined behaviour (UB). Using uninitialized variables invokes UB.

test(const char *cstr)
{
    size_t j=0; //<--- here you want to do : n = 0;
    while(cstr[n++])
        ;
    //j = n;
 }

Or better yet, you should use member-initialization-list as:

test(const char *cstr) : n(0)
{                    //^^^^^^ it is member-initialization-list
    while(cstr[n++]) ; 
}

Make sure cstr is null-terminated string, otherwise your code would still have UB.

The private member size_t n in the first example is never initialized. Its value is undefined. Add n=0 before your while loop and it should work the same.