Problem of two consecutive ++operators , always works the second ?

[tonnot]

I'm passing values of an array by pointer to a funtcion, and as you can see I use ++prefix to increase the index,

w_math->Visual_cent(&w3d_p3d_af[++w3d_p3d],&w3d_p3d_af[++w3d_p3d]) 

w_math->Visual_cent(*x,*y) 
*x=10;
*y=20;

I dont know what is happen but I have the values
0, 20
That is , the first value never receives data .
Are this a compile error ? Have I to change by w3d_p3d+1, wp3d+2 ?
Thanks

[iLUjLyTq]

Try changing pre-incrementation to post-incrementation:

w_math->Visual_cent(&w3d_p3d_af[w3d_p3d++],&w3d_p3d_af[w3d_p3d++])

[Zhuge]

The problem is you are incrementing the variable twice between two sequence points. That results in the weird behavior you are seeing.

[LB]

The order in which parameters are evaluated is not defined by the C++ standard, so it could be any order. Usually it's right to left, but you should never rely on undefined behavior.