Why must my C++ function have different pointer arguments? -

i made c function pointer-passing gives right result if pointers different. instance:

void dotransform(point *pout, const point *pin, transform mat) {     pout->x = mat[0][0] * pin->x + mat[1][0] * pin->y + mat[2][0] * pin->z + mat[3][0] * 1.0;     pout->y = mat[0][1] * pin->x + mat[1][1] * pin->y + mat[2][1] * pin->z + mat[3][1] * 1.0;     pout->z = mat[0][2] * pin->x + mat[1][2] * pin->y + mat[2][2] * pin->z + mat[3][2] * 1.0; } 

dotransform() supposed called this:

//... transform toworldmat; point local; point world; dotransform(&world, &local, toworldmat ); 

the problem is, in team called this:

point p; dotransform(&p, &p, toworldmat); 

it took me 1 week figure out why final output of program changed.

what best style declare kind of function, avoid case? wrong write that?

seeing you're passing transform object directly (though not const&: should instead) there reason can't write code so:

point dotransform(point const& pin, transform const& mat) {     point pout     pout.x = mat[0][0] * pin.x + mat[1][0] * pin.y + mat[2][0] * pin.z + mat[3][0] * 1.0;     pout.y = mat[0][1] * pin.x + mat[1][1] * pin.y + mat[2][1] * pin.z + mat[3][1] * 1.0;     pout.z = mat[0][2] * pin.x + mat[1][2] * pin.y + mat[2][2] * pin.z + mat[3][2] * 1.0;     return pout; } 

which allow write code this:

transform toworldmat; point local; point world = dotransform(local, toworldmat); 

or this:

point p; p = dotransform(p, toworldmat); 

these correct, ideomatic c++ ways write code.